E:\data\efiles.dir ##END_OF_RECORD Absolute Maximum Ratings Peak ratings, that, if exceeded, may cause permanent damage to a power supply. When specified, they are not given as continuous ratings, and proper operation is not implied. ##END_OF_RECORD AC Front-End Part of a distributed power system that converts ac line voltage to a semiregulated dc voltage level. AC front-ends typically provide power factor correction and universal (~85 VAC to 265 VAC) ac compatibility. An ac front-end's output voltage is usually 350 VDC to 400 VDC. See Power Factor Correction. ##END_OF_RECORD AC Line The ac power distribution lines. See Alternating Current. ##END_OF_RECORD AC Line Filter A filter placed in the ac line to condition high frequency noise variations. ##END_OF_RECORD Aging The operation of a power supply under tightly controlled conditions (input voltage, output load, ambient temperature, etc.) for a predetermined time. Used in reliability testing to screen out or stabilize marginal units (infant mortality). See Burn-in. ##END_OF_RECORD Alternating Current (Voltage) (ac) A periodic current (voltage) that symmetrically switches direction of flow from positive to negative values. Over a given period of time, the average value of an ac waveform is zero. The number of cycles completed per second is the ac frequency. See Direct Current. ##END_OF_RECORD Ambient Air The air mass immediately surrounding an operating power supply. Reliable operation of power supplies requires sufficient surrounding air mass and flow to prevent thermal runaway caused by heat dissipated by the supply. ##END_OF_RECORD Ambient Temperature The average temperature of air immediately surrounding a power system. Temperature measurements should be made approximately 0.5 inches from the body of the power supply. See Operating Temperature Range and Storage Temperature Range. ##END_OF_RECORD American Wire Gauge (AWG) A standard for sizing wire by cross sectional area. ##END_OF_RECORD Ampere (A) The base unit of current, equal to: ##END_OF_RECORD Amplifier See Error Amplifier. ##END_OF_RECORD Apparent Power The product of input rms voltage times input rms current. High rms values in AC input, switch-mode power systems, where the input current is distorted results in high apparent power. ##END_OF_RECORD Autoranging Input An ac power supply input circuit that senses the input voltage level and automatically switches the input to the appropriate range (i.e. 90 VAC to 130 VAC or 180 VAC to 264 VAC). Sometimes called an autoselect input. ##END_OF_RECORD Average Power In an ac circuit, the average value of ac power. For resistive circuits, this equals the square of the rms current times the circuit resistance, as given by: PAVG = (Irms)2R ##END_OF_RECORD Average Voltage The average value of an ac voltage is equal to 0.637 times the maximum value. EAVG = 0.637 EMAX ##END_OF_RECORD B British Approvals Board for Telecommunications (BABT) An independent organization with approval authority for telecommunications equipment sold in the UK. The BABT grants approvals and accredits test laboratories. ##END_OF_RECORD Back Ripple Current See Input Reflected Ripple Current. ##END_OF_RECORD Bakelite A plastic resin used as a case material for power conversion modules. Part of the family phenol-formaldehyde plastics. ##END_OF_RECORD Balun Filter An input line filter that include a differential wound transformer often used within power supplies. Balun filters a high impedance to common mode signals and a low impedance to differential mode signals. ##END_OF_RECORD Baseplate A metal plate or substrate to which circuit components are mounted or the power supply is attached. Used to draw heat away from critical circuit components or mechanical stability). See Heat Sink. ##END_OF_RECORD Baseplate Temperature See Case Temperature. ##END_OF_RECORD Battery A multicell device that produces DC electricity via an electrochemical action. Battery cells are connected in parallel (to produce a required current) and/or series (to produce a required voltage). ##END_OF_RECORD Battery Backup An electronic equipment subsystem that provides temporary power in the event of input power loss. Battery backed systems range from short term options for AC/DC power supplies to high VA Uninterruptible Power Systems. See Uninterruptible Power Supplies (UPS). ##END_OF_RECORD Battery Charger A power supply or power supply output that is used to charge a storage battery or battery system. ##END_OF_RECORD Bench Power Supply A cased power supply with displays and controls that can be used to precisely adjust its operating characteristics. Usually an ac input device, bench supplies are sometimes referred to as Laboratory Supplies. ##END_OF_RECORD Bleeder Resistor A resistor that provides a path for current drain. Often used in filter circuits provide a discharge path for capacitors. ##END_OF_RECORD Bobbin A spool (typically nonmetallic) used in the winding of transformers and inductors. Bobbins provide a physical frame that supports the transformer or inductor windings and keeps the windings isolated from the core. ##END_OF_RECORD Bode Plot For a control loop, a plot that shows the relationship of gain and phase vs frequency. The loop stability of a power supply is verified using a bode plot. ##END_OF_RECORD Boost Regulator A basic switching converter topology wherein energy is stored in an input inductor. When the shunt switch is turned off, this energy is transferred to the output. Boost regulators take an unregulated input voltage, and produce a higher, regulated output voltage. See Buck Regulator, Bridge Converter, Flyback Converter, Forward Converter, Push-Pull Converter and Resonant Converter. ##END_OF_RECORD Booster See Slave. ##END_OF_RECORD Breakdown Voltage The maximum ac or dc voltage that can be applied from the input to output (or chassis) of a power supply without causing damage. ##END_OF_RECORD Bridge Converter A switching converter topology that utilizes a full bridge circuit (four switching elements) or half bridge circuit (two switching elements). This topology is more common in off-line (ac input) supplies than DC/DC converters. Bridge converters can provide high output power and low output ripple, but tend to be more complex than other power converter topologies. The added circuit complexity increases cost and may reduce reliability. See Boost Regulator, Buck Regulator, Flyback Converter, Forward Converter, Push-Pull Converter and Resonant Converter. ##END_OF_RECORD British Standards Institution The B.S.I. is an organization that develops standards and tests products/systems for compliance. An organization associated with B.S.I. called Technical Help to Exporters (T.H.E.) provides reference material on world wide standards. ##END_OF_RECORD Brownout A reduction or sag in the ac line or system power bus. If not protected against sags (by an adequate hold-up time or extended input ranges), an ac input power system could inadvertently shutdown. ##END_OF_RECORD Brownout Rated An ac input power supply that has been designed to operate within specified limits in the event of a brownout (sag) in the input ac mains or system power bus. ##END_OF_RECORD Brute Force Supply A basic power supply that provides an unfiltered and unregulated output. ##END_OF_RECORD Buck Regulator A basic switching converter topology that uses a series switch to chop the input voltage. The resulting pulses are applied to an averaging LC filter. Buck regulators will only produce an output voltage lower than the input voltage level. See Boost Regulator, Bridge Converter, Flyback Converter, Forward Converter, Push-Pull Converter and Resonant Converter. ##END_OF_RECORD Buck-Boost Converter See Flyback Converter. ##END_OF_RECORD Buck-Derived Converter See Forward Converter. ##END_OF_RECORD Bulk Capacitor A storage capacitor often recommended for use on the input of a distributed power system. ##END_OF_RECORD Burn-In To stabilize power supply operation and eliminate infant mortality, newly manufactured power supplies are often operated for a period of time prior to shipment. This "aging" of the unit is tightly controlled, including the time period, temperature and any special conditions (input power cycling, load switching, etc.). ##END_OF_RECORD Bus The primary conductor path (wires, cables, etc.) used for routing power to various components within a (sub)system. ##END_OF_RECORD C Canadian Standards Association Established in 1919, CSA is an independent organization that sets safety standards (and tests for compliance) for electronic components and systems that are to be sold in Canada. ##END_OF_RECORD Capacitance An electrical property that opposes a change in voltage level and stores energy in an electrostatic field. ##END_OF_RECORD Capacitive Coupling The coupling of a signal between two circuits or components caused by discrete or parasitic capacitance. ##END_OF_RECORD Capacitive Input Filter A filter that uses a capacitor as its input component. See Input Line Filter, Balun Filter, Choke Input Filter, Pi Filter and Output Filter. ##END_OF_RECORD Case See Enclosure. ##END_OF_RECORD Case Temperature When the power supply and surrounding system are operating normally, the temperature of the unit case. Often used as a specification (as opposed to ambient temperature) for power supplies with extended temperature ranges or high power densities. Case temperature is sometimes referred to as Baseplate Temperature. ##END_OF_RECORD Cell The basic unit used in the generation and storage of electricity. A battery consists of a number of connected cells. ##END_OF_RECORD Central Power System A power system in which all the components are contained in one box (or on one card). Often, a central power system consists of a medium or high power switching power supply that takes an ac input and generates multiple, regulated dc outputs. These dc outputs are bused throughout the system (via a wiring harness or backplane) to provide power to various system components. ##END_OF_RECORD Certified Europe (CE) A mark applied to electronic products (including some power supplies) to be sold in Europe. The mark certifies the power supplies meet the applicable EMI/RFI and safety directives. ##END_OF_RECORD Chassis See Enclosure. ##END_OF_RECORD Chassis Ground The voltage potential present on the chassis, frame or enclosure surrounding a power supply. ##END_OF_RECORD Charge Rate The rate at which current is applied to a battery cell to restore its capacity. Typically expressed as ampere-hours or milliampere-hours. ##END_OF_RECORD Choke See Inductor. ##END_OF_RECORD Choke Input Filter A filter that uses an inductor as its input component. Often used with a capacitor (as an L/C filter), choke input filters offer improved line regulation and are more resistant to input variations than capacitor filters. See Input Line Filter, Balun Filter, Capacitor Input Filter, Pi Filter and Output Filter. ##END_OF_RECORD Chopper See Inverter. ##END_OF_RECORD Circuit Breaker A fuse like protection device that opens when a circuit is overloaded. Once tripped, circuit breakers can be reset. ##END_OF_RECORD Circulating Current See Ground Loop. ##END_OF_RECORD Clamp Diode A diode found in clipper or clamp circuits. These circuits are often used as part of a power supply protection circuit. ##END_OF_RECORD Clearance Distance The shortest distance (through air) separating two live conductors or circuit components. See Creepage Distance. ##END_OF_RECORD Clock Timing pulses used in electronic systems to synchronize circuit operation. In a power supply, clock pulses synchronize operation of the pulse width modulator (PWM). ##END_OF_RECORD Comite International Special des Perturbations Radioelectriques (C.I.S.P.R.) The International Special Committee on Radioelectronic Interference is an IEC committee that develops standards for EMC (CISPR 22). Requirements are similar to those of the FCC, with significant changes in frequency limits and the specification of "average" limits for testing. See Electromagnetic Compatibility, Electromagnetic Interference, Federal Communications Commission and International Electrotechnical Commission. ##END_OF_RECORD Comite pour Europeen de Normalisation Electronic (CEN/CENELEC) The European Committee for Electrotechnical Standardization is a technical committee that recommends standards to be adopted by the European Community (EC). Standards cover EMI/RFI interference, intrinsic product safety, immunity, etc. ##END_OF_RECORD Common A conductive path used as a return for two or more circuits. When connected to earth, a common is the same as ground. See Ground. ##END_OF_RECORD Common Mode Noise The noise component that is equal and in phase on conductors with respect to a common reference point. ##END_OF_RECORD Common Mode Output Electrical energy that is unintentionally supplied to an external impedance that exists between the output connection of a floating power supply output and a system common point (chassis, frame, shield, etc.) ##END_OF_RECORD Compliance Range The range of voltage required to maintain a given constant current output for a specified range of load resistance. ##END_OF_RECORD Compliance Voltage The maximum dc output voltage of a constant current power supply. ##END_OF_RECORD Conductance The ability of a circuit or circuit element to conduct electricity. Conductance, measured in mhos (English) or siemens (metric), is the reciprocal of resistivity and is given by the formula: ##END_OF_RECORD G = 1/R ##END_OF_RECORD Conductor A material that allows the easy flow of electricity, such as metals, electrolytes, etc. See Insulation. ##END_OF_RECORD Conduction Cooling The transfer of heat through a solid material. Used to cool electrical components or subassemblies (such as a power supply) by the addition of heat sinking. This effectively increases the component or subassembly case surface area, lowering the thermal resistance. Thermal resistance equals: ##END_OF_RECORD q = L/KA Where: q = Thermal Resistance L = Length of material K = Thermal conductivity of material A = Cross sectional area of material ##END_OF_RECORD See Cooling, Convection Cooling, Free Convection and Forced Air Cooling. ##END_OF_RECORD Constant Current Load An electronic load that maintains a constant current drain on a power supply. ##END_OF_RECORD Constant Current Power Supply A power supply designed to regulate output current to within a specified range for changes in output load, input line and ambient temperature. ##END_OF_RECORD Constant Voltage Power Supply A power supply designed to regulate output voltage to within a specified range for changes in output load, input line and ambient temperature. ##END_OF_RECORD Continuous Shield See Six-Sided Shielding. ##END_OF_RECORD Control Circuit Circuit that controls certain operating parameters of the power supply. Used to maintain output regulation. ##END_OF_RECORD Control Loop A feedback loop used to control a power supply output. ##END_OF_RECORD Convection Cooling The transfer of heat via a fluid motion (typically air). In power subsystems this is normally accomplished by the forced movement of air over the power supply case and/or heat sink surface area. See Cooling and Conduction Cooling, Free Convection and Forced Air Cooling. ##END_OF_RECORD Converter (AC/DC) A device that accepts an ac line input voltage and produces a dc output(s). Often referred to as a "switcher" (although linear converters are available), switching regulated supplies are used in the majority of applications. AC/DC supplies are available in a variety of form factors, power levels and feature/performance envelopes. ##END_OF_RECORD Converter (DC/DC) A device that accepts a regulated or unregulated dc input voltage and produces a dc output that is typically at another voltage level. At times, to provide noise isolation, power bus regulation, etc., converters will have the same input and output level. ##END_OF_RECORD Cooling The transfer of heat dissipated by an operating power supply into the surrounding ambient air mass. See Convection Cooling and Conduction Cooling. ##END_OF_RECORD Core The material that a transformer or an inductor is wound on. In many cases, the core consists of a type of magnetic material (iron, etc.) and provides a path for magnetic flux. ##END_OF_RECORD Core loss Power dissipated within the core of a transformer/inductor due to eddy currents or hysteresis. Adversely affects the efficiency of a power supply. ##END_OF_RECORD COS J For undistorted sinusoidal waveforms, COSJ is the shift in load current as compared to the ac mains voltage. Equal to the difference between apparent power and true power (for an undistorted sinusoidal waveform). See Apparent Power, and True Power. ##END_OF_RECORD Creepage Distance The shortest distance between two conductors (typically, one primary, one secondary). ##END_OF_RECORD Crest Factor For an ac waveform, the ratio of peak value to RMS value. If the waveform is pure sinusoidal, this value is 2. Crest factor was used to approximate the current stress in an AC mains circuit. Today, the use of power factor is more common. See Power Factor. ##END_OF_RECORD Crest Value The maximum value of a waveform (excluding transients). ##END_OF_RECORD Cross Regulation In a multiple output power supply, the change in voltage level on one output, caused by a load change on another output, expressed as a percentage. ##END_OF_RECORD Crowbar A protection circuit that rapidly shuts down a power supply's output when a preset voltage level is exceeded. A crowbar circuit places a low resistance shunt across the output when an overvoltage condition exists. ##END_OF_RECORD Cubic Feet per Minute (CFM) A measure of the volume of air flowing in a system. The conversion of cubic feet per minute to linear feet per minute is dependent upon the cross-sectional area through which the air flows: ##END_OF_RECORD CFM = LFM x Area ##END_OF_RECORD See Linear Feet per Minute. ##END_OF_RECORD Cuk Converter A "buck-boost" converter variation that produces very low output ripple. Typically used in applications that do not require isolation. See Flyback Converter. ##END_OF_RECORD Current Foldback An overload protection circuit that reduces the output current as the load approaches short circuit. This minimizes internal power dissipation under short circuit conditions. Also called foldback current limiting. ##END_OF_RECORD Current Limiting An overload protection circuit that limits the maximum power supply output current to a predetermined, safe value. See Current Limit Knee. ##END_OF_RECORD Current Limit Knee The point at which current begins to foldback or limit on a plot of output voltage vs current. See Current Limiting, and Current Foldback. ##END_OF_RECORD Current Mode Control A control method used with switching converter topologies. A dual loop control circuit adjusts the PWM operation in response to a measured output current. ##END_OF_RECORD Current Share To increase system reliability or power, multiple power supplies are often used in parallel. When connected this way, the power supply outputs are strapped together and each unit supplies approximately equal "shares" of the load current. Current sharing can be controlled passively, by synchronizing multiple converters and trimming the outputs to within a tight margin; or actively with internal circuits to monitor and adjust output current. The most popular redundant topology is the "N+1" circuit. See Master/Slave, N+1 and Dual Redundant. ##END_OF_RECORD D DC/AC Converter See Inverter. ##END_OF_RECORD DC/DC Converter See Converter (DC/DC). ##END_OF_RECORD DC OK Signal See Power Good Signal. ##END_OF_RECORD Decay Time The time required for a pulse to decrease from 90% to 10% of its maximum amplitude (either positive or negative). ##END_OF_RECORD Decibel A decibel (dB) is the fundamental measurement unit used in EMI measurements. Given by the equation: ##END_OF_RECORD 1dB = 10log10 PO/PI Where: PO = Output Power PI = Input Power ##END_OF_RECORD Decoupling To reduce system noise, it is often recommended that filter components (typically mF capacitors) be connected between the power terminals of the load in a power system. ##END_OF_RECORD Density The ratio of power supply output power to the volume of the power supply. Commonly referred to as power density. ##END_OF_RECORD Derate See Derating ##END_OF_RECORD Derating The reduction in output power, voltage or current required to operate a power supply in an environment beyond that typically specified. Normally used to characterize power supply operation at extended temperatures. See Cooling, Convection Cooling, Conduction Cooling, Forced Air Cooling and Free Convection. ##END_OF_RECORD Derating Curve The curve or graph that illustrates the amount of derating required to operate a power supply in an abnormal environment. ##END_OF_RECORD Dielectric A material that prevents two points in an electrical circuit from becoming conductively connected. At times referred to as a dielectric barrier. See Conductor. ##END_OF_RECORD Dielectric Breakdown The sudden, destructive, conduction of current through a dielectric that occurs when the applied voltage exceeds the breakdown value. See Dielectric Withstand Voltage and Breakdown Voltage. ##END_OF_RECORD Dielectric Strength Test See High Potential Test. ##END_OF_RECORD Dielectric Withstand Voltage The maximum voltage an insulating material can withstand before suffering punch through or arcing across. See Breakdown Voltage and High Potential Test. ##END_OF_RECORD Differential Mode Noise The noise component measured between two circuit points with respect to a common point (minus common mode noise). ##END_OF_RECORD Differential Voltage The difference in voltage levels measured between two points. ##END_OF_RECORD Direct Current A current that flows in one direction. See Alternating Current. ##END_OF_RECORD Discrete Circuit A circuit or subassembly comprised of discrete components (resistors, diodes, FETs, etc.) as opposed to an assembly comprised of integrated components. ##END_OF_RECORD Dissipation See Internal Power Dissipation. ##END_OF_RECORD Distributed Power A system level architecture in which a central supply or battery system powers an intermediate power bus (i.e. 24 or 48 VDC). This intermediate bus, routed throughout the system (normally via a backplane), provides the input power to a number of DC/DC converters. These converters in turn provide localized power (and various voltage levels) to individual subassemblies and/or components. The type of power distribution system used is dependent upon the requirements of a particular application. ##END_OF_RECORD Drift With all operating parameters (load, line, etc.) held constant, the gradual (and undesirable) change in the output voltage of a power supply over a specified period of time. Typically specified as starting after a warm-up period. ##END_OF_RECORD Driver See Master. ##END_OF_RECORD Drop Out Voltage The minimum input voltage level required to operate a power supply to within specified operating limits. ##END_OF_RECORD Dual-In-line-Package (DIP) Popular packaging footprint for low power DC/DC converter modules. ##END_OF_RECORD Dual-Redundant A fully redundant power system topology. The circuit consists of two complete power supplies connected in parallel. Power supply #1 runs "hot", supplying all the load current, while power supply #2 runs "cool" (disconnected from the load via oring diodes). If the "hot" supply fails, the oring diodes reverse bias and power supply #2 starts to provide full load current. Dual redundant systems are expensive and not as flexible in comparison to some other redundant approaches, but they do allow separate input power sources to be used. Sometimes referred to as a "100% redundant" system. See Master/Slave and N+1. ##END_OF_RECORD Duty Cycle For power supplies, the ratio of "on" time to "off" time of the semiconductor switch (in PWM systems) or clock signal. ##END_OF_RECORD Dynamic Load An output load that changes rapidly. Normally specified as a load change value as well as a rate of change. ##END_OF_RECORD Dynamic Response The output overshoot that occurs when the output load of a power supply is either cycled on/off or abruptly changed. This overshoot gives the high frequency output impedance of the power supply. See Output Impedance. ##END_OF_RECORD E Earth See Ground. ##END_OF_RECORD Eddy Current A circulating current induced by variations in a magnetic field. ##END_OF_RECORD Eddy Current Loss The energy losses in a system/component ##END_OF_RECORD Efficiency (h) The ratio of total output power to input power expressed as a percentage. Expressed as a percentage, efficiency is derived by the equation: ##END_OF_RECORD Efficiency (%) = Output Power/Input Power x 100 ##END_OF_RECORD Efficiency may vary under differing operating conditions, and is normally specified at full rated output power, nominal input line conditions and +251C. Efficiency has become an increasingly important power supply specification with the rising demand for higher power density systems. ##END_OF_RECORD Electrolytic Capacitor A type of capacitor used in power supplies that consists of two plates (or electrodes) separated by an electrolyte. ##END_OF_RECORD Electrostatic Shield See Faraday Shield. ##END_OF_RECORD Electromagnetic Compatibility (EMC) Power supplies and systems must be designed to be electomagnetically compatible. In other words, they cannot be adversely affected by emissions generated internal to the power supply or externally within the system. ##END_OF_RECORD Electromagnetic Interference - Conducted Noise generated by a power supply (typically by the switching action of the more popular power conversion topologies) and reflected back onto the input power bus. Acceptable limits for conducted EMI are set by various agencies (FCC, VDE, etc.). Switch-mode power supplies typically include input filters to reduce noise to within agency limits. ##END_OF_RECORD Electromagnetic Interference - Radiated Noise generated by a power supply (typically by the switching action of the more popular power conversion topologies) and emitted into the area surrounding a power system. Radiated EMI, consisting of broadband radio frequencies and narrow band emissions is limited by various agencies (FCC, VDE, etc.). Typically, it is controlled by shielding. ##END_OF_RECORD Electrostatic Discharge (ESD) Current produced by two objects having a static charge when they are brought close enough to produce an arc or discharge. ##END_OF_RECORD Enclosure The case/container used to package a power supply. Enclosures, which range from small plastic shells to sophisticated metal cases, protect internal components from the outside environment; provide mechanical stability and in some applications improve thermal and noise performance. See Open Frame and L-Frame. ##END_OF_RECORD Equivalent Series Inductance (ESL) The inductance in series with an "ideal" capacitor. ESL sources include terminals, electrodes etc. ##END_OF_RECORD Equivalent Series Resistance (ESR) The resistance in series with an "ideal" capacitor. ESR sources include lead resistance, terminal losses, etc. ESR is an important specification for high frequency applications. ##END_OF_RECORD Error Amplifier An amplifier used in the control feedback loop of a power supply. It produces an error voltage when the sensed output (tapped off a voltage divider network) differs from a reference voltage. The error voltage adjusts the PWM operation to correct the sensed output voltage. Sometimes called a Reference Amplifier. ##END_OF_RECORD F Failure Mode The reason a power supply or system either does not meet or stops meeting specified parameters. ##END_OF_RECORD Fan Cooled See Forced Air Cooling. ##END_OF_RECORD Faraday Shield An electrostatic shield that reduces coupling capacitance in transformers. The shield, which reduces output common mode noise, is placed between the primary and secondary windings of a transformer. ##END_OF_RECORD Fault Mode Input Current The input current drawn by a power supply when the output is shorted. ##END_OF_RECORD Fault Tolerant A power system configuration optimized for continuing operation without shutdown. Typically involves the use of a redundant topology (i.e. N+1, etc.) and the ability to remove and replace power modules without disturbing system operation (hot-plug capability). Thus, the failure of any one power module will not cause a system failure. ##END_OF_RECORD Federal Communications Commission (FCC) A US government agency that sets standards for, and governs the testing of conducted and radiated emissions. These are system level standards, but they are typically used in specifying converters. See Electromagnetic Interference. ##END_OF_RECORD Feedback The process of returning part of the output signal of a power supply to its input, where it is used to control or regulate the output. ##END_OF_RECORD Feed Forward A method of improving line regulation by directly sensing the input voltage of the power converter. See Line Regulation. ##END_OF_RECORD Ferroresonant Transformer A transformer in which part of the core is driven into saturation by a resonant tank circuit. The output of the transformer, taken from the saturated portion, is relatively immune to variations in input voltage. Used in ferroresonant power supplies. ##END_OF_RECORD Filter A circuit that passes one frequency or band of frequencies while blocking others. See Input Line Filter, Balun Filter, Pi Filter and Output Filter. ##END_OF_RECORD Flag Signal A status or alarm signal generated by a power system. Often displayed as a front panel LED or output logic signals (normally TTL/CMOS compatible), typical signals would indicate "Power Good", "Power On", "Over temperature", etc. ##END_OF_RECORD Floating Output A power supply output that is ungrounded and not referenced to another output. Typically, floating outputs are fully isolated and may be referenced positive or negative by the user. Outputs that are not floating share a common return and as such, are referenced to one another. ##END_OF_RECORD Flyback Converter Sometimes called a "buck-boost" converter, this topology minimizes the required components by using a single transistor switch and eliminating the need for an output inductor. During the first half of the switching period, when the transistor switch is "ON", energy is stored in the transformer primary. During the second half or "flyback" period when the switch is "OFF", this energy is transferred to the transformer secondary and load. This technique is cost effective because of the minimum number of components required. See Boost Regulator, Buck Regulator, Bridge Converter, Forward Converter, Push Pull Converter and Resonant Converter. ##END_OF_RECORD Foldback Current See Current Limiting and Current Limit Knee. ##END_OF_RECORD Forced Air Cooling The use of a fan (or other air moving equipment) within a (sub)system to move air across heat producing components in order to reduce the ambient temperature. See Convection Cooling, Free Convection and Forced Convection. ##END_OF_RECORD Forced Convection An operating environment in which air movement induced by a fan, blower, etc. is used to maintain power modules within operating limits. See Convection Cooling, Free Convection and Forced Air Cooling. ##END_OF_RECORD Forward Converter Like the flyback circuit, a forward converter uses a single transistor switch. However, in the forward converter, energy is transferred to the transformer secondary while the transistor switch is "ON", and stored in an output inductor. Also called a "Buck-Derived" converter. See Boost Regulator, Buck Regulator, Bridge Converter, Flyback Converter, Push Pull Converter and Resonant Converter. ##END_OF_RECORD Free Convection An operating environment in which the natural movement of air (unassisted by fans or blowers) is sufficient to maintain the power module within its' operating limits. Also called "natural convection". See Convection Cooling, Forced Air Cooling and Forced Convection. ##END_OF_RECORD Frequency of Operation See Switching Frequency. ##END_OF_RECORD Front End See AC Front End. ##END_OF_RECORD Full Bridge Converter A power supply topology, typically configured as a forward converter, that uses a bridge circuit, consisting of four switching transistors, to drive the transformer primary. See Bridge Converter. ##END_OF_RECORD Full Load The maximum value of output load specified for a converter under continuous operating conditions. ##END_OF_RECORD Full Wave Rectifier A circuit (bridged or center tapped) that rectifies both halves of an ac waveform. ##END_OF_RECORD Fuse A circuit protection device, consisting primarily of a low melting point wire. If current passing through the wire exceeds a set level (as in the case of a fault condition), the wire melts and the circuit opens. ##END_OF_RECORD G Galvanic Isolation Two circuits which have no significant ohmic connection are "galvanically isolated" from each other. Galvanic isolation (separation) is achieved by using a transformer, optocoupler, etc. ##END_OF_RECORD Ground An electrical connection that is made to earth (or a conductor that is connected to earth). A power converter "common" is not actually ground unless it is somehow connected to earth. See Common. ##END_OF_RECORD Ground Bus A common grounded conductor to which various system components are returned. ##END_OF_RECORD Ground Connection A low resistance connection to earth. ##END_OF_RECORD Ground Fault Circuit failure caused by the loss of a ground connection. ##END_OF_RECORD Ground Loop When two or more system components share a common ground line, a feedback (ground) loop is induced. This can cause unwanted voltage levels within the system. ##END_OF_RECORD Ground Noise Circuit noise caused by a faulty ground connection. ##END_OF_RECORD Ground Plane A conductive pad on the printed circuit board, set at ground potential, upon which a power supply is mounted. Shields system components from possible RFI noise generated by the power supply. ##END_OF_RECORD Ground Potential Potential of the ground point in a circuit as referenced to other circuit voltage levels. ##END_OF_RECORD Ground Resistance The DC resistance of a connection to ground or earth. ##END_OF_RECORD Ground Return The circuit point or path used as a return to ground (to compete the circuit). ##END_OF_RECORD H Half Bridge Converter A power supply topology that uses a bridge circuit, consisting of two switching transistors, to drive the transformer primary. Half bridge converters are typically configured as a forward converter. See Bridge Converter. ##END_OF_RECORD Half Wave Rectifier A single diode circuit that rectifies 1/2 of an AC waveform. ##END_OF_RECORD Harmonic Distortion For sinusoidal AC current waveforms, the load current distortion caused by the presence of multiple harmonics of the fundamental frequency (the frequency of the AC mains voltage). This decomposition is calculated through Fourier analysis: ##END_OF_RECORD I(t)=I0 x sin(w0t+J0)+I1 x sin(w1t+J1)+ ...In x sin(wnt+Jn) Where: IO = Amplitude of the fundamental In = Amplitude of the nth harmonic wO = 2p x fO fo = Frequency of the fundamental (50/60 Hz) wn = Frequency of the nth harmonic JO = Phase shift of the fundamental Jn = Phase shift of the nth harmonic ##END_OF_RECORD Based on this, the distortion factor is calculated as: ##END_OF_RECORD K = Contents of Harmonics/rms value ##END_OF_RECORD Head Room For a series pass regulator, refers to the difference between the input and output voltages. ##END_OF_RECORD Heat Flux The flow rate of heat across or through a material. Given in units of W/cm2. ##END_OF_RECORD Heat Rise The increase in component/subassembly temperature caused by self-heating or heat absorption. ##END_OF_RECORD Heat Sink A metal extrusion, plate etc. used to transfer heat away from sensitive components and/or circuits. See Baseplate. ##END_OF_RECORD Heat Sink Resistance The resistance to heat flow within a heat sink. ##END_OF_RECORD Heat Sink Temperature The average temperature of a heat sink attached to a power system module or component during normal operation. Under typical operating conditions, heat sink temperature will be lower than the module/component case (or baseplate) temperature. ##END_OF_RECORD Hiccup Mode An operating mode in which an output fault condition (short-circuit) causes the power supply to cycle on and off. The duty cycle of the on/off cycle is set to maintain the internal power dissipation at a safe level until the fault condition is corrected. ##END_OF_RECORD High Line The maximum input line voltage specified for normal power supply operation. See also Low Line and Input Voltage Range. ##END_OF_RECORD High Potential Test (Hipot Test) A test that determines whether a power supply passes its minimum break down voltage specification. See Breakdown Voltage. ##END_OF_RECORD Hipot Test Abbreviation for High Potential Test. ##END_OF_RECORD Hold-up Capacitor A capacitor on the input of a distributed power system. The hold-up capacitor will maintain or "hold-up" the input voltage to the power system in the event a fault causes a loss of the input bus voltage. The capacitor has to be selected to have sufficient capacity to maintain the input until the fault condition is removed (typically the time required for a fuse to blow). ##END_OF_RECORD Hold-Up Time Following a loss of input power, the period of time a power supply output will remain operating to within specification. This is a more common specification for AC/DC power supplies (typically specified at >16 mS). ##END_OF_RECORD Hot-Plug A fault tolerant feature that allows power supplies to be inserted or removed from a system while the system is powered and operating. Power supplies must be specifically designed for this feature to avoid without disturbing other modules or subassemblies connected to the system power bus. ##END_OF_RECORD Hot-Swap See Hot-Plug. ##END_OF_RECORD I Impedance The total opposition offered by a circuit/component to the flow of ac current. ##END_OF_RECORD Inductance An electrical property that opposes the flow of current in a circuit when a voltage is applied (or a change in an established current). Measured in henries. ##END_OF_RECORD Inductive Load A load that is inductive in nature, such as electric motors. ##END_OF_RECORD Inductor A coil or winding with inductive properties. ##END_OF_RECORD Inhibit See Logic Inhibit/Enable. ##END_OF_RECORD Input Capacitor The first capacitor in a capacitor-input filter. ##END_OF_RECORD Input Current The current drawn from the input power bus by a power converter when operating under nominal conditions. ##END_OF_RECORD Input Line Filter A low-pass or band-reject filter on the power supply input (internal or external) that attenuates noise from the system power bus. See Balun Filter, Filter and Pi Filter. ##END_OF_RECORD Input/Output Isolation See Isolation. ##END_OF_RECORD Input Reflected Ripple Current The AC component (typically generated by the switching circuit) measured at the input of a power supply. Given as a peak to peak or rms value. ##END_OF_RECORD Input Surge Current See Inrush Current. ##END_OF_RECORD Input Transient A spike in the input line to a power supply. Input circuits/components are used to absorb or clip transients, shielding sensitive components from possible damage. ##END_OF_RECORD Input Voltage Range The minimum and maximum input voltage limits within which a power supply will operate to within specifications. For DC/DC converters, it is typically given as a percentage or ratio of high line to low line (i.e. a range of 18 VDC to 36 VDC is 2:1 and a range of 9 VDC to 36 VDC is 4:1). For AC/DC power supplies, operation in Europe and North America requires an input range that accepts nominal levels of 110 VAC and 220 VAC and it is normally specified as "universal" (90 VAC to 264 VAC), autoranging or user selectable. ##END_OF_RECORD Inrush Current At turn on, the maximum, instantaneous input current drawn by a power supply. Also called Input Surge Current. ##END_OF_RECORD Inrush Current Limiting A protection circuit that limits the current a power system draws at turn on. ##END_OF_RECORD Insulation A nonconductive material used to protect and separate electronic components or circuits. ##END_OF_RECORD Insulation Resistance Resistance offered by an insulating material to current flow. ##END_OF_RECORD Internal Power Dissipation The power dissipated as heat within the power supply during normal operation. Primarily a function of the power handling capability and efficiency of the power supply. Internal power dissipation is normally given as a maximum specification that cannot be exceeded without risking damage to the power supply. ##END_OF_RECORD International Electrotechnical Commission (IEC) An organization based in Switzerland that sets standards for electronic products and components. IEC does not conduct any testing, however, their standards have been adopted by many of the national safety/standards agencies. ##END_OF_RECORD International Organization for Standards (ISO) Established in 1947 ISO is a worldwide federation of standards bodies that works to promote the harmonization of international standards. Thousands of companies now comply with ISO 9000 and ISO 14000 standards. ##END_OF_RECORD Inverter A circuit that converts DC power to AC power. ##END_OF_RECORD Isolated Output See Floating Output. ##END_OF_RECORD Isolation The electrical separation between the input and output of a power converter. Normally determined by transformer characteristics and component spacing, isolation is specified in values of resistance (typically megohms) and capacitance (typically picofarads). ##END_OF_RECORD Isolation Capacitance See Isolation. ##END_OF_RECORD Isolation Resistance See Isolation. ##END_OF_RECORD Isolation Voltage The maximum voltage (ac or dc) that can be continuously applied between isolated circuits without a breakdown occurring. For power supplies, it is normally specified as input-output or input-case isolation. Minimum isolation voltage levels must be maintained to meet most safety regulations. See Breakdown Voltage, High Potential Test and Isolation. ##END_OF_RECORD L L-C Filter A low pass filter consisting of a series inductor followed by a shunt capacitor. Often used as an input filter in low power converters. ##END_OF_RECORD L-Frame A type of power supply construction in which the assembled power unit is mounted onto an "L" shaped metal frame. This frame is used as heatsinking for critical components and gives mechanical stability to the entire power supply assembly. In the application, the L-frame is typically attached directly to the users equipment chassis. See Open Frame and Enclosure. ##END_OF_RECORD Lagging Load An inductive load (current lags voltage). ##END_OF_RECORD Lead Frame A metal frame that holds the pins or components of a power supply in place during encapsulation (after which it is cut away). Very common in the manufacture of semiconductors, lead frames are now used on low power DC/DC converters. ##END_OF_RECORD Leading Load A capacitive load (current leads voltage). ##END_OF_RECORD Leakage Current At a set voltage level, the current flowing from input to output or input to case of an isolated power supply. For an ac input power supply, the current that flows from the primary circuit to earth ground. Excessive leakage current may pose a hazard to system operators if the earth ground is disconnected. For this reason, safety agencies regulate allowable levels of leakage current. ##END_OF_RECORD Life Test A power supply reliability test in which units are operated (typically under accelerated conditions) over some period of time in order to approximate life expectancy. ##END_OF_RECORD Light Load A power supply load that is a small fraction of its normal level. ##END_OF_RECORD Line Bus used to deliver power to the input (terminals, pins, etc.) of a power supply. See Bus, High Line, AC Line and Low Line. ##END_OF_RECORD Line Cord A two or three wire insulated conductor used to connect equipment to the power line. ##END_OF_RECORD Line Current The ac input current to a power system, taken off the mains. ##END_OF_RECORD Line Effect See Line Regulation. ##END_OF_RECORD Line Regulation The change in output voltage caused by varying the input voltage over a specified range (with output load, temperature, etc. remaining constant), expressed as a percentage. ##END_OF_RECORD Line Transient See Input Transient. ##END_OF_RECORD Line Frequency Regulation The change in output voltage caused by a variation in input line frequency, all other factors remaining constant. In switching and linear power supplies, this is not significant, but it is an important specification in ferroresonant power supplies. ##END_OF_RECORD Line Voltage The ac input voltage to a power system, taken off the mains. ##END_OF_RECORD Linear Feet per Minute (LFM) A measure of the velocity of air. Used in distributed power systems to give the air flow over a baseplate or heatsink surface area. LFM is equal to: m/s multiplied by 196.8, Where, m/s = meters per second ##END_OF_RECORD Linear Power Supply A power supply that uses linear regulation. Linears provide excellent regulation, low output noise and fast transient response. However, they are typically much heavier, larger and less efficient then "switchers", which are now much more popular. See Switching Power Supply. ##END_OF_RECORD Linear Regulation A regulation technique in which the regulating device (typically a transistor) is placed in series or parallel with the load. Voltage variations across the load are then controlled by changing the effective resistance of the regulating device to dissipate unused power. See Series Regulator, Shunt Regulator and Post Regulation. ##END_OF_RECORD Load The electronic components or circuitry powered by a power system. The load characteristics (resistance, reactance, etc.) determine the amount of power drawn from the power supply. ##END_OF_RECORD Load Decoupling See Decoupling. ##END_OF_RECORD Load Effect See Load Regulation. ##END_OF_RECORD Load Regulation Change in output voltage caused by varying the output load over a specified range (with input line, temperature, etc. remaining constant), expressed as a percentage. ##END_OF_RECORD Load Sharing See Current Share. ##END_OF_RECORD Local Sensing Using the output terminals of the power supply to provide feedback to voltage regulation circuits. See Remote Sensing. ##END_OF_RECORD Logic Inhibit/Enable A signal (typically TTL/CMOS compatible) used to turn a power supply output on/off. Also called a Remote On/Off. ##END_OF_RECORD Long Term Stability The change, over time, in the output voltage of a power supply with all other factors (line, load, temp, etc.) remaining constant, expressed as a percentage. This output change is primarily due to component aging. ##END_OF_RECORD Low Line The minimum input line voltage specified for normal power supply operation. See High Line and Input Voltage Range. ##END_OF_RECORD M Magnetic Core See Core. ##END_OF_RECORD Mains See ac Line. ##END_OF_RECORD Margining A system performance test, in which the power supply output is adjusted up and down (typically by 5%). Used to verify the resilience of a system to power supply voltage fluctuations. ##END_OF_RECORD Master In a master/slave system, the intelligent power supply. The master power supply contains the feedback loop for the power subsystem, and thus controls output regulation. Sometimes called a driver. ##END_OF_RECORD Master/Slave Operation The connection of two or more power supplies in which one (master) controls the operation of the others (slaves). Master/slave configurations provide higher output power, wider input voltage ranges, synchronized operation, etc. ##END_OF_RECORD Maximum Current When in current limiting mode, the highest value of output current delivered by a power supply. ##END_OF_RECORD Maximum Load When running under continuous operating specifications, the highest amount of output load allowable for a power supply. ##END_OF_RECORD Maximum Operating Temperature The maximum temperature at which a power supply will operate to within specified operating parameters. Normally specified as either ambient or case. ##END_OF_RECORD Mean Time Between Failure (MTBF) A unit of measure, that gives the relative reliability of a power supply. MTBF data is either calculated per the conditions of MIL-HDBK-217F or demonstrated using actual operating data. MTBF is expressed in hours. ##END_OF_RECORD Mean Time To Repair (MTTR) For a repairable power supply, MTTR refers to the average time required to perform the repair work. Encapsulated power modules are typically not repaired. ##END_OF_RECORD Mil-Specifications US government standards that specify operating, test and environmental standards for equipment to be used in military/aerospace applications. ##END_OF_RECORD Minimum Load The minimum output load required to maintain the normal continuous operating specifications of a power supply. Usually associated with PWM (Pulse Width Modulation) controlled power supplies. ##END_OF_RECORD Minimum Operating Temperature The minimum temperature at which a power supply will operate within specified operating parameters. Normally specified as either ambient or case. ##END_OF_RECORD Modular Term used to describe the physical construction of power systems that consists of separate subassemblies. Modular construction tends to lower the design turnaround for custom products, but increases cost and lowers MTBF. ##END_OF_RECORD Modulator See Pulse Width Modulation. ##END_OF_RECORD Module An encapsulated or potted power supply. ##END_OF_RECORD Molded A component or subsystem that is completely enclosed in an insulating material (such as plastic). Some low power converters are now available as molded units. ##END_OF_RECORD Mylar A plastic material (polyethylene terephathalate) often used as an insulator in the design and assembly of power supplies. A registered trademark of EI Dupont DeNemours & Co. ##END_OF_RECORD N N+1 A redundant power system topology used to achieve high reliability levels. The circuit consists of a number of power supplies connected in parallel, sharing the load power. One more supply than is necessary to provide full load current is used (i.e. for a 1200W load, three 600W power supplies are used). Thus, if any single power supply fails, the remaining units will continue to power the load. See Master/Slave and Dual Redundant. ##END_OF_RECORD N+x An "N+1" redundant circuit in which more than one extra power supply is used (i.e. N+2, N+3, ...N+x). ##END_OF_RECORD Natural Convection See Free Convection. ##END_OF_RECORD Negative Temperature Coefficient A function (voltage, resistance, etc.) that decreases as temperature increases. ##END_OF_RECORD No Load Voltage The voltage level present at the output of a power supply when 0% load is applied. ##END_OF_RECORD Noise Undesirable variations in the power supply output that are unrelated to the switching frequency. Noise is typically specified as "Ripple and Noise" and given as a peak to peak value with a specified bandwidth. See Electromagnetic Interference, and Periodic and Random Deviation. ##END_OF_RECORD Nominal Value An ideal value that is used as a reference point. Typically it is not the same as the value actually measured. ##END_OF_RECORD O Off Line Power Supply A power supply (linear or switching) that operates directly off the ac line. The input voltage is rectified and filtered prior to any isolation transformer. ##END_OF_RECORD Open-Circuit Voltage See No Load Voltage. ##END_OF_RECORD Open Frame A power supply that is made without an enclosure. Very common in low/medium power ac switching supplies, the power supply is typically constructed on a printed circuit board which is then mounted in the end-user application. See Enclosure and L-Frame. ##END_OF_RECORD Operating Temperature Range The temperature range over which a power supply can be safely used within specified operating limits. May be specified as ambient, case or base plate temperature. ##END_OF_RECORD Oring Diodes Diodes, typically, connected externally in the output circuit of a power supply that isolate a faulty power supply from the load and other power converters. ##END_OF_RECORD Output Capacitance When a power supply is off, the capacitance present between the output terminals. ##END_OF_RECORD Output Current Limiting See Current Limiting. ##END_OF_RECORD Output Filter A circuit that attenuates noise and ripple current present on a power supply output. Normally, a power supply includes internal output filtering. For critical applications, external filter components may be added. ##END_OF_RECORD Output Impedance The ratio of change in output voltage to a change in output load current. Sometimes referred to as Dynamic Response. ##END_OF_RECORD Output Power The maximum output power a power supply is designed to provide. ##END_OF_RECORD Output Ripple & Noise See Noise and Periodic and Random Distribution. ##END_OF_RECORD Output Voltage DC voltage level measured at the power supply output pins. ##END_OF_RECORD Output Voltage Accuracy The maximum deviation (from the ideal or nominal value) allowed in the dc output voltage of a power supply. Also called Output Voltage Tolerance, it is expressed as a percentage of output voltage. ##END_OF_RECORD Output Voltage Range The minimum and maximum output voltage limits within which a power supply will operate within specified limits. ##END_OF_RECORD Output Voltage Tolerance See Output Voltage Accuracy. ##END_OF_RECORD Overcurrent Protection See Current Limiting. ##END_OF_RECORD Overload A condition under which excessive power is drawn from the power supply. May lead to a fault condition. See Current Limiting. ##END_OF_RECORD Overload Protection See Current Limiting. ##END_OF_RECORD Overshoot A momentary change in output voltage that exceeds specified limits. A transient overshoot will typically occur when a power supply is turned on/off; or with a step change in output load or input line. ##END_OF_RECORD Overvoltage Protection (OVP) An output monitoring circuit that activates whenever a preset output voltage level is exceeded. Differing OVP circuits are used, with features that will shut the power supply down, "crowbar" the faulty output or switch the supply to a different operating mode. ##END_OF_RECORD P Parallel Operation A power system in which the output currents of two or more power supplies are connected together to provide a higher level of output power. A power supply must be designed to share output loads to be used in a parallel connection. Although parallel circuits are used in redundant systems; power system redundancy is not necessarily achieved through a simple parallel connection. See Redundant Operation. ##END_OF_RECORD Periodic and Random Deviation (PARD) The noise and ripple voltage super imposed on the output of a power supply. Expressed in peak to peak or rms volts over a given bandwidth and typically specified at full load. ##END_OF_RECORD Pi Filter A filter consisting of two capacitors connected in parallel with a series inductance. Often used in DC/DC converters to reduce input reflected ripple current. ##END_OF_RECORD Pinout Diagram A diagram that shows the pin location and functionality for a power module. ##END_OF_RECORD Plate See Base Plate. ##END_OF_RECORD Post Regulation A power supply output circuit that uses a linear regulator to reduce output ripple and noise and improve line/load regulation. On the negative side, post regulation adds expense and degrades power supply efficiency. ##END_OF_RECORD Power Density The ratio of power supply output power to the volume of the power supply, expressed as watts per in3. ##END_OF_RECORD Power Factor For an ac input power supply, the ratio of true input power to apparent input power. A measure of the input current that is in phase with the input voltage (and thus contributing to the average power). ##END_OF_RECORD Power Factor Correction (PFC) A power supply feature in which the ac input current is forced to be approximately in phase with the input voltage. This reduces the harmonic distortion of the power system and increases the useful power drawn from the mains. PFC circuits can be active or passive. See Power Factor. ##END_OF_RECORD Power Fail Signal An output signal (typically TTL compatible) that indicates the DC input to the power supply has failed. This housekeeping signal is typically used to trigger the execution of an orderly shutdown during the hold-up time of the power supply. ##END_OF_RECORD Power Good Signal A signal that indicates the primary DC output of a power supply is still present. Often brought out to a system front panel as an LED indicator. ##END_OF_RECORD Power Supply See Converter (AC/DC) and Converter (DC/DC). ##END_OF_RECORD Power Rating The specified power available at the power supply output pins. ##END_OF_RECORD Primary Circuit The input side of an isolated power supply. See Secondary Circuit. ##END_OF_RECORD Pulse Width Modulation (PWM) A circuit used in power supplies to regulate output voltage. Regulation is achieved by varying the conduction time of the transistor switches. ##END_OF_RECORD Puncture Voltage See Breakdown Voltage. ##END_OF_RECORD Push Pull Converter A power supply circuit that uses two transistor switches and a center tapped transformer. Normally configured as a forward converter, the transistor switches turn on and off alternately. See Boost Regulator, Buck Regulator, Bridge Converter, Flyback Converter, Forward Converter and Resonant Converter. ##END_OF_RECORD Q Quasi-Regulation Auxiliary outputs on a multiple output power supply that are regulated via the primary output (which is controlled by a direct feedback loop). Auxiliary output voltages are set by the turns ratio of the isolation transformer. Quasi-regulated outputs are significantly affected by variations in the primary output. Sometimes referred to as semi-regulation. ##END_OF_RECORD R Radiation Cooling The transfer of heat between two materials at different temperature levels. Radiant heat does not play a significant role in the cooling of distributed power systems. ##END_OF_RECORD Radio Frequency Interference (RFI) Undesired noise generated by the power supply. See Electromagnetic Interference. ##END_OF_RECORD Rated Output Current The maximum output current that can be continuously drawn from a power supply under specified conditions. ##END_OF_RECORD Recovery Time The time required for a measured parameter to return to within specified limits following an abnormal variation. ##END_OF_RECORD Rectified AC Current The unfiltered dc output of a rectifying circuit. It consists of the unidirectional half cycles passed by the full or half bridge rectifiers. ##END_OF_RECORD Redundant Operation Power system configuration used to increase system reliability. Often used in distributed power systems, to achieve redundancy, power supplies are connected in parallel (typically configured in an "N+1", dual redundant or master/slave architecture). ##END_OF_RECORD Reflected Ripple Current See Input Reflected Ripple Current. ##END_OF_RECORD Regulated Power Supply A power supply whose output is held to within a tight error band regardless of changes in line and load. ##END_OF_RECORD Regulation A power supplies ability to maintain an output voltage to within specified limits under varying conditions of input line and output load. See Linear Regulation. ##END_OF_RECORD Regulatory Agencies Organizations (both independent and government supported) that develop specifications and/or test power supplies to specifications that define product performance or intrinsic safety. ##END_OF_RECORD Relative Humidity (rh) The ratio of the amount of moisture in the air to the amount the air could contain at a given temperature, expressed as a percentage. A common environmental specification for power supplies is the ability to withstand high levels of relative humidity. ##END_OF_RECORD Relative Power A power level that is specified with respect to a reference power level. ##END_OF_RECORD Remote Sensing An external output connection wherein sense leads connected to the output load provide feedback to a power supply. Used to compensate for voltage losses from long leads to a load. See Local Sensing. ##END_OF_RECORD Remote Shutdown See Logic Inhibit/Enable. ##END_OF_RECORD Reset After a power supply has been shut down by a fault condition, a manual operation or automatic signal (normally TTL compatible) that brings it back into operation. ##END_OF_RECORD Resistive Load A power supply load that exhibits the characteristics of a pure resistance. ##END_OF_RECORD Resonant Converter A power supply topology in which a resonant tank circuit operating at very high frequencies is used to transfer energy to the output. Often found in higher power DC/DC converter applications. ##END_OF_RECORD Response Time See Transient Recovery Time. ##END_OF_RECORD Return The common terminal on the output of a power supply. It is the return current path for the output. See Common. ##END_OF_RECORD Reverse Current See Leakage Current. ##END_OF_RECORD Reverse Voltage Protection A power supply feature that protects internal components from damage in the event a reverse voltage is accidentally applied to the either the input or output terminals. ##END_OF_RECORD Ripple and Noise See Periodic and Random Deviation (PARD). ##END_OF_RECORD Ripple Voltage The periodic AC component imposed on the output voltage of a power supply. Typically specified as part of "Ripple & Noise" and given as a peak to peak value over a specified bandwidth. See Periodic and Random Deviation (PARD). ##END_OF_RECORD Room Temperature A term sometimes used to describe a benign operating environment. Although it typically implies a temperature of about 77°F (25°C), room temperature has no actual quantitative meaning. ##END_OF_RECORD Root Mean Square (rms) For a sine wave, the rms value is equal to: rms = 0.707 x IPEAK ##END_OF_RECORD Royer Converter A self-oscillating push-pull switching topology often used in low cost, low power DC/DC converter designs. Also called the classical converter. ##END_OF_RECORD S Safety Compliance Certification that a power supply (or family of power supplies) has been approved by a safety agency such as UL, TUV, CSA, VDE, etc. ##END_OF_RECORD Safety Extra Low Voltage. (SELV) A term used by safety and regulatory agencies to describe the highest voltage level that can be contacted by a person without causing injury. It is currently defined as 60 VDC (42.4 Vpk). ##END_OF_RECORD Safety Ground A conductive path to earth ground designed to protect equipment operators from potential electrical shocks. ##END_OF_RECORD Secondary Circuit The output side of an isolated power supply. See Primary Circuit. ##END_OF_RECORD Secondary Current The current flowing in the secondary side of the isolation transformer of a power supply. Also called transformer output current. ##END_OF_RECORD Secondary Outputs The outputs not directly controlled by the output feedback loop in a multiple output power supply. These outputs are quasi-regulated, with the output voltage being set by the turns ratio of the isolation transformer. Also called auxiliary outputs. ##END_OF_RECORD Sense Line An input to the power supply that "senses" the output voltage level at the load. Used in remote sensing connections to compensate for line losses incurred when powering a load some distance from the power supply. See Remote Sensing and Local Sensing. ##END_OF_RECORD Series Operation A power system configuration in which multiple isolated power supplies are configured to obtain a higher output voltage level (inputs connected in parallel) or wider input voltage range (inputs connected in series) than that obtainable from one power supply. ##END_OF_RECORD Series Regulator A linear regulator placed in series with the load to achieve a constant voltage across the load. This is the most popular method of linear regulation. See Linear Regulation, Post Regulation and Shunt Regulator. ##END_OF_RECORD Set Point Accuracy The ratio of the actual output voltage of a power supply to its specified output voltage. ##END_OF_RECORD Short Circuit Protection See Current Limiting. ##END_OF_RECORD Shunt Regulator A linear regulator placed in parallel with the load to achieve a constant voltage across the load. See Linear Regulation, Post Regulation and Series Regulator. ##END_OF_RECORD Six-Sided Shielding A packaging technique in which the power supply is placed into a metal case. The metal shielding minimizes noise radiation from the switching components of the power supply. A continuous shielded case has the base (or header) welded on, further reducing potential noise leakage. ##END_OF_RECORD Slave A power supply that has no internal feedback loop. The output regulation of a slave unit is controlled by a master power supply to which its' operation is synchronized. Sometimes called a "booster" See Master. ##END_OF_RECORD Snubber A resistor-capacitor network that reduces the voltage rise in a power supply switching circuit. ##END_OF_RECORD Soft Start A power system input circuit that limits the inrush current at turn on. ##END_OF_RECORD Source The device (battery, generator, utility mains, etc.) that provides input power to a power supply. ##END_OF_RECORD Source Effect See Line Regulation. ##END_OF_RECORD Specific Power See Power Density. ##END_OF_RECORD Split Bobbin Winding A transformer winding technique in which the primary and secondary windings are wound on a bobbin side by side with an isolation layer in between. See Bobbin. ##END_OF_RECORD Stability See Long Term Stability. ##END_OF_RECORD Standby Current The input current drawn by a power supply when it has no load and has been shut down by a remote ON/OFF signal. ##END_OF_RECORD Standby Power The power dissipated by a power supply when it has no load and has been shut down by a remote ON/OFF signal. ##END_OF_RECORD Start-Up Delay After input power is applied to a power supply, the delay that occurs before the output voltages are within regulation. ##END_OF_RECORD Static Load A load that remains constant for a specified period of time. Typically specified as a percentage of full load. ##END_OF_RECORD Status Signals See Flag Signal. ##END_OF_RECORD Step Change A sudden change in a power supply parameter. Normally used when referring to changes in output load or input line during power supply testing. ##END_OF_RECORD Stiff Line A condition wherein there is no significant impedance present on the AC input mains. The input voltage to the power supply under these conditions will not change appreciably with load. ##END_OF_RECORD Still Air A power supply operating environment in which the surrounding air restricted. Often occurs in small enclosures (sometimes sealed) where air cannot move freely. Because it does not have sufficient mass or flow, still air is prone to thermal runaway caused by the thermal output of the power system. ##END_OF_RECORD Storage Temperature Range The range of ambient temperatures over which a power supply can be safely stored. See Operating Temperature Range. ##END_OF_RECORD Switching Frequency The rate at which the input voltage is switched or "chopped" in a power supply. Sometimes referred to as frequency of operation. ##END_OF_RECORD Surface Mount Devices (SMD) Components (including some power supplies) designed to be assembled using surface mount technology. ##END_OF_RECORD Surface Mount Technology (SMT) An automated manufacturing process in which components are designed for mounting on the surface of a substrate or PC board. Often used in the design/assembly of low power DC/DC converter modules. ##END_OF_RECORD Switching Power Supply A power supply that uses switching regulation. Switchers are typically smaller, more efficient and lighter than linear supplies. ##END_OF_RECORD Switching Regulator A circuit (typically a pulse width modulator) that uses a closed loop design to regulate the output voltage. ##END_OF_RECORD T Back To Top Technical Help to Exporters (T.H.E.) A British government agency that works with the British Standards Institution (BSI), T.H.E. publishes English translations of various international standards as well as a regular newsletter. See BSI. ##END_OF_RECORD Technischer Uberwachungs-Verein (TUV) An organization approved for testing products to VDE and DIN standards. ##END_OF_RECORD Temperature Coefficient The average change in power supply output voltage per degree centigrade. Expressed as a percentage over a specified temperature range. ##END_OF_RECORD Temperature Derating See Derating. ##END_OF_RECORD Temperature Effect See Temperature Coefficient. ##END_OF_RECORD Temperature Range See Operating Temperature Range and Storage Temperature Range. ##END_OF_RECORD Thermal Adhesive An epoxy (rather than oil) based thermal joint compound. When cured, thermal adhesives provide a mechanical bond between the base plate/chassis of a power supply module and heat sink as well as insuring high thermal conductivity across the junction. See Thermal Joint Compound. ##END_OF_RECORD Thermal Conductivity For a given material, the time required for heat to transfer (via conduction) across a unit material thickness of 1 meter when the temperature differential of the two opposite faces is 1K. Given in joules per second per square centimeter. ##END_OF_RECORD Thermal Gasket A flexible pad or wafer that is placed between a power supply baseplate/chassis and a heat sink. Thermal gaskets have a very low thermal resistance, insuring high thermal conductivity across the junction. Use of a 'dry' gasket may eliminate some problems associated with thermal joint compounds (inconsistent coating, contaminants, etc.). Gaskets are available in a number of standard pre-cut forms. ##END_OF_RECORD Thermal Grease See Thermal Joint Compound. ##END_OF_RECORD Thermal Joint Compound A fluid or paste that is spread between a power supply baseplate\chassis and a heat sink. Normally, compounds consist of a silicon or synthetic oil that carries a low thermal resistance filler (aluminum, etc.). The compound fills the voids caused by irregularities in the mating surfaces insuring high thermal conductivity across the junction. ##END_OF_RECORD Thermal Resistance (q) A measure of the opposition a material will have to the flow of heat. Used to calculate the temperature drop that occurs when power flows through a material or across the junction of two materials. Given in units of degrees Celsius per watt (°C/W). ##END_OF_RECORD Thermal Resistivity (r) A measure of the ability of a material to impede the flow of heat. Typically given in units of (°C)(T)/W, where T equals the thickness of the material and W equals the power flowing through the material in watts. ##END_OF_RECORD Thermal Runaway A circuit condition in which an increase in the ambient temperature surrounding a power supply causes an increase in the internal power dissipated. This increases the case temperature, which in turn increases the ambient temperature of the surrounding air. The spiraling effect of these temperature increases will eventually lead to failure of the power system. This condition occurs when inadequate measures (air flow, system venting, power derating, etc.) are taken to get heat away from critical components. ##END_OF_RECORD Thermal Protection A protection feature that shuts a power supply down if its internal temperature exceeds a preset limits. Also called thermal shutdown. ##END_OF_RECORD Three Terminal Regulator A linear regulator packaged in a standard 3-terminal transistor package. These devices can be configured as either a shunt or series regulator. ##END_OF_RECORD Tracking For a multiple output power supply, the parameter that gives the change in one output voltage caused by a change in the voltage level or load on another output. ##END_OF_RECORD Transient A spike or step change in a power supply parameter. Commonly used in describing input line and output load characteristics. ##END_OF_RECORD Transient Recovery Time The time required for a power supply output to return to within specified limits following a step change in output load current. Expressed as a percentage of rated value. ##END_OF_RECORD Transient Response A power supply's ability to return to a constant output voltage level following a step change in output current. ##END_OF_RECORD True Power The actual power consumed by an ac circuit. ##END_OF_RECORD U Underwriters Laboratories (UL) is an independent, organization that conducts safety testing of products to established standards. ##END_OF_RECORD Undershoot A transient change in a power supply output voltage that does not meet the lower limit of the voltage accuracy specification. Typically occurs at the power supply turn on/off or with some step change in output load or input line. ##END_OF_RECORD Undervoltage Protection A protection circuit that shuts a power supply off if the output voltage falls below a preset level. Sometimes used as an input protection circuit in wide input range power supplies (ac and dc) to prevent overheating if the input voltage sags below a predetermined level. Sometimes called undervoltage shutdown or undervoltage lockout. ##END_OF_RECORD Uninterruptible Power Supply (UPS) A power supply that will continue to operate after the loss of ac input power. A UPS normally uses some type of automatic battery backup system. ##END_OF_RECORD Universal Input An AC input to a power system that accepts all the standard voltage levels available from AC mains. Typically specified at 85 VAC to 265 VAC. ##END_OF_RECORD V Verband Deutsher Electrotechniker (VDE) is a German organization that sets standards for product safety and noise emissions; and tests and certifies products to those standards. ##END_OF_RECORD Voltage Balance For a multiple output power supply, the percentage difference in voltage level of two outputs with opposite polarities and equal nominal values. ##END_OF_RECORD Voltage Regulation See Regulation. ##END_OF_RECORD W Warm-up Drift See Drift. ##END_OF_RECORD Warm-up Time The time required for a power supply to operate to within specifications after turn-on. This time normally precedes a long-term drift specification. ##END_OF_RECORD Withstand Voltage The maximum voltage level that can be applied between circuits or components without causing a breakdown. See Breakdown Voltage and Isolation. ##END_OF_RECORD Definition MCM multi-chip module. Term (Index) Definition Multichip Module, MCM package containing more than one IC chip. ##END_OF_RECORD Failure rate can be defined as the anticipated number of times that an item fails in a specified period of time. It is a calculated value that provides a measure of reliability for a product. This value is normally expressed as failures per million hours, but can also be expressed as a FIT (Failures in time) Rate or failures per billion hours. For example, if a component has a failure rate of 2 failures per million hours, then it is anticipated that the component fails 2 times in a million hour time period. A calculated failure rate is generally based on an established reliability prediction model (like MIL-HDBK-217 or Telcordia). Calculations are based on component data such as temperature, environment, and stress. Calculated failure rates for assemblies are a sum of the failure rates for components within the assembly. A component manufacturer may sometimes provide a specified failure rate. This failure rate is usually based on field or laboratory test data. Similarly, a manufacturer can also provide a specified failure rate for an assembly. Failure rate is also the inverse of the Mean Time Between Failures (MTBF) value for constant rate failure systems. For example: If a component has an MTBF value of 500,000 hours, and the failure rate is desired in failures per million hours, the failure rate would be: Failure Rate = (1,000,000 hours) / (500,000 hours) = failures per million hours ##END_OF_RECORD conversion To Convert Into Multiply By Centigrate Fahrenheit (°C + 32) x 1.8 Centimeters Inches 0.3937 Centimeters Millimeters 10 Circular Mils Square Inches 7.854 x 10 -7 Circular Mils Square Millimeters 5.067 x 10 -4 Fahrenheit Centigrade (°F - 32) 1.8 Horsepower (electric) Watts 746 Inches Centimeters 2.54 Inches Millimeters 25.4 Inches Mils 1,000 Millimeters Centimeters 0.1 Millimeters Inches 0.03937 Millimeters Mils 39.37 Mils Centimeters 2.54 x 10 -3 Mils Inches 0.001 Mils Millimeters 2.54 x 10 -4 Square Inches Circular Mils 1.273 x 10 6 Square Inches Square Millimeters 645.16 Square Millimeters Circular Mils 1,973.5 Square Millimeters Square Inches 1.550 x 10 -3 Watts Horsepower (electric) 1.340 x 10 -3 ##END_OF_RECORD Approximate # of Wires in Conduit Chart for Thhn wire Check Code Book for actual types and charts; **************************************************************************** | 1/2 | 3/4 | 1 | 1-1/4 | 1-1/2 | 2 | 2-1/2 | 3 | |---------|---------|-------|---------|---------|-------|----------|-------| |AWG #wire|AWG #wire|AWG#wir|AWG#wires|AWG#wires|AWG#wir|AWG#wires |AWG#WRs| | 14 (13) | 14 (24) |14 (39)| 14 (69) | 14 (94) | 14(154)| 14 | | | 12 (10) | 12 (18) |12 (29)| 12 (51) | 12 (70) | 12(114)| 12(164)|12( )| | 10 (6) | 10 ( 11) |10 (18)| 10 (32) | 10 (44) | 10(73)| 10 (104)|10(160)| | 8 (3) | 8 ( 5) | 8 ( 9)| 8 (16) | 8 (22) | 8(36)| 8 ( 51) | 8( 79)| | 6 (1) | 6 ( 2) | 6 ( 6)| 6 (11) | 6 (15) | 6(26)| 6 ( 37) | 6( 57)| | 4 (1) | 4 ( 1) | 4 ( 4)| 4 ( 7) | 4 ( 9) | 4(16)| 4 ( 22) | 4( 35)| | 3 (1) | 3 ( 1) | 3 ( 3)| 3 ( 6) | 3 ( 8) | 3(13)| 3 ( 19) | 3( 29)| | 2 (1) | 2 ( 1) | 2 ( 3)| 2 ( 5) | 2 ( 7) | 2(11)| 2 ( 16) | 2( 25)| | 1 | 1 | 1 ( 1)| 1 ( 3) | 1 ( 5) | 1( 8)| 1 ( 12) | 1( 18)| **************************************************************************** The number of conductors shall not exceed the percentage fill specified in table 1, chapter 9. (fpn) for conductor cross sectional area see tables: 5,5a,6,7,8 and the applicable notes to tables at the beginning of chapter 9. The maximum percentage of conduit fill for more than three conductors in a conduit or raceway is 40 percent. (all conductors except lead covered) Units of area: The circular mil is the unit of cross section used in the American wire gauge (AWG). The term Mil means one thousandth of an inch (0.001 inch). A circular mil is the area of a circular wire with a diameter of one mil, 0.001". The circular mil area of any solid cylindrical wire is equal to its diameter (expressed in mils) squared. For example, the area in circular mils of a wire having a diameter of 3/8 of an inch (0.375) equals 375 x 375 = 140,625 CM. The diameter in mils of a solid circular wire is equal to the square root of its circular mil area. Assuming that a conductor has an area of 500,000 CM, its diameter in mils is the square root of 500,000, which is equal to 707 mils, or 0.707 inch (approximately). The area in square inches of a wire whose diameter is one mil is Pi D squared /4 = 0.7854 x 0.001 squared = 0.0000007854 sq.in. The square mil is the area of a square whose sides are each one mil (0.001 inch). Hence, the area of a square mil is 0.001 squared, or 0.000001 square inch. With reference to the previous definitions of the circulare mil and the square mil, it is obvious that in order to convert a unit of circular area into its equivalent area in square mils the circular mil must be multiplied by Pi/4, or 0.7854, which is the same as multiplying by 1.273. These relations may be written as: Area = diameter squared = circular mils (CM) Square Mils = circular mils times 0.7854 = circular mils divided by 1.273. Circular mils = square mils divided by 0.7854 = square mils times 1.273. By this method any circular conductor may be converted into a rectangular conductor (a bus bar) containing an equivalent area or current carrying capacity. Table 5 of the NEC gives the approximate area in circular mils and square mils of different sizes of wire. ##END_OF_RECORD Multiply By To Obtain Acres x 43560=Square Feet Acres x 4840=Square Yards Circular Mils x 7.854x10e7 =Square Inches Circular Mils x 0.7854=Square Mils Squarebr Centimeters x 0.155=Square Inches Sq. Feet x 144=Square Inches Sq. Feet x 0.0929=Square Meters Sq. Inches x 6.452=Square Centimeters Sq. Meters x 1.196=Square Yards Sq. Miles x 640=Acres Sq. Mils x 1.273=Circular Mils MSI x 1.55=Square Meters Sq. Yards x 0.8361=Square Meters EnergyEnergy Or Work Multiply By To Obtain Btu x 778.2=Foot-pounds Btu x 252=Gram-calories ForceForce and WeightWeight Multiply By To Obtain Grams x 0.0353=Ounces Kilograms x 2.205=Pounds Newtons x 0.00248=Pounds(force) Ounces x 28.35=Grams Pounds (U.S.avoirdupox 453.59=Grams Poundsbr Force x 4.448=Newtons Tonsbr (short) x 907.2=Kilograms Tonsbr (short) x 2000=Pounds LengthLength Multiply By To Obtain Centimeters x 0.3937=Inches Fathoms x 6=Feet Feet x 12=Inches Feet x 0.3048=Meters Inches x 2.54=Centimeters Kilometers x 0.6214=Miles Meters x 3.281=Feet Meters x 39.37=Inches Meters x 1.094=Yards Miles x 5280=Feet Miles x 1.609=Kilometers Rods x 5.5=Yards Yards x 0.9144=Meters PlanePlane Angle Multiply By To Obtain Degrees x 0.0175=Radians Minutes x 0.01667=Degrees Minutes x 2.9x10sup-4/sup=Radians Quadrants x 90=Degrees Quadrants x 1.5708=Radians Radians x 57.3=Degrees PowerPower Multiply By To Obtain Btu per hour x 0.293=Watts Horsepower x 33000=Foot-poundsbr per minute Horsepower x 550=Foot-poundsbr per second Horsepower x 746=Watts Kilowatts x 1.341=Horsepower TorqueTorque Multiply By To Obtain Gram-centimeters x 0.0139=Ounce-inches Newton-meters x 0.7376=Pound-feet Newton-meters x 8.851=Pound-inches Ounce-inches x 72=Gram-centimeters Pound-feet x 1.3558=Newton-meters Pound-inches x 0.113=Newton-meters VolumeVolume p(Gallons and quarts are U.S.) Multiply By To Obtain Cubic Feet x 0.0283=Cubic Meters Cubic Feet x 7.481=Gallons Cubic Inches x 0.5541=Ounces (fluid) Cubic Meters x 35.31=Cubic Feet Cubic Meters x 1.308=Cubic Yards Cubic Yards x 0.7646=Cubic Meters Gallons x 0.1337=Cubic Feet Gallons x 3.785=Liters Liters x 0.2642=Gallons Liters x 1.057=Quarts (liquid) Ounces (fluid) x 1.805=Cubic Inches Quarts (liquid) x 0.9463=Liters ##END_OF_RECORD electron volt = 1.6021917E-19 joule joule = 6.2414503832469E+18 electron volt ' = " foot " " = " inch " $ (US dollar) = $ (currency; base unit) % (percent) = 0.01 *approx = "" *avg = "" *exact = "" / = replacement for " per " A (ampere) = A (electric current; base unit) a (atto) = 1E-18 (prefix) a = are (area) abA = abampere (electric current) abamp = abampere (electric current) abampere = 10 A (electric current) abC = abcoulomb (electric charge) abcoulomb = 10 C (electric charge) abF = abfarad (capacitance) abfarad = 1000000000 F (capacitance) abH = abhenry (inductance) abhenry = 0.000000001 H (inductance) abmho = 1000000000 mho (conductance) abohm = 0.000000001 ohm (electric resistance) absolute temperature = thermodynamic temperature (thermodynamic temperature) absolute viscosity = dynamic viscosity (dynamic viscosity) Absorbed Dose gray = J/kg (derived unit) Gy = gray Rad = 0.01 Gy absorbed dose = specific energy (specific energy) abV = abvolt (electric potential) abvolt = 0.00000001 V (electric potential) Acceleration acceleration = m/s^2 (category unit) acceleration linear = acceleration freefall = g g (gravitational acceleration) = 9.80665 m/s^2 galileo = 0.01 m/s^2 gravitational acceleration = g linear acceleration = acceleration acceleration = m/s^2 (acceleration; category unit) acceleration angular = angular acceleration (angular acceleration) acceleration linear = acceleration (acceleration) acoustical energy = energy (energy) acoustical intensity = heat flux density (heat flux density) acoustical power = power (power) acre = 4840 yard^2 (area) Activity becquerel = 1/s (derived unit) Bq = becquerel Ci = curie curie = 37000000000 Bq rutherford = 1000000 Bq activity = frequency (frequency) actus (Roman actus) = 116.4 foot (length) Ad = angstrom (length) admiralty knot = 6080 ft/hour (velocity) admittance = conductance (conductance) Adsorption adsorption = mol/m^2 (category unit) surface concentration = adsorption adsorption = mol/m^2 (adsorption; category unit) agate (typography agate) = 7.14285714285714E-02 in (length) amount of magnetization = magnetic flux (magnetic flux) Amount Of Substance amount of substance = mol (category unit) mol (mole) = mol (base unit) mole = mol amount of substance = mol (amount of substance; category unit) amp = A (electric current) ampere = A (electric current) ampere turn = A*turn (magnetic force; derived unit) amphora (Greek amphora) = 10.3 gallon (volume) AMU (atomic mass unit) = 1.66044E-27 kg*approx (mass) angstrom = 0.0000000001 m (length) Angular Acceleration acceleration angular = angular acceleration angular acceleration = rad/s^2 (category unit) rotational acceleration = angular acceleration angular acceleration = rad/s^2 (angular acceleration; category unit) angular impulse = angular momentum (angular momentum) Angular Momentum angular impulse = angular momentum angular momentum = N*m*s (category unit) planck (Planck constant) = 6.626196E-34 J*s angular momentum = N*m*s (angular momentum; category unit) Angular Power angular power = W*rad (category unit) angular power = W*rad (angular power; category unit) angular speed = angular velocity (angular velocity) Angular Velocity angular speed = angular velocity angular velocity = rad/s (category unit) circular frequency = angular velocity rotational frequency = angular velocity rpm (revolutions per minute) = revolution/minute (derived unit) velocity angular = angular velocity angular velocity = rad/s (angular velocity; category unit) annual = yr (time) ap = replacement for "apoth" ap = replacement for "apothecary" ap dr = dram ap (mass) ap dram = dram ap (mass) ap gr = grain ap (mass) ap grain = grain ap (mass) ap lb = pound ap (mass) ap oz = oz ap (mass) ap pound = pound ap (mass) ap scruple = scruple (mass) apostilb = cd/pi*m^2 (luminance; derived unit) apoth = "ap" apothecary = "ap" approx = "" arc deg = degree (plane angle) arc degree = degree (plane angle) arc min = arc minute (plane angle) arc minute (minute of arc) = 1.66666666666667E-02 arc degree (plane angle) arc s (second of arc) = 1.66666666666667E-02 arc minute (plane angle) arc sec = arc s (plane angle) arc second = arc s (plane angle) archin (Russian archin) = 28 in (length) are = 100 m^2 (area) Area a = are acre = 4840 yard^2 are = 100 m^2 area = m^2 (category unit) barn (abbreviation b) = 1E-28 m^2 cho (Japanese cho) = 2.45 acre circular inch = 0.25 pi*in^2 circular mil = 0.00000025 pi*in^2 desyatina (Russian desyatina) = 2.7 acre hectare = 100 are homestead = 0.25 mile^2 linear displacement = area permeability2 = area rood = 40 rod^2 sabin = ft^2 (derived unit) section (of land) = mile^2 (derived unit) square = 100 ft^2 surface area = area township = 36 mile^2 volume to length = area area = m^2 (area; category unit) area to volume = wave number (wave number) arpentcan = 27.52 mile (length) arpentlin = 191.835 ft (length) arroba (Mexican arroba) = 26.367 pound (mass) arroba oil = oil arroba (volume) arroba wine = wine arroba (volume) as (Roman as) = 12 uncia (mass) assay ton = 29.167 gram (mass) astronomical unit = 149597871000 m*approx (length) At = ampere turn (magnetic force) AT = assay ton (mass) at wt = atomic weight (force) atm = atmosphere (pressure) atmosphere = 101325 Pa (pressure) atomic mass = AMU (mass) atomic mass unit = AMU (mass) atomic weight = AMU*g (force; derived unit) atto = 1E-18 (prefix) au = astronomical unit (length) autumn = 89.6666666666667 day*approx (time) avdp = "" avg = "" avogadro (Avogadro's number) = 6.022169E+23 1/mol*approx (unknown units) avoirdupois = "" b = byte (data) Babylonian shekel = 8.37 gram (mass) bag = 94 pound (mass) bakers dozen = 13 balthazar = 8 magnum (volume) bar = 100000 Pa (pressure) barie = 0.1 Pa (pressure) barleycorn = 0.333333333333333 in (length) barn (abbreviation b) = 1E-28 m^2 (area) barrel = 31.5 gallon (volume) barye = 0.1 Pa (pressure) basis point = 0.01 percent bath (Israeli bath) = 2.25 in^3 (volume) baud = 0.1 byte/s (data flow rate) bbl = petroleum barrel (volume) bearing capacity mass basis = surface density (surface density) becquerel = 1/s (activity; derived unit) beer gallon (English beer gallon) = 282 in^3 (volume) bending moment = energy (energy) bending moment to length = force (force) bes (Roman bes) = 0.48 pound (mass) bev (billion electric volts) = 1000000000 electron volt (energy) bi = 2 (prefix) Biblical cubit = 18 in (length) billion = 1000000000 (prefix) biot = 10 A (electric current) bit = bit (data; base unit) blondel = cd/pi*m^2 (luminance; derived unit) board foot = 144 in^3 (volume) bolt (of cloth) = 40 yard (length) boltzmann (Boltzmann constant) = 1.380622E-23 J/K (heat capacity) bottom measure = 0.025 in (length) bowling ball = 16 pound (mass) bps (bits per second) = 0.1 byte/s (data flow rate) Bq = becquerel (activity) British = "UK" BTU (International Table BTU) = 1055.05585262 J*approx (energy) bu = bushel (volume) buck = $ (currency) bundle (of paper) = 2 ream bushel (dry bushel) = 4 peck (volume) byte = 8 bit (data) c (centi) = 0.01 (prefix) C = coulomb (electric charge) c = speed of light (velocity) cable = cable length (length) cable length = 720 ft (length) cal = calorie (energy) calendar yr (calendar year) = 365 day (time) caliber (gun barrel caliber) = 0.01 in (length) caloric value mole basis = molar energy (molar energy) calorie (International Table calorie) = 4.1868 J (energy) calorific value mass basis = specific energy (specific energy) calorific value volume basis = pressure (pressure) Canadian gallon = 0.00454609 m^3 (volume) candela = cd (luminous intensity) candle power (spherical) = 12.556 lumen (luminous flux) Capacitance abF = abfarad abfarad = 1000000000 F capacitance = F (category unit) electric capacitance = capacitance F = farad farad = s/ohm (derived unit) sF = statfarad statfarad = 1.11265E-12 F capacitance = F (capacitance; category unit) capacity = volume (volume) capture unit = wave number (wave number) carat = carat troy (mass) carat t = carat troy (mass) carat troy (troy carat) = 0.0002 kg (mass) case (of paper) = 4 bundle catty (Chinese catty) = 1.33333333333333 pound (mass) cc (cubic centimeters) = cm^3 (volume; derived unit) cd (candela) = cd (luminous intensity; base unit) celsius = dC (thermodynamic temperature) cen = century (time) cental (British cental) = 100 pound (mass) centennial = century (time) centesimal minute = 0.01 grade (plane angle) centesimal second = 0.0001 grade (plane angle) centi = 0.01 (prefix) centigrade = dC (thermodynamic temperature) centimilli = 0.00001 (prefix) century = 100 yr (time) Cesium frequency = Sc frequency (frequency) CFM = LFM x Area - (CFM A measure of the volume of air - Cubic Feet Minute) cfs (cubic feet per second) = ft^3/s (volume flow rate; derived unit) ch = chain (length) chain (surveyors chain) = 4 rod (length) char = byte (data) character = byte (data) charge = electric charge (electric charge) charge density = electric charge density (electric charge density) chemical potential = molar energy (molar energy) chetvert (Russian chetvert) = 5.96 bushel (volume) chin = catty (mass) cho (Japanese cho) = 2.45 acre (area) Ci = curie (activity) circle = 2 pi*rad (plane angle) circular frequency = angular velocity (angular velocity) circular inch = 0.25 pi*in^2 (area) circular mil = 0.00000025 pi*in^2 (area) city block (informal) = 100 yard*approx (length) cloth finger = 4.5 in (length) cloth quarter = 9 in (length) cm (centimilli) = 0.00001 (prefix) Compressibility compressibility = 1/Pascal (category unit) specific fuel consumption volume basis = compressibility stretchability = compressibility twistability = compressibility volume basis specific fuel consumption = compressibility compressibility = 1/Pascal (compressibility; category unit) concentration (of amount of substance) = mol/m^3 (concentration; category unit) Concentration concentration (of amount of substance) = mol/m^3 (category unit) concentration mole to volume = concentration ionic strength = concentration mole to volume concentration = concentration solubility = concentration concentration mass to mass = dimensionless unit concentration mass to volume = mass density (mass density) concentration mole to volume = concentration (concentration) Concentration Rate concentration rate = mol/m^3*s (category unit) reaction rate = concentration rate concentration rate = mol/m^3*s (concentration rate; category unit) concentration volume to mole = molar volume (molar volume) concentration volume to volume = dimensionless unit Conductance abmho = 1000000000 mho admittance = conductance conductance = S (category unit) mho = siemens modulus of admittance = conductance S = siemens siemens = A/V (derived unit) susceptance = conductance conductance = S (conductance; category unit) Conductivity conductivity = S/m (category unit) specific conductance = conductivity volume conductivity = conductivity conductivity = S/m (conductivity; category unit) conversion change rate = mole flow rate (mole flow rate) cooling duty = dimensionless unit coomb = 4 bushel (volume) cord (of wood) = 128 ft^3 (volume) cord foot (of wood) = 16 ft^3 (volume) corrosion rate = velocity (velocity) cotton bale (US) = 500 pound (mass) cotton bale Egypt = 750 pound (mass) coul = coulomb (electric charge) coulomb = A*s (electric charge; derived unit) cps (characters per second) = byte/s (data flow rate; derived unit) crith = 0.0906 gram (mass) ct = carat troy (mass) cu = cup (volume) cubed = "^3" cubit = Biblical cubit (length) cup = 0.5 pint (volume) curie = 37000000000 Bq (activity) Currency $ (US dollar) = $ (base unit) buck = $ currency = $ (category unit) dollar = $ tick = 0.03125 $ US dollar = $ currency = $ (currency; category unit) current = electric current (electric current) Current Density current density = A/m^2 (category unit) electric current density = current density current density = A/m^2 (current density; category unit) curvature of a curve = wave number (wave number) cwt = short hundredweight (mass) d (deci) = 0.1 (prefix) D (deka) = 10 (prefix) d = day (time) da (deka) = 10 (prefix) dalton = AMU (mass) damping viscosity = mass flow rate (mass flow rate) Data b = byte bit = bit (base unit) byte = 8 bit char = byte character = byte data = byte (category unit) gb = gigabyte gigabyte = 1024 megabyte kb = kilobyte kilobyte = 1024 byte mb = megabyte megabyte = 1024 kilobyte nibble = 0.5 byte tb = terabyte terabyte = 1024 gigabyte data = byte (data; category unit) Data Flow Rate baud = 0.1 byte/s bps (bits per second) = 0.1 byte/s cps (characters per second) = byte/s (derived unit) data flow rate = byte/s (category unit) data flow rate = byte/s (data flow rate; category unit) day = 24 hour (time) dC (Celsius degree) = K (thermodynamic temperature) deadweight ton = long ton (mass) dec = 10 (prefix) decade = 10 yr (time) deci = 0.1 (prefix) decillion = 1E+33 (prefix) decimilli = 0.0001 (prefix) deg = degree (plane angle) degree (Pi rad / 180) = 1.74532925199433E-02 rad (plane angle) deka = 10 (prefix) demi = 0.5 (prefix) denarius (Roman denarius) = 60 grain (mass) density = mass density (mass density) Density Of States density of states = 1/J*m^3 (category unit) density of states = 1/J*m^3 (density of states; category unit) Density Of Vibrational Modes density of vibrational modes = s/m^3 (category unit) density of vibrational modes = s/m^3 (density of vibrational modes; category unit) depth = length (length) desyatina (Russian desyatina) = 2.7 acre (area) dF (Fahrenheit degree) = 0.555555555555556 K (thermodynamic temperature) dialectric permittivity = permittivity (permittivity) dialectric polarization = electric flux density (electric flux density) dialectric strength = permittivity (permittivity) diffusivity = kinematic viscosity (kinematic viscosity) digitus (Roman digitus) = 0.73 in (length) Dimensionless Unit % (percent) = 0.01 bakers dozen = 13 basis point = 0.01 percent bundle (of paper) = 2 ream case (of paper) = 4 bundle concentration mass to mass = dimensionless unit concentration volume to volume = dimensionless unit cooling duty = dimensionless unit dimensionless unit = 1 (category unit) dozen = 12 dry = 1.16364718614719 approx googol = 1E+100 great gross = 12 gross gross = 12 dozen karat = 4.16666666666667E-02 length to length = dimensionless unit mass to mass concentration = dimensionless unit percent = 0.01 pi = 3.14159265358979 proof = 0.005 quire (of paper) = 25 ream (of paper) = 20 quire score = 20 scotch = 86 proof UK (British) = 1.000002 UK caliber (British caliber) = 0.001 volume to volume concentration = dimensionless unit dimensionless unit = 1 (category unit) dinar (Arabian dinar) = 4.2 gram (mass) diopter (lens power) = 1/m (wave number; derived unit) displacement = electric flux density (electric flux density) displacement ton = 35 ft^3 (volume) diurnal = day (time) dK = K (thermodynamic temperature) dm (decimilli) = 0.0001 (prefix) dollar = $ (currency) doppelzentner = 100 kg (mass) Dose Equiv rem = 0.01 Sv sievert = m^2/s^2 (derived unit) Sv = sievert dose equiv = specific energy (specific energy) double = 2 (prefix) dozen = 12 dR (Rankine degree) = 0.555555555555556 K (thermodynamic temperature) dr = dram (mass) dr ap = dram ap (mass) dr fl = dram fluid (volume) dr fluid = dram fluid (volume) dr t = dram troy (mass) drachma2 (Greek drachma) = 4.2923 gram (mass) draft = pressure (pressure) dram (avoirdupois dram) = 0.0625 oz (mass) dram ap (apothecary dram) = 3 scruple (mass) dram fl = dram fluid (volume) dram fluid (fluid dram) = 0.125 oz fluid (volume) dram troy (troy dram) = 60 grain (mass) dRe (Reaumur degree) = 1.25 K (thermodynamic temperature) drop = 0.03 cm^3 (volume) dry = 1.16364718614719 approx dry barrel = 7056 in^3 (volume) dry gallon = 268.8025 in^3*approx (volume) dry pint = 0.5 dry quart (volume) dry quart = 0.25 dry gallon (volume) duodec = 12 (prefix) dw t = pennyweight troy (mass) dyn = dyne (force) Dynamic Viscosity absolute viscosity = dynamic viscosity dynamic viscosity = Pa*s (category unit) flow rate to length mass basis = dynamic viscosity mass basis flow rate to length = dynamic viscosity P = poise poise = gram/cm*s (derived unit) reyn = lbf*s/in^2 (derived unit) viscosity absolute = dynamic viscosity viscosity dynamic = dynamic viscosity dynamic viscosity = Pa*s (dynamic viscosity; category unit) dyne (cm*gm/s^2) = 0.00001 N (force) E (exa) = 1E+18 (prefix) e = elementary charge (electric charge) earth mass = 5.983E+24 kg*approx (mass) earth to moon (mean distance earth to moon) = 238860 mile (length) EEC therm (European Economic Community therm) = 105506000 J (energy) eighth = 0.125 (prefix) Elastance elastance = 1/F (category unit) elastance = 1/F (elastance; category unit) elasticity = surface tension (surface tension) electric capacitance = capacitance (capacitance) Electric Charge abC = abcoulomb abcoulomb = 10 C C = coulomb charge = electric charge coul = coulomb coulomb = A*s (derived unit) e = elementary charge electric charge = C (category unit) electric flux = electric charge electric induction flux = electric charge electronic charge = elementary charge elementary charge = 1.6021917E-19 C flux of displacement = electric charge franklin current = statcoulomb quantity of electricity = electric charge sC = statcoulomb statcoulomb = 0.000000000333564 C electric charge = C (electric charge; category unit) Electric Charge Density charge density = electric charge density electric charge density = C/m^3 (category unit) volume density of charge = electric charge density electric charge density = C/m^3 (electric charge density; category unit) Electric Current A (ampere) = A (base unit) abA = abampere abamp = abampere abampere = 10 A amp = A ampere = A biot = 10 A current = electric current electric current = A (category unit) intl ampere ampere (international as of 1948) = 0.999835 A magnetic potential difference = electric current sA = statampere statampere = 0.000000000333564 A electric current = A (electric current; category unit) electric current density = current density (current density) Electric Dipole Moment electric dipole moment = C*m (category unit) electric dipole moment = C*m (electric dipole moment; category unit) electric displacement = electric flux density (electric flux density) electric field (constant) = 8.8542E-12 F/m (permittivity) Electric Field Strength electric field strength = V/m (category unit) electric intensity = electric field strength electric field strength = V/m (electric field strength; category unit) electric flux = electric charge (electric charge) Electric Flux Density dialectric polarization = electric flux density displacement = electric flux density electric displacement = electric flux density electric flux density = C/m^2 (category unit) electric polarization = electric flux density surface density of charge = electric flux density electric flux density = C/m^2 (electric flux density; category unit) electric inductance = inductance (inductance) electric induction flux = electric charge (electric charge) electric intensity = electric field strength (electric field strength) Electric Mobility electric mobility = m^2/V*s (category unit) mobility = electric mobility electric mobility = m^2/V*s (electric mobility; category unit) electric polarization = electric flux density (electric flux density) Electric Potential abV = abvolt abvolt = 0.00000001 V electric potential = V (category unit) electromotive force = electric potential induced emf = electric potential potential = electric potential potential difference = electric potential statvolt = 299.7925 V sV = statvolt V = volt volt = W/A (derived unit) voltage = electric potential voltage potential = electric potential electric potential = V (electric potential; category unit) Electric Resistance abohm = 0.000000001 ohm electric resistance = ohm (category unit) impedance = electric resistance modulus of impedance = electric resistance ohm = V/A (derived unit) omega = ohm reactance = electric resistance resistance = electric resistance statohm = 898755400000 ohm electric resistance = ohm (electric resistance; category unit) electrical power = power (power) electrochemical potential = molar energy (molar energy) Electromagnetic Moment electromagnetic moment = A*m^2 (category unit) magnetic moment = electromagnetic moment electromagnetic moment = A*m^2 (electromagnetic moment; category unit) electromotive force = electric potential (electric potential) electron mass (electron rest mass) = 9.109558E-31 kg (mass) electron volt = 1.6021917E-19 J*approx (energy) electronic charge = elementary charge (electric charge) elementary charge = 1.6021917E-19 C (electric charge) ell = 45 in (length) em (typography em) = point (length) emitted radiant flux = heat flux density (heat flux density) en (typography en) = 0.5 em (length) Energy acoustical energy = energy bending moment = energy bev (billion electric volts) = 1000000000 electron volt BTU (International Table BTU) = 1055.05585262 J*approx cal = calorie calorie (International Table calorie) = 4.1868 J EEC therm (European Economic Community therm) = 105506000 J electron volt = 1.6021917E-19 J*approx energy = J (category unit) erg = 0.0000001 J eV = electron volt frigorie = 1000 calorie heat = energy impact energy = energy J = joule joule = N*m (derived unit) moment of force = energy quantity of heat = energy radiant energy = energy rydberg = 1.36054 electron volt surface energy = energy therm = 100000 BTU thermie = 1000000 calorie thermochemical calorie = 4.184 J torque = energy UK thermal unit = BTU Wh (watt hour) = W*hour (derived unit) work = energy energy = J (energy; category unit) energy density = pressure (pressure) engineers chain = 100 ft (length) engineers link = ft (length) English carat = 0.000205 kg (mass) ennial = yr (time) enthalpy mass basis = specific energy (specific energy) enthalpy mole basis = molar energy (molar energy) entropy = heat capacity (heat capacity) ephah (Israeli ephah) = 10 omer (volume) epsilon0 = electric field (permittivity) equiv = replacement for "equivalent" equiv footcandle (equivalent footcandle) = lm/pi*ft^2 (illuminance; derived unit) equiv lux (equivalent lux) = lx/pi (illuminance; derived unit) equiv phot (equivalent phot) = cd/pi*cm^2 (luminance; derived unit) equivalent = "equiv" erg = 0.0000001 J (energy) eV = electron volt (energy) exa = 1E+18 (prefix) exact = "" f (femto) = 0.000000000000001 (prefix) F = farad (capacitance) fahrenheit = dF (thermodynamic temperature) farad = s/ohm (capacitance; derived unit) faraday (Faraday constant) = 96486.7 C/mol (unknown units) fath = fathom (length) fathom = 6 ft (length) fbm = board foot (volume) fc = footcandle (illuminance) Fdy = faraday (unknown units) feet = "foot" femto = 0.000000000000001 (prefix) fermi = 0.000000000000001 m (length) fifth = 0.8 quart (volume) finger = 0.875 in (length) firkin = 9 gallon (volume) fiscal yr (fiscal year) = 360 day (time) fl = replacement for "liq" fl = replacement for "liquid" fl dr = dram fluid (volume) fl dram = dram fluid (volume) fl head = length (length) fl oz = oz fluid (volume) flam = footlambert (luminance) flow rate area mass basis (flow rate/area mass basis) = kg*s/m^2 (flow rate area mass basis; category unit) Flow Rate Area Mass Basis flow rate area mass basis (flow rate/area mass basis) = kg*s/m^2 (category unit) mass basis flow rate to area = flow rate area mass basis flow rate mass basis = mass flow rate (mass flow rate) flow rate mole basis = mole flow rate (mole flow rate) flow rate to area volume basis = velocity (velocity) flow rate to length mass basis = dynamic viscosity (dynamic viscosity) flow rate to length volume basis = kinematic viscosity (kinematic viscosity) flow rate volume basis = volume flow rate (volume flow rate) fluid dr = dram fluid (volume) fluid dram = dram fluid (volume) fluid oz = oz fluid (volume) Fluidity fluidity = m*s/kg (category unit) rhe = 10 m^2/N*s fluidity = m*s/kg (fluidity; category unit) flux linkage = magnetic flux (magnetic flux) flux of displacement = electric charge (electric charge) foot (" foot ") = replacement for "'" foot = 12 in (length) foot = replacement for "feet" football field = 100 yard (length) footcandle = lm/ft^2 (illuminance; derived unit) footlambert = cd/pi*ft^2 (luminance; derived unit) Force at wt = atomic weight atomic weight = AMU*g (derived unit) bending moment to length = force dyn = dyne dyne (cm*gm/s^2) = 0.00001 N force = N (category unit) gf = gram force gram force = gram*g (derived unit) kip (kilopound force) = 1000 pound*g lb force = pound force lbf = pound force N = newton newton = kg*m/s^2 (derived unit) Nt = newton pdl = poundal pound force = pound*g (derived unit) poundal = ft*pound/s^2 (derived unit) sthene = 1000 N ton force = metric ton*g (derived unit) weight = force force = N (force; category unit) fortnight = 2 week (time) fps (feet per second) = ft/s (velocity; derived unit) franklin current = statcoulomb (electric charge) freefall = g (acceleration) freight ton = 40 ft^3 (volume) Frequency activity = frequency Cesium frequency = Sc frequency frequency = 1/s (category unit) hertz = 1/s (derived unit) Hz = hertz pitch = frequency radioactivity = frequency Sc frequency (cesium atom frequency) = 9192631770 1/s frequency = 1/s (frequency; category unit) frigorie = 1000 calorie (energy) ft = foot (length) fuel consumption = length (length) Fuel Efficiency fuel efficiency = 1/m^2 (category unit) length to volume = fuel efficiency mpg (miles per gallon) = mile/gallon (derived unit) fuel efficiency = 1/m^2 (fuel efficiency; category unit) funt (Russian funt) = 0.9 pound (mass) fur = furlong (length) furlong (surveyors furlong) = 10 chain (length) fy = fiscal yr (time) G (giga) = 1000000000 (prefix) g (gravitational acceleration) = 9.80665 m/s^2 (acceleration) G = gauss (magnetic flux density) gal = gallon (volume) galileo = 0.01 m/s^2 (acceleration) gallon = 231 in^3 (volume) gamma = 0.000000001 T (magnetic flux density) gas = universal gas (molar heat capacity) gauss = 0.0001 T (magnetic flux density) gb = gigabyte (data) geodetic foot = survey foot (length) geographical mile = nautical mile (length) gf = gram force (force) giga = 1000000000 (prefix) gigabyte = 1024 megabyte (data) gilbert = 0.795775 ampere turn (magnetic force) gill = 0.5 cup (volume) gm = gram (mass) googol = 1E+100 googol = 1E+100 (prefix) gpm (gallons per minute) = gal/min (volume flow rate; derived unit) gr = grain (mass) gr ap = grain ap (mass) gr t = grain troy (mass) grad = grade (plane angle) grade = 0.015707963267949 rad (plane angle) gradus (Roman gradus) = 2.43 foot (length) grain (avoirdupois grain) = 1.42857142857143E-04 pound (mass) grain ap (apothecary grain) = grain (mass) grain troy (troy grain) = grain (mass) gram = 0.001 kg (mass) gram force = gram*g (force; derived unit) gravitation (constant) = 0.00000000006664 N*m^2/kg^2 (unknown units) gravitational acceleration = g (acceleration) gray = J/kg (absorbed dose; derived unit) great gross = 12 gross Greek cubit = 18.22 in (length) Greek fathom = 4 Greek cubit (length) Greek obolos = 715.38 milligram (mass) Greek palm = 0.333333333333333 Greek span (length) Greek span = 0.5 Greek cubit (length) gross = 12 dozen gross cwt = long hundredweight (mass) gross hundredweight = long hundredweight (mass) gross ton = long ton (mass) gunters chain = chain (length) gunters link = link (length) Gy = gray (absorbed dose) h (hecto) = 100 (prefix) H = henry (inductance) h = hour (time) H20 density (water weight density) = 0.0885472440945 kg/m^2*s^2 (weight density) half = 0.5 (prefix) hand = 4 in (length) hd = hogshead (volume) heat = energy (energy) Heat Capacity boltzmann (Boltzmann constant) = 1.380622E-23 J/K entropy = heat capacity heat capacity = J/K (category unit) k = boltzmann heat capacity = J/K (heat capacity; category unit) heat conductivity = thermal conductivity (thermal conductivity) heat flow rate = power (power) heat flux = heat flux density (heat flux density) Heat Flux Density acoustical intensity = heat flux density emitted radiant flux = heat flux density heat flux = heat flux density heat flux density = W/m^2 (category unit) irradiance = heat flux density power to area = heat flux density radiant exitance = heat flux density radiant flux received = heat flux density sound intensity = heat flux density thermal flux = heat flux density heat flux density = W/m^2 (heat flux density; category unit) Heat Release Rate heat release rate = W/m^3 (category unit) mixing power = heat release rate heat release rate = W/m^3 (heat release rate; category unit) Heat Transfer Coefficient heat transfer coefficient = W/m^2*K (category unit) heat transfer coefficient = W/m^2*K (heat transfer coefficient; category unit) hectare = 100 are (area) hecto = 100 (prefix) hectokilo = 100000 (prefix) hefner candle = 0.92 cd (luminous intensity) height = length (length) hekat (Israeli hekat) = 291 in^3 (volume) henry = Wb/A (inductance; derived unit) hertz = 1/s (frequency; derived unit) Hg weight density = 133322.368421 kg/m^2*s^2 (weight density) hhd = hogshead (volume) hk (hectokilo) = 100000 (prefix) HK = hefner candle (luminous intensity) hogshead = 2 barrel (volume) homestead = 0.25 mile^2 (area) horsepower = 550 ft*pound*g/sec (power) hour = 60 minute (time) hp = horsepower (power) hr = hour (time) hundred = 100 (prefix) hundredweight = short hundredweight (mass) hyl = gf*s^2/m (mass; derived unit) Hz = hertz (frequency) ies ("ies ") = "y " Illuminance equiv footcandle (equivalent footcandle) = lm/pi*ft^2 (derived unit) equiv lux (equivalent lux) = lx/pi (derived unit) fc = footcandle footcandle = lm/ft^2 (derived unit) illuminance = lx (category unit) illumination = illuminance luminous exitance = illuminance lux = lm/m^2 (derived unit) lx = lux nox = 0.001 lux illuminance = lx (illuminance; category unit) illumination = illuminance (illuminance) imp ("imp ") = "UK " impact energy = energy (energy) impedance = electric resistance (electric resistance) imperial = "UK" impulse = momentum (momentum) in = inch (length) inch (" inch ") = replacement for """ inch = 0.0254 m (length) inch = replacement for "inches" inches = "inch" induced emf = electric potential (electric potential) Inductance abH = abhenry abhenry = 0.000000001 H electric inductance = inductance H = henry henry = Wb/A (derived unit) inductance = H (category unit) mutual inductance = inductance permeance = inductance self inductance = inductance sH = stathenry stathenry = 898755400000 H inductance = H (inductance; category unit) intensity of force = surface tension (surface tension) interfacial tension = surface tension (surface tension) international = "intl" intl = replacement for "international" intl ampere ampere (international as of 1948) = 0.999835 A (electric current) intl foot = foot (length) intl mile = mile (length) intl yard = yard (length) Ionic Conductivity ionic conductivity = S*m^2/mol (category unit) ionic conductivity = S*m^2/mol (ionic conductivity; category unit) ionic strength = concentration (concentration) ips (inches per second) = in/s (velocity; derived unit) irradiance = heat flux density (heat flux density) Israeli cubit = 21.8 inch (length) Israeli shekel mass = 14.1 gram (mass) J = joule (energy) jeroboam = 0.8 gallon (volume) jigger = 1.5 oz fluid (volume) joule = N*m (energy; derived unit) K (Kelvin degree) = K (thermodynamic temperature; base unit) k (kilo) = 1000 (prefix) k = boltzmann (heat capacity) karat = 4.16666666666667E-02 kayser = 100 1/m (wave number) kb = kilobyte (data) kelvin = K (thermodynamic temperature) ken (Japanese ken) = 6.95 foot (length) key = kg (mass) kg (kilogram) = kg (mass; base unit) kilderkin = 18 gallon (volume) kilo = 1000 (prefix) kilobyte = 1024 byte (data) kin (Japanese kin) = 1.323 pound (mass) Kinematic Viscosity diffusivity = kinematic viscosity flow rate to length volume basis = kinematic viscosity kinematic viscosity = m^2/s (category unit) St = stoke stoke = 0.0001 m^2/s thermal diffusion = kinematic viscosity transmissivity = kinematic viscosity viscosity kinematic = kinematic viscosity volume basis flow rate to length = kinematic viscosity kinematic viscosity = m^2/s (kinematic viscosity; category unit) kip (kilopound force) = 1000 pound*g (force) knot (nautical miles per hour) = nmi/hour (velocity; derived unit) koku (Japanese koku) = 47.6 gallon (volume) kph (kilometers per hour) = km/hour (velocity; derived unit) kwan (Japanese kwan) = 8.27 pound (mass) L = avogadro (unknown units) l = liter (volume) lam = lambert (luminance) lambert = cd/pi*cm^2 (luminance; derived unit) land league = league (length) land mile = mile (length) langley = calorie/cm^2 (surface tension; derived unit) last = 80 bushel (volume) lb = pound (mass) lb ap = pound ap (mass) lb force = pound force (force) lb t = pound troy (mass) lb troy = pound troy (mass) lbf = pound force (force) league = 3 mile (length) leap yr (leap year) = 366 day (time) Length actus (Roman actus) = 116.4 foot Ad = angstrom agate (typography agate) = 7.14285714285714E-02 in angstrom = 0.0000000001 m archin (Russian archin) = 28 in arpentcan = 27.52 mile arpentlin = 191.835 ft astronomical unit = 149597871000 m*approx au = astronomical unit barleycorn = 0.333333333333333 in Biblical cubit = 18 in bolt (of cloth) = 40 yard bottom measure = 0.025 in cable = cable length cable length = 720 ft caliber (gun barrel caliber) = 0.01 in ch = chain chain (surveyors chain) = 4 rod city block (informal) = 100 yard*approx cloth finger = 4.5 in cloth quarter = 9 in cubit = Biblical cubit depth = length digitus (Roman digitus) = 0.73 in earth to moon (mean distance earth to moon) = 238860 mile ell = 45 in em (typography em) = point en (typography en) = 0.5 em engineers chain = 100 ft engineers link = ft fath = fathom fathom = 6 ft fermi = 0.000000000000001 m finger = 0.875 in fl head = length foot = 12 in football field = 100 yard ft = foot fuel consumption = length fur = furlong furlong (surveyors furlong) = 10 chain geodetic foot = survey foot geographical mile = nautical mile gradus (Roman gradus) = 2.43 foot Greek cubit = 18.22 in Greek fathom = 4 Greek cubit Greek palm = 0.333333333333333 Greek span Greek span = 0.5 Greek cubit gunters chain = chain gunters link = link hand = 4 in height = length in = inch inch = 0.0254 m intl foot = foot intl mile = mile intl yard = yard Israeli cubit = 21.8 inch ken (Japanese ken) = 6.95 foot land league = league land mile = mile league = 3 mile length = m (category unit) li (Chinese li) = 705 yard light yr (light year) = speed of light*yr (derived unit) line = 8.33333333333333E-02 in link (surveyors link) = 7.92 in lyr = light yr m (meter) = m (base unit) marathon = 46145 yard marine league = nautical league meter = m metre = m mi = mile micron = 0.000001 m mil = 0.001 in mile = 5280 foot nail (cloth nail) = 0.0625 yard naut mi = nautical mile nautical league = 3 nautical mile nautical mile = 1852 m naval shot = 15 fathom nmi = nautical mile nmile = nautical mile pace = 30 in palm = 3 in parasang = 3.5 mile parsec = au*rad/arc sec (derived unit) pc = parsec perch = rod pica (typography pica) = 0.166666666666667 in point (typography point) = 0.013837 in pole = rod rd = rod ri (Japanese ri) = 2.44 mile rod (surveyors rod) = 25 link Roman cubit = 17.5 in Roman foot = 0.971 foot Roman mile = 5000 Roman foot rope = 20 ft sazhen (Russian sazhen) = 6.9 foot shaku (Japanese shaku) = 11.93 in skein = 120 yd soccer field = 100 yard span (cloth span) = 9 in spindle = 14400 yard stadia (Greek stadia) = 400 Greek cubit stadium (Roman stadium) = 202 yard statute league = league statute mile = mile sun (Japanese sun) = 1.193 in survey foot = 0.304800609601219 m surveyors chain = chain surveyors furlong = furlong surveyors link = link surveyors rod = rod U = micron UK mile (British mile) = 1609.34721869 m vara (Mexican vara) = 32.99 in verst (Russian verst) = 3500 foot wavelength = length width = length yard = 3 ft yd = yard length = m (length; category unit) length to length = dimensionless unit length to volume = fuel efficiency (fuel efficiency) lens power = wave number (wave number) LFM = Area/CFM - (LFM) A measure of the velocity of air - Linear Feet per minute LFM = meters/sec x 196.8 li (Chinese li) = 705 yard (length) liang = tael (mass) libra (Roman libra) = as (mass) librae = libra (mass) Light Exposure light exposure = lx*s (category unit) light exposure = lx*s (light exposure; category unit) light yr (light year) = speed of light*yr (length; derived unit) line = 8.33333333333333E-02 in (length) linear acceleration = acceleration (acceleration) linear current density = magnetic field strength (magnetic field strength) linear displacement = area (area) linear electric current density = magnetic field strength (magnetic field strength) linear impulse = momentum (momentum) linear momentum = momentum (momentum) linear power = power (power) linear speed = velocity (velocity) linear velocity = velocity (velocity) link (surveyors link) = 7.92 in (length) liq = "fl" liquid = "fl" liter = 0.001 m^3 (volume) livre (French livre) = 490 gram (mass) lm = lumen (luminous flux) load = 1.9 yd^3 (volume) long hundredweight (avoirdupois long hundredweight) = 112 pound (mass) long quarter = 28 pound (mass) long ton (avoirdupois long ton) = 2240 pound (mass) loschmidt (Loschmidt's constant) = 2.68719E+25 1/m^3 (number concentration) lumen = cd*sr (luminous flux; derived unit) Luminance apostilb = cd/pi*m^2 (derived unit) blondel = cd/pi*m^2 (derived unit) equiv phot (equivalent phot) = cd/pi*cm^2 (derived unit) flam = footlambert footlambert = cd/pi*ft^2 (derived unit) lam = lambert lambert = cd/pi*cm^2 (derived unit) luminance = cd/m^2 (category unit) nit = cd/m^2 (derived unit) ph = phot phot = cd/cm^2 (derived unit) sb = stilb skot = 0.001 apostilb stilb = cd/cm^2 (derived unit) luminance = cd/m^2 (luminance; category unit) Luminous Efficacy luminous efficacy = lm/W (category unit) luminous efficacy = lm/W (luminous efficacy; category unit) Luminous Energy luminous energy = lm*s (category unit) luminous energy = lm*s (luminous energy; category unit) luminous exitance = illuminance (illuminance) Luminous Flux candle power (spherical) = 12.556 lumen lm = lumen lumen = cd*sr (derived unit) luminous flux = lm (category unit) luminous flux = lm (luminous flux; category unit) Luminous Intensity candela = cd cd (candela) = cd (base unit) hefner candle = 0.92 cd HK = hefner candle luminous intensity = cd (category unit) luminous intensity = cd (luminous intensity; category unit) lunation = synodic month (time) lusec = 0.000001 torr*m^3/s (power) lux = lm/m^2 (illuminance; derived unit) lx = lux (illuminance) lyr = light yr (length) M (mega) = 1000000 (prefix) m (meter) = m (length; base unit) m (milli) = 0.001 (prefix) ma (myria) = 10000 (prefix) Ma = mach (velocity) mach (Mach at sea level & 32 dF) = 331.46 m/s (velocity) Magnetic Dipole Moment magnetic dipole moment = J*m/A (category unit) magnetic dipole moment = J*m/A (magnetic dipole moment; category unit) magnetic field (constant) = 0.0000004 pi*H/m (permeability) Magnetic Field Strength linear current density = magnetic field strength linear electric current density = magnetic field strength magnetic field strength = A/m (category unit) magnetic intensity = magnetic field strength magnetization = magnetic field strength Oe = oersted oersted = 79.5774715459477 A/m magnetic field strength = A/m (magnetic field strength; category unit) Magnetic Flux amount of magnetization = magnetic flux flux linkage = magnetic flux magnetic flux = Wb (category unit) magnetic mass = magnetic flux maxwell = 0.00000001 Wb Mx = maxwell Wb = weber weber = V*s (derived unit) magnetic flux = Wb (magnetic flux; category unit) Magnetic Flux Density G = gauss gamma = 0.000000001 T gauss = 0.0001 T magnetic flux density = T (category unit) magnetic induction flux = magnetic flux density magnetic polarization = magnetic flux density T = tesla tesla = kg/A*s^2 (derived unit) magnetic flux density = T (magnetic flux density; category unit) Magnetic Force ampere turn = A*turn (derived unit) At = ampere turn gilbert = 0.795775 ampere turn magnetic force = ampere turn (category unit) magnetic potential = magnetic force magnetomotive force = magnetic force magnetic force = ampere turn (magnetic force; category unit) magnetic induction flux = magnetic flux density (magnetic flux density) magnetic intensity = magnetic field strength (magnetic field strength) magnetic mass = magnetic flux (magnetic flux) magnetic moment = electromagnetic moment (electromagnetic moment) magnetic permeability = permeability (permeability) magnetic polarization = magnetic flux density (magnetic flux density) magnetic potential = magnetic force (magnetic force) magnetic potential difference = electric current (electric current) Magnetic Vector Potential magnetic vector potential = Wb/m (category unit) magnetic vector potential = Wb/m (magnetic vector potential; category unit) magnetization = magnetic field strength (magnetic field strength) magnetomotive force = magnetic force (magnetic force) magnum = 2 quart (volume) marathon = 46145 yard (length) marine league = nautical league (length) Mass AMU (atomic mass unit) = 1.66044E-27 kg*approx ap dr = dram ap ap dram = dram ap ap gr = grain ap ap grain = grain ap ap lb = pound ap ap oz = oz ap ap pound = pound ap ap scruple = scruple arroba (Mexican arroba) = 26.367 pound as (Roman as) = 12 uncia assay ton = 29.167 gram AT = assay ton atomic mass = AMU atomic mass unit = AMU Babylonian shekel = 8.37 gram bag = 94 pound bes (Roman bes) = 0.48 pound bowling ball = 16 pound carat = carat troy carat t = carat troy carat troy (troy carat) = 0.0002 kg catty (Chinese catty) = 1.33333333333333 pound cental (British cental) = 100 pound chin = catty cotton bale (US) = 500 pound cotton bale Egypt = 750 pound crith = 0.0906 gram ct = carat troy cwt = short hundredweight dalton = AMU deadweight ton = long ton denarius (Roman denarius) = 60 grain dinar (Arabian dinar) = 4.2 gram doppelzentner = 100 kg dr = dram dr ap = dram ap dr t = dram troy drachma2 (Greek drachma) = 4.2923 gram dram (avoirdupois dram) = 0.0625 oz dram ap (apothecary dram) = 3 scruple dram troy (troy dram) = 60 grain dw t = pennyweight troy earth mass = 5.983E+24 kg*approx electron mass (electron rest mass) = 9.109558E-31 kg English carat = 0.000205 kg funt (Russian funt) = 0.9 pound gm = gram gr = grain gr ap = grain ap gr t = grain troy grain (avoirdupois grain) = 1.42857142857143E-04 pound grain ap (apothecary grain) = grain grain troy (troy grain) = grain gram = 0.001 kg Greek obolos = 715.38 milligram gross cwt = long hundredweight gross hundredweight = long hundredweight gross ton = long ton hundredweight = short hundredweight hyl = gf*s^2/m (derived unit) Israeli shekel mass = 14.1 gram key = kg kg (kilogram) = kg (base unit) kin (Japanese kin) = 1.323 pound kwan (Japanese kwan) = 8.27 pound lb = pound lb ap = pound ap lb t = pound troy lb troy = pound troy liang = tael libra (Roman libra) = as librae = libra livre (French livre) = 490 gram long hundredweight (avoirdupois long hundredweight) = 112 pound long quarter = 28 pound long ton (avoirdupois long ton) = 2240 pound mass = kg (category unit) me = electron mass metric ton = 1000 kg Mexican libra = 1.015 pound mg (milligram) = milligram mina (Greek mina) = 0.9463 pound mite (English mite) = 0.05 grain mn = neutron mass mp = proton mass net hundredweight = short hundredweight net ton = short ton neutron mass (neutron rest mass) = 1.67492E-27 kg obol (Greek obol) = 11.2 grain oz (avoirdupois ounce) = 0.0625 pound oz ap (apothecary ounce) = 8 dram ap oz t = oz troy oz troy (troy ounce) = 480 grain pennyweight = pennyweight troy pennyweight troy (troy pennyweight) = 24 grain pfund (German pfund) = 500 gram picul (Chinese picul) = 100 catty pondus (Roman pondus) = as pood (Russian pood) = 36.11 pound pound (avoirdupois pound) = 0.45359237 kg pound ap (apothecary pound) = 12 oz ap pound t = pound troy pound troy (troy pound) = 12 oz troy proton mass (proton rest mass) = 1.672614E-27 kg quintal = 100 kg Roman obolus = 8.77 grain Roman talent = 125 libra s ap = scruple scruple (apothecary scruple) = 20 grain scruple ap = scruple scrupulum (Roman scrupulum) = 17.53 grain short hundredweight (avoirdupois short hundredweight) = 100 pound short quarter = 25 pound short ton (avoirdupois short ton) = 2000 pound slug = lbf*s^2/ft (derived unit) Spanish libra = 1.014 pound Spanish quintal = 101.4 pound stone = 14 pound t = metric ton tael (Chinese tael) = 0.0625 catty talent (Greek talent) = 60 mina tan = picul tn = short ton ton = short ton tonne = metric ton troy carat = carat troy troy dr = dram troy troy dram = dram troy troy gr = grain troy troy grain = grain troy troy lb = pound troy troy oz = oz troy troy pennyweight = pennyweight troy troy pound = pound troy u = AMU UK quintal (British quintal) = 100 pound UK quintal2 (British quintal) = 112 pound uncia (Roman uncia) = 420 grain unciae = uncia wey mass = 252 pound mass = kg (mass; category unit) mass basis bearing capacity = surface density (surface density) mass basis calorific value = specific energy (specific energy) mass basis enthalpy = specific energy (specific energy) mass basis flow rate = mass flow rate (mass flow rate) mass basis flow rate to area = flow rate area mass basis (flow rate area mass basis) mass basis flow rate to length = dynamic viscosity (dynamic viscosity) mass basis specific fuel consumption = specific fuel consumption mass basis (specific fuel consumption mass basis) mass basis specific heat capacity = specific heat capacity (specific heat capacity) mass basis throughput = mass flow rate (mass flow rate) mass concentration = mass density (mass density) Mass Density concentration mass to volume = mass density density = mass density mass concentration = mass density mass density = kg/m^3 (category unit) mass to volume concentration = mass density mass density = kg/m^3 (mass density; category unit) Mass Flow Rate damping viscosity = mass flow rate flow rate mass basis = mass flow rate mass basis flow rate = mass flow rate mass basis throughput = mass flow rate mass flow rate = kg/s (category unit) throughput mass basis = mass flow rate mass flow rate = kg/s (mass flow rate; category unit) mass to area structural loading = surface density (surface density) Mass To Length mass to length = kg/m (category unit) mass to length = kg/m (mass to length; category unit) mass to mass concentration = dimensionless unit mass to volume concentration = mass density (mass density) mass transfer coefficient = velocity (velocity) maxwell = 0.00000001 Wb (magnetic flux) mb = megabyte (data) me = electron mass (mass) mean calendar yr = calendar yr (time) mean solar day = day (time) mean solar yr = yr (time) measurement ton = freight ton (volume) mechanical power = power (power) mega = 1000000 (prefix) megabyte = 1024 kilobyte (data) meter = m (length) methuselah = 4 magnum (volume) metre = m (length) metric ton = 1000 kg (mass) Mexican libra = 1.015 pound (mass) mg (milligram) = milligram (mass) mgd (megagallons per day) = 1000000 gallon/day (volume flow rate) mho = siemens (conductance) mi = mile (length) micro = 0.000001 (prefix) micron = 0.000001 m (length) mil = 0.001 in (length) mile = 5280 foot (length) millenium = 1000 yr (time) millennial = millenium (time) milli = 0.001 (prefix) million = 1000000 (prefix) min = minute (time) mina (Greek mina) = 0.9463 pound (mass) miners inch = 1.5 ft^3/minute (volume flow rate) minim = 1.66666666666667E-02 dram fluid (volume) minute = 60 s (time) mite (English mite) = 0.05 grain (mass) mixing power = heat release rate (heat release rate) mmHg (millimeters of mercury) = mm*Hg weight density (pressure; derived unit) mn = neutron mass (mass) mo = month (time) mobility = electric mobility (electric mobility) modulus of admittance = conductance (conductance) modulus of impedance = electric resistance (electric resistance) mol (mole) = mol (amount of substance; base unit) molality (of a solute) = mol/kg (molality; category unit) Molality molality (of a solute) = mol/kg (category unit) molar = universal gas (molar heat capacity) Molar Conductivity molar conductivity = S/m^2*mol (category unit) molar conductivity = S/m^2*mol (molar conductivity; category unit) Molar Energy caloric value mole basis = molar energy chemical potential = molar energy electrochemical potential = molar energy enthalpy mole basis = molar energy molar energy = J/mol (category unit) mole basis caloric value = molar energy mole basis enthalpy = molar energy molar energy = J/mol (molar energy; category unit) molar entropy = molar heat capacity (molar heat capacity) Molar Heat Capacity gas = universal gas molar = universal gas molar entropy = molar heat capacity molar heat capacity = J/mol*K (category unit) mole basis specific heat capacity = molar heat capacity specific heat capacity mole basis = molar heat capacity universal gas (constant) = 8.31434 J/mol*K molar heat capacity = J/mol*K (molar heat capacity; category unit) Molar Mass molar mass = kg/mol (category unit) molar mass = kg/mol (molar mass; category unit) molar refraction = molar volume (molar volume) Molar Volume concentration volume to mole = molar volume molar refraction = molar volume molar volume = m^3/mol (category unit) mole basis specific volume = molar volume specific volume mole basis = molar volume standard kilomole volume = 22.414 m^3/kmol Vm = standard kilomole volume volume to mole concentration = molar volume molar volume = m^3/mol (molar volume; category unit) mole = mol (amount of substance) mole basis caloric value = molar energy (molar energy) mole basis enthalpy = molar energy (molar energy) mole basis flow rate = mole flow rate (mole flow rate) mole basis specific heat capacity = molar heat capacity (molar heat capacity) mole basis specific volume = molar volume (molar volume) mole basis throughout = mole flow rate (mole flow rate) Mole Flow Rate conversion change rate = mole flow rate flow rate mole basis = mole flow rate mole basis flow rate = mole flow rate mole basis throughout = mole flow rate mole flow rate = mol/s (category unit) throughput mole basis = mole flow rate mole flow rate = mol/s (mole flow rate; category unit) mole to volume concentration = concentration (concentration) moment of force = energy (energy) Moment Of Inertia moment of inertia = kg*m^2 (category unit) moment of inertia of mass = moment of inertia polar moment of inertia of mass = moment of inertia product of inertia of mass = moment of inertia moment of inertia = kg*m^2 (moment of inertia; category unit) Moment Of Inertia Of Area moment of inertia of area = m^4 (category unit) moment of section = moment of inertia of area polar moment of inertia of area = moment of inertia of area product of inertia of area = moment of inertia of area static moment of volume = moment of inertia of area moment of inertia of area = m^4 (moment of inertia of area; category unit) moment of inertia of mass = moment of inertia (moment of inertia) Moment Of Inertia Of Volume moment of inertia of volume = m^5 (category unit) polar moment of inertia of volume = moment of inertia of volume product of inertia of volume = moment of inertia of volume moment of inertia of volume = m^5 (moment of inertia of volume; category unit) moment of section = moment of inertia of area (moment of inertia of area) Momentum impulse = momentum linear impulse = momentum linear momentum = momentum momentum = N*s (category unit) momentum = N*s (momentum; category unit) mono = 1 (prefix) month = 8.33333333333333E-02 yr*approx (time) monthly = month (time) mp = proton mass (mass) mpg (miles per gallon) = mile/gallon (fuel efficiency; derived unit) mph (miles per hour) = mile/hour (velocity; derived unit) mu (micro) = 0.000001 (prefix) mutual inductance = inductance (inductance) Mx = maxwell (magnetic flux) myria = 10000 (prefix) n (nano) = 0.000000001 (prefix) N = newton (force) Na = avogadro (unknown units) nail (cloth nail) = 0.0625 yard (length) nano = 0.000000001 (prefix) naut mi = nautical mile (length) nautical league = 3 nautical mile (length) nautical mile = 1852 m (length) naval shot = 15 fathom (length) nebuchadnezzar = 10 magnum (volume) net hundredweight = short hundredweight (mass) net ton = short ton (mass) neutron mass (neutron rest mass) = 1.67492E-27 kg (mass) newton = kg*m/s^2 (force; derived unit) nibble = 0.5 byte (data) nit = cd/m^2 (luminance; derived unit) NL = loschmidt (number concentration) nmi = nautical mile (length) nmile = nautical mile (length) noggin = 0.125 quart (volume) nonillion = 1E+30 (prefix) novennial = 9 yr (time) nox = 0.001 lux (illuminance) Nt = newton (force) Number Concentration loschmidt (Loschmidt's constant) = 2.68719E+25 1/m^3 NL = loschmidt number concentration = 1/m^3 (category unit) number concentration = 1/m^3 (number concentration; category unit) obol (Greek obol) = 11.2 grain (mass) octant = 0.25 pi*rad (plane angle) octennial = 8 yr (time) octillion = 1E+27 (prefix) Oe = oersted (magnetic field strength) oersted = 79.5774715459477 A/m (magnetic field strength) ohm = V/A (electric resistance; derived unit) oil arroba (Spanish oil arroba) = 3.32 gallon (volume) oil barrel = petroleum barrel (volume) olympiad = 4 yr*approx (time) omega = ohm (electric resistance) omer (Israeli omer) = 0.45 peck (volume) one = 1 (prefix) ounce = "oz" oz (avoirdupois ounce) = 0.0625 pound (mass) oz = replacement for "ounce" oz ap (apothecary ounce) = 8 dram ap (mass) oz fl = oz fluid (volume) oz fluid (fluid ounce) = 0.25 gill (volume) oz t = oz troy (mass) oz troy (troy ounce) = 480 grain (mass) P (peta) = 1E+15 (prefix) p (pico) = 0.000000000001 (prefix) P = poise (dynamic viscosity) Pa = pascal (pressure) pace = 30 in (length) palm = 3 in (length) parasang = 3.5 mile (length) parsec = au*rad/arc sec (length; derived unit) pascal = N/m^2 (pressure; derived unit) pc = parsec (length) pdl = poundal (force) peck (dry peck) = 2 dry gallon (volume) pennyweight = pennyweight troy (mass) pennyweight troy (troy pennyweight) = 24 grain (mass) per (" per ") = "/" percent = 0.01 perch = rod (length) perennial = yr (time) perigon = circle (plane angle) Permeability magnetic field (constant) = 0.0000004 pi*H/m magnetic permeability = permeability permeability = H/m (category unit) u0 = magnetic field permeability = H/m (permeability; category unit) permeability2 = area (area) permeance = inductance (inductance) Permittivity dialectric permittivity = permittivity dialectric strength = permittivity electric field (constant) = 8.8542E-12 F/m epsilon0 = electric field permittivity = F/m (category unit) permittivity = F/m (permittivity; category unit) peta = 1E+15 (prefix) petroleum barrel = 42 gallon (volume) pfund (German pfund) = 500 gram (mass) ph = phot (luminance) phot = cd/cm^2 (luminance; derived unit) pi = 3.14159265358979 pica (typography pica) = 0.166666666666667 in (length) pico = 0.000000000001 (prefix) picul (Chinese picul) = 100 catty (mass) pieze = 1000 Pa (pressure) pint (fluid pint) = 0.5 quart (volume) pipe = 2 hogshead (volume) pitch = frequency (frequency) pk = peck (volume) planck (Planck constant) = 6.626196E-34 J*s (angular momentum) Plane Angle arc deg = degree arc degree = degree arc min = arc minute arc minute (minute of arc) = 1.66666666666667E-02 arc degree arc s (second of arc) = 1.66666666666667E-02 arc minute arc sec = arc s arc second = arc s centesimal minute = 0.01 grade centesimal second = 0.0001 grade circle = 2 pi*rad deg = degree degree (Pi rad / 180) = 1.74532925199433E-02 rad grad = grade grade = 0.015707963267949 rad octant = 0.25 pi*rad perigon = circle plane angle = rad (category unit) quadrant = 0.5 pi*rad r = rad rad (radian) = rad (base unit) radian = rad rev = revolution revolution = 2 pi*rad sextant = 0.333333333333333 pi*rad sign = 0.166666666666667 pi*rad turn = 2 pi*rad plane angle = rad (plane angle; category unit) point (typography point) = 0.013837 in (length) poise = gram/cm*s (dynamic viscosity; derived unit) polar moment of inertia of area = moment of inertia of area (moment of inertia of area) polar moment of inertia of mass = moment of inertia (moment of inertia) polar moment of inertia of volume = moment of inertia of volume (moment of inertia of volume) pole = rod (length) pondus (Roman pondus) = as (mass) pony = 0.5 jigger (volume) pood (Russian pood) = 36.11 pound (mass) potential = electric potential (electric potential) potential difference = electric potential (electric potential) pound (avoirdupois pound) = 0.45359237 kg (mass) pound ap (apothecary pound) = 12 oz ap (mass) pound force = pound*g (force; derived unit) pound t = pound troy (mass) pound troy (troy pound) = 12 oz troy (mass) poundal = ft*pound/s^2 (force; derived unit) Power acoustical power = power electrical power = power heat flow rate = power horsepower = 550 ft*pound*g/sec hp = horsepower linear power = power lusec = 0.000001 torr*m^3/s mechanical power = power power = W (category unit) radiant flux = power radiant power = power W = watt watt = J/s (derived unit) power = W (power; category unit) power to area = heat flux density (heat flux density) PREFIXES a (atto) = 1E-18 atto = 1E-18 bi = 2 billion = 1000000000 c (centi) = 0.01 centi = 0.01 centimilli = 0.00001 cm (centimilli) = 0.00001 d (deci) = 0.1 D (deka) = 10 da (deka) = 10 dec = 10 deci = 0.1 decillion = 1E+33 decimilli = 0.0001 deka = 10 demi = 0.5 dm (decimilli) = 0.0001 double = 2 duodec = 12 E (exa) = 1E+18 eighth = 0.125 exa = 1E+18 f (femto) = 0.000000000000001 femto = 0.000000000000001 G (giga) = 1000000000 giga = 1000000000 googol = 1E+100 h (hecto) = 100 half = 0.5 hecto = 100 hectokilo = 100000 hk (hectokilo) = 100000 hundred = 100 k (kilo) = 1000 kilo = 1000 M (mega) = 1000000 m (milli) = 0.001 ma (myria) = 10000 mega = 1000000 micro = 0.000001 milli = 0.001 million = 1000000 mono = 1 mu (micro) = 0.000001 myria = 10000 n (nano) = 0.000000001 nano = 0.000000001 nonillion = 1E+30 octillion = 1E+27 one = 1 P (peta) = 1E+15 p (pico) = 0.000000000001 peta = 1E+15 pico = 0.000000000001 quadr = 4 quadri = 4 quadrillion = 1E+15 quadruple = 4 quarter = 0.25 quin = 5 quintillion = 1E+18 semi = 0.5 septillion = 1E+24 sesqu = 1.5 sesqui = 1.5 sex = 6 sextillion = 1E+21 T (tera) = 1000000000000 ten = 10 tera = 1000000000000 thousand = 1000 thrice = 3 tri = 3 trillion = 1000000000000 triple = 3 u (micro) = 0.000001 undec = 11 vic = 20 Y (myria) = 10000 pregnancy = 9 month*approx (time) Pressure atm = atmosphere atmosphere = 101325 Pa bar = 100000 Pa barie = 0.1 Pa barye = 0.1 Pa calorific value volume basis = pressure draft = pressure energy density = pressure mmHg (millimeters of mercury) = mm*Hg weight density (derived unit) Pa = pascal pascal = N/m^2 (derived unit) pieze = 1000 Pa pressure = N/m^2 (category unit) psi (pounds per square inch) = pound*g/in^2 (derived unit) radiant energy density = pressure sound pressure = pressure standard atmosphere = atmosphere stress = pressure torr = mm*Hg weight density (derived unit) vacuum = pressure volume basis calorific value = pressure pressure = N/m^2 (pressure; category unit) pressure drop to length = weight density (weight density) product of inertia of area = moment of inertia of area (moment of inertia of area) product of inertia of mass = moment of inertia (moment of inertia) product of inertia of volume = moment of inertia of volume (moment of inertia of volume) proof = 0.005 proton mass (proton rest mass) = 1.672614E-27 kg (mass) psi (pounds per square inch) = pound*g/in^2 (pressure; derived unit) pt = pint (volume) qt = quart (volume) quadr = 4 (prefix) quadrant = 0.5 pi*rad (plane angle) quadri = 4 (prefix) quadrillion = 1E+15 (prefix) quadruple = 4 (prefix) quantity of electricity = electric charge (electric charge) quantity of heat = energy (energy) quart (fluid quart) = 0.25 gallon (volume) quarter = 0.25 (prefix) quin = 5 (prefix) quindecennial = 15 yr (time) quinquennial = 5 yr (time) quintal = 100 kg (mass) quintillion = 1E+18 (prefix) quire (of paper) = 25 r = rad (plane angle) R = roentgen (radiation exposure) rad (radian) = rad (plane angle; base unit) Rad = 0.01 Gy (absorbed dose) radian = rad (plane angle) Radiance radiance = W/m^2*sr (category unit) radiance = W/m^2*sr (radiance; category unit) radiant energy = energy (energy) radiant energy density = pressure (pressure) radiant exitance = heat flux density (heat flux density) radiant flux = power (power) radiant flux received = heat flux density (heat flux density) Radiant Intensity radiant intensity = W/sr (category unit) radiant intensity = W/sr (radiant intensity; category unit) radiant power = power (power) Radiation Exposure R = roentgen roentgen = 0.000258 A*s/kg radiation exposure = specific charge (specific charge) radioactivity = frequency (frequency) rankine = dR (thermodynamic temperature) rd = rod (length) reactance = electric resistance (electric resistance) reaction rate = concentration rate (concentration rate) ream (of paper) = 20 quire reaumur = dRe (thermodynamic temperature) refrigeration = 12000 BTU/ton*hour (unknown units) register ton = 100 ft^3 (volume) rehoboam = 3 magnum (volume) Reluctance reluctance = 1/H (category unit) reluctance = 1/H (reluctance; category unit) Reluctivity reluctivity = 1/H*m (category unit) reluctivity = 1/H*m (reluctivity; category unit) rem = 0.01 Sv (dose equiv) Replaced Values / = replacement for " per " ^2 = replacement for "squared" ^3 = replacement for "cubed" ap = replacement for "apoth" ap = replacement for "apothecary" equiv = replacement for "equivalent" fl = replacement for "liq" fl = replacement for "liquid" foot (" foot ") = replacement for "'" foot = replacement for "feet" inch (" inch ") = replacement for """ inch = replacement for "inches" intl = replacement for "international" oz = replacement for "ounce" UK ("UK ") = replacement for "imp " UK = replacement for "British" UK = replacement for "imperial" y ("y ") = replacement for "ies " yr = replacement for "year" Replacements ' = " foot " " = " inch " *approx = "" *avg = "" *exact = "" apoth = "ap" apothecary = "ap" approx = "" avdp = "" avg = "" avoirdupois = "" British = "UK" cubed = "^3" equivalent = "equiv" exact = "" feet = "foot" ies ("ies ") = "y " imp ("imp ") = "UK " imperial = "UK" inches = "inch" international = "intl" liq = "fl" liquid = "fl" ounce = "oz" per (" per ") = "/" squared = "^2" year = "yr" resistance = electric resistance (electric resistance) Resistivity resistivity = ohm*m (category unit) volume resistivity = resistivity resistivity = ohm*m (resistivity; category unit) rev = revolution (plane angle) revolution = 2 pi*rad (plane angle) reyn = lbf*s/in^2 (dynamic viscosity; derived unit) rhe = 10 m^2/N*s (fluidity) ri (Japanese ri) = 2.44 mile (length) rod (surveyors rod) = 25 link (length) roentgen = 0.000258 A*s/kg (radiation exposure) Roman amphora = 6.84 gallon (volume) Roman cubit = 17.5 in (length) Roman foot = 0.971 foot (length) Roman mile = 5000 Roman foot (length) Roman obolus = 8.77 grain (mass) Roman talent = 125 libra (mass) rood = 40 rod^2 (area) rope = 20 ft (length) rotational acceleration = angular acceleration (angular acceleration) rotational frequency = angular velocity (angular velocity) rpm (revolutions per minute) = revolution/minute (angular velocity; derived unit) rutherford = 1000000 Bq (activity) rydberg = 1.36054 electron volt (energy) s (second) = s (time; base unit) S = siemens (conductance) s ap = scruple (mass) sA = statampere (electric current) sabin = ft^2 (area; derived unit) sack = 3 bushel (volume) salmanazar = 6 magnum (volume) sazhen (Russian sazhen) = 6.9 foot (length) sb = stilb (luminance) sC = statcoulomb (electric charge) Sc frequency (cesium atom frequency) = 9192631770 1/s (frequency) score = 20 scotch = 86 proof scruple (apothecary scruple) = 20 grain (mass) scruple ap = scruple (mass) scrupulum (Roman scrupulum) = 17.53 grain (mass) seam = 8 bushel (volume) sec = s (time) second = s (time) section (of land) = mile^2 (area; derived unit) section modulus = volume (volume) self inductance = inductance (inductance) semi = 0.5 (prefix) septennial = 7 yr (time) septillion = 1E+24 (prefix) sesqu = 1.5 (prefix) sesqui = 1.5 (prefix) sex = 6 (prefix) sextant = 0.333333333333333 pi*rad (plane angle) sextillion = 1E+21 (prefix) sF = statfarad (capacitance) sH = stathenry (inductance) shaku (Japanese shaku) = 11.93 in (length) shipping ton = freight ton (volume) short hundredweight (avoirdupois short hundredweight) = 100 pound (mass) short quarter = 25 pound (mass) short ton (avoirdupois short ton) = 2000 pound (mass) shot = oz fluid (volume) sidereal day = 86164.091 s (time) sidereal month = 27.3217 day (time) sidereal yr (sidereal year) = 365.2564 day (time) siemens = A/V (conductance; derived unit) sievert = m^2/s^2 (dose equiv; derived unit) sigma = us (time) sign = 0.166666666666667 pi*rad (plane angle) skein = 120 yd (length) skot = 0.001 apostilb (luminance) slug = lbf*s^2/ft (mass; derived unit) soccer field = 100 yard (length) SOL = speed of light (velocity) Solid Angle solid angle = sr (category unit) spat = 4 pi*sr sphere = 4 pi*sr sr (steradian) = sr (base unit) steradian = sr solid angle = sr (solid angle; category unit) solubility = concentration (concentration) sound intensity = heat flux density (heat flux density) sound pressure = pressure (pressure) span (cloth span) = 9 in (length) Spanish libra = 1.014 pound (mass) Spanish quintal = 101.4 pound (mass) spat = 4 pi*sr (solid angle) Specific Charge radiation exposure = specific charge specific charge = C/kg (category unit) specific charge = C/kg (specific charge; category unit) specific conductance = conductivity (conductivity) Specific Energy absorbed dose = specific energy calorific value mass basis = specific energy dose equiv = specific energy enthalpy mass basis = specific energy mass basis calorific value = specific energy mass basis enthalpy = specific energy specific energy = m^2/s^2 (category unit) TNT = 4600000 m^2/s^2 specific energy = m^2/s^2 (specific energy; category unit) specific entropy = specific heat capacity (specific heat capacity) Specific Fuel Consumption Mass Basis mass basis specific fuel consumption = specific fuel consumption mass basis specific fuel consumption mass basis = kg/J (category unit) specific fuel consumption mass basis = kg/J (specific fuel consumption mass basis; category unit) specific fuel consumption volume basis = compressibility (compressibility) Specific Heat Capacity mass basis specific heat capacity = specific heat capacity specific entropy = specific heat capacity specific heat capacity = J/kg*K (category unit) specific heat capacity mass basis = specific heat capacity specific heat capacity = J/kg*K (specific heat capacity; category unit) specific heat capacity mass basis = specific heat capacity (specific heat capacity) specific heat capacity mole basis = molar heat capacity (molar heat capacity) specific impact energy = surface tension (surface tension) Specific Surface Area specific surface area = m^2/kg (category unit) specific surface area = m^2/kg (specific surface area; category unit) Specific Volume specific volume = m^3/kg (category unit) yield = specific volume specific volume = m^3/kg (specific volume; category unit) specific volume mole basis = molar volume (molar volume) speed = velocity (velocity) speed of light (in vacuum) = 299792458 m/s*approx (velocity) sphere = 4 pi*sr (solid angle) spindle = 14400 yard (length) spring = 92.9583333333333 day*approx (time) square = 100 ft^2 (area) squared = "^2" sr (steradian) = sr (solid angle; base unit) st = stere (volume) St = stoke (kinematic viscosity) stadia (Greek stadia) = 400 Greek cubit (length) stadium (Roman stadium) = 202 yard (length) standard atmosphere = atmosphere (pressure) standard barrel = dry barrel (volume) standard kilomole volume = 22.414 m^3/kmol (molar volume) statampere = 0.000000000333564 A (electric current) statcoulomb = 0.000000000333564 C (electric charge) statfarad = 1.11265E-12 F (capacitance) stathenry = 898755400000 H (inductance) static moment of area = volume (volume) Static Moment Of Mass static moment of mass = kg*m (category unit) static moment of mass = kg*m (static moment of mass; category unit) static moment of volume = moment of inertia of area (moment of inertia of area) statohm = 898755400000 ohm (electric resistance) statute league = league (length) statute mile = mile (length) statvolt = 299.7925 V (electric potential) stefan-boltzmann (Stefan-Boltzmann constant) = 0.000000056697 W/m^2*K^4 (unknown units) steradian = sr (solid angle) stere = m^3 (volume; derived unit) sthene = 1000 N (force) stilb = cd/cm^2 (luminance; derived unit) Stimulated Absorption stimulated absorption = s/kg (category unit) stimulated emission = stimulated absorption stimulated absorption = s/kg (stimulated absorption; category unit) stimulated emission = stimulated absorption (stimulated absorption) stoke = 0.0001 m^2/s (kinematic viscosity) stone = 14 pound (mass) stress = pressure (pressure) stretchability = compressibility (compressibility) strike = 2 bushel (volume) summer = 93.5416666666667 day*approx (time) sun (Japanese sun) = 1.193 in (length) surface area = area (area) surface concentration = adsorption (adsorption) Surface Density bearing capacity mass basis = surface density mass basis bearing capacity = surface density mass to area structural loading = surface density surface density = kg/m^2 (category unit) surface density = kg/m^2 (surface density; category unit) surface density of charge = electric flux density (electric flux density) surface energy = energy (energy) surface pressure = surface tension (surface tension) Surface Tension elasticity = surface tension intensity of force = surface tension interfacial tension = surface tension langley = calorie/cm^2 (derived unit) specific impact energy = surface tension surface pressure = surface tension surface tension = N/m (category unit) tension = surface tension surface tension = N/m (surface tension; category unit) survey foot = 0.304800609601219 m (length) surveyors chain = chain (length) surveyors furlong = furlong (length) surveyors link = link (length) surveyors rod = rod (length) susceptance = conductance (conductance) Sv = sievert (dose equiv) sV = statvolt (electric potential) synodic month = 29.5306 day (time T (tera) = 1000000000000 (prefix) t = metric ton (mass) T = tesla (magnetic flux density) tablespoon = 0.5 oz fluid (volume) tael (Chinese tael) = 0.0625 catty (mass) talent (Greek talent) = 60 mina (mass) tan = picul (mass) tb = terabyte (data) tbsp = tablespoon (volume) tea cup = 6 oz fluid (volume) teaspoon = 0.333333333333333 tablespoon (volume) temperature = thermodynamic temperature (thermodynamic temperature) temperature difference = thermodynamic temperature (thermodynamic temperature) ten = 10 (prefix) tension = surface tension (surface tension) tera = 1000000000000 (prefix) terabyte = 1024 gigabyte (data) tesla = kg/A*s^2 (magnetic flux density; derived unit) therm = 100000 BTU (energy) Thermal Conductance thermal conductance = W/K (category unit) thermal conductance = W/K (thermal conductance; category unit) Thermal Conductivity heat conductivity = thermal conductivity thermal conductivity = W/m*K (category unit) thermal conductivity = W/m*K (thermal conductivity; category unit) thermal diffusion = kinematic viscosity (kinematic viscosity) thermal flux = heat flux density (heat flux density) Thermal Resistance thermal resistance = K*m^2/W (category unit) thermal resistance = K*m^2/W (thermal resistance; category unit) thermie = 1000000 calorie (energy) thermochemical calorie = 4.184 J (energy) Thermodynamic Temperature absolute temperature = thermodynamic temperature celsius = dC centigrade = dC dC (Celsius degree) = K 555555555555556 K dK = K dR (Rankine degree) = 0.555555555555556 K dRe (Reaumur degree) = 1.25 K fahrenheit = dF K (Kelvin degree) = K (base unit) kelvin = K rankine = dR reaumur = dRe temperature = thermodynamic temperature temperature difference = thermodynamic temperature thermodynamic temperature = K (category unit) thermodynamic temperature = K (thermodynamic temperature; category unit) thousand = 1000 (prefix) thrice = 3 (prefix) throughput mass basis = mass flow rate (mass flow rate) throughput mole basis = mole flow rate (mole flow rate) throughput volume basis = volume flow rate (volume flow rate) tick = 0.03125 $ (currency) timber foot = ft^3 (volume; derived unit) Time annual = yr autumn = 89.6666666666667 day*approx calendar yr (calendar year) = 365 day cen = century centennial = century century = 100 yr d = day day = 24 hour decade = 10 yr diurnal = day ennial = yr fiscal yr (fiscal year) = 360 day fortnight = 2 week fy = fiscal yr h = hour hour = 60 minute hr = hour leap yr (leap year) = 366 day lunation = synodic month mean calendar yr = calendar yr mean solar day = day mean solar yr = yr millenium = 1000 yr millennial = millenium min = minute minute = 60 s mo = month month = 8.33333333333333E-02 yr*approx monthly = month novennial = 9 yr octennial = 8 yr olympiad = 4 yr*approx perennial = yr pregnancy = 9 month*approx quindecennial = 15 yr quinquennial = 5 yr s (second) = s (base unit) sec = s second = s septennial = 7 yr sidereal day = 86164.091 s sidereal month = 27.3217 day sidereal yr (sidereal year) = 365.2564 day sigma = us spring = 92.9583333333333 day*approx summer = 93.5416666666667 day*approx synodic month = 29.5306 day time = s (category unit) week = 7 day weekend = 2 day weekly = week winter = 89.0416666666667 day*approx wk = week yr (year) = 365.24219879 day*approx time = s (time; category unit) tn = short ton (mass) TNT = 4600000 m^2/s^2 (specific energy) ton = short ton (mass) ton force = metric ton*g (force; derived unit) tonne = metric ton (mass) torque = energy (energy) torr = mm*Hg weight density (pressure; derived unit) torsion of a curve = wave number (wave number) tou (Chinese tou) = 2.74 gallon (volume) township = 36 mile^2 (area) transmissivity = kinematic viscosity (kinematic viscosity) tri = 3 (prefix) trillion = 1000000000000 (prefix) triple = 3 (prefix) troy carat = carat troy (mass) troy dr = dram troy (mass) troy dram = dram troy (mass) troy gr = grain troy (mass) troy grain = grain troy (mass) troy lb = pound troy (mass) troy oz = oz troy (mass) troy pennyweight = pennyweight troy (mass) troy pound = pound troy (mass) tsp = teaspoon (volume) tun (English tun) = 8 barrel (volume) turn = 2 pi*rad (plane angle) twistability = compressibility (compressibility) u (micro) = 0.000001 (prefix) u = AMU (mass) U = micron (length) XC= 1/(2 x pie x Capacitance x frequency) u0 = magnetic field (permeability) UK ("UK ") = replacement for "imp " UK (British) = 1.000002 UK = replacement for "British" UK = replacement for "imperial" UK bushel (British bushel) = 4 UK peck (volume) UK caliber (British caliber) = 0.001 UK fl oz = UK oz fluid (volume) UK fluid oz = UK oz fluid (volume) UK gallon (British gallon) = 277.41945 in^3*approx (volume) UK mile (British mile) = 1609.34721869 m (length) UK oz = UK oz fluid (volume) UK oz fl = UK oz fluid (volume) UK oz fluid (British fluid ounce) = 0.05 UK pint (volume) UK peck (British peck) = 554.84 in^3*approx (volume) UK pint (British pint) = 0.5 UK quart (volume) UK quart (British quart) = 0.25 UK gallon (volume) UK quintal (British quintal) = 100 pound (mass) UK quintal2 (British quintal) = 112 pound (mass) UK thermal unit = BTU (energy) XL = (2 x pie x Inductance x frequency) uncia (Roman uncia) = 420 grain (mass) unciae = uncia (mass) undec = 11 (prefix) universal gas (constant) = 8.31434 J/mol*K (molar heat capacity) Unknown Units avogadro (Avogadro's number) = 6.022169E+23 1/mol*approx faraday (Faraday constant) = 96486.7 C/mol Fdy = faraday gravitation (constant) = 0.00000000006664 N*m^2/kg^2 L = avogadro Na = avogadro refrigeration = 12000 BTU/ton*hour stefan-boltzmann (Stefan-Boltzmann constant) = 0.000000056697 W/m^2*K^4 US dollar = $ (currency) V = volt (electric potential) vacuum = pressure (pressure) vara (Mexican vara) = 32.99 in (length) vedro (Russian vedro) = 3.25 gallon (volume) Velocity admiralty knot = 6080 ft/hour c = speed of light corrosion rate = velocity flow rate to area volume basis = velocity fps (feet per second) = ft/s (derived unit) ips (inches per second) = in/s (derived unit) knot (nautical miles per hour) = nmi/hour (derived unit) kph (kilometers per hour) = km/hour (derived unit) linear speed = velocity linear velocity = velocity Ma = mach mach (Mach at sea level & 32 dF) = 331.46 m/s mass transfer coefficient = velocity mph (miles per hour) = mile/hour (derived unit) SOL = speed of light speed = velocity speed of light (in vacuum) = 299792458 m/s*approx velocity = m/s (category unit) velocity linear = velocity volume basis flow rate to area = velocity velocity = m/s (velocity; category unit) velocity angular = angular velocity (angular velocity) velocity linear = velocity (velocity) verst (Russian verst) = 3500 foot (length) vic = 20 (prefix) viscosity absolute = dynamic viscosity (dynamic viscosity) viscosity dynamic = dynamic viscosity (dynamic viscosity) viscosity kinematic = kinematic viscosity (kinematic viscosity) Vm = standard kilomole volume (molar volume) volt = W/A (electric potential; derived unit) voltage = electric potential (electric potential) voltage potential = electric potential (electric potential) Volume amphora (Greek amphora) = 10.3 gallon arroba oil = oil arroba arroba wine = wine arroba balthazar = 8 magnum barrel = 31.5 gallon bath (Israeli bath) = 2.25 in^3 bbl = petroleum barrel beer gallon (English beer gallon) = 282 in^3 board foot = 144 in^3 bu = bushel bushel (dry bushel) = 4 peck Canadian gallon = 0.00454609 m^3 capacity = volume cc (cubic centimeters) = cm^3 (derived unit) chetvert (Russian chetvert) = 5.96 bushel coomb = 4 bushel cord (of wood) = 128 ft^3 cord foot (of wood) = 16 ft^3 cu = cup cup = 0.5 pint displacement ton = 35 ft^3 dr fl = dram fluid dr fluid = dram fluid dram fl = dram fluid dram fluid (fluid dram) = 0.125 oz fluid drop = 0.03 cm^3 dry barrel = 7056 in^3 dry gallon = 268.8025 in^3*approx dry pint = 0.5 dry quart dry quart = 0.25 dry gallon ephah (Israeli ephah) = 10 omer fbm = board foot fifth = 0.8 quart firkin = 9 gallon fl dr = dram fluid fl dram = dram fluid fl oz = oz fluid fluid dr = dram fluid fluid dram = dram fluid fluid oz = oz fluid freight ton = 40 ft^3 gal = gallon gallon = 231 in^3 gill = 0.5 cup hd = hogshead hekat (Israeli hekat) = 291 in^3 hhd = hogshead hogshead = 2 barrel jeroboam = 0.8 gallon jigger = 1.5 oz fluid kilderkin = 18 gallon koku (Japanese koku) = 47.6 gallon l = liter last = 80 bushel liter = 0.001 m^3 load = 1.9 yd^3 magnum = 2 quart measurement ton = freight ton methuselah = 4 magnum minim = 1.66666666666667E-02 dram fluid nebuchadnezzar = 10 magnum noggin = 0.125 quart oil arroba (Spanish oil arroba) = 3.32 gallon oil barrel = petroleum barrel omer (Israeli omer) = 0.45 peck oz fl = oz fluid oz fluid (fluid ounce) = 0.25 gill peck (dry peck) = 2 dry gallon petroleum barrel = 42 gallon pint (fluid pint) = 0.5 quart pipe = 2 hogshead pk = peck pony = 0.5 jigger pt = pint qt = quart quart (fluid quart) = 0.25 gallon register ton = 100 ft^3 rehoboam = 3 magnum Roman amphora = 6.84 gallon sack = 3 bushel salmanazar = 6 magnum seam = 8 bushel section modulus = volume shipping ton = freight ton shot = oz fluid st = stere standard barrel = dry barrel static moment of area = volume stere = m^3 (derived unit) strike = 2 bushel tablespoon = 0.5 oz fluid tbsp = tablespoon tea cup = 6 oz fluid teaspoon = 0.333333333333333 tablespoon timber foot = ft^3 (derived unit) tou (Chinese tou) = 2.74 gallon tsp = teaspoon tun (English tun) = 8 barrel UK bushel (British bushel) = 4 UK peck UK fl oz = UK oz fluid UK fluid oz = UK oz fluid UK gallon (British gallon) = 277.41945 in^3*approx UK oz = UK oz fluid UK oz fl = UK oz fluid UK oz fluid (British fluid ounce) = 0.05 UK pint UK peck (British peck) = 554.84 in^3*approx UK pint (British pint) = 0.5 UK quart UK quart (British quart) = 0.25 UK gallon vedro (Russian vedro) = 3.25 gallon volume = m^3 (category unit) wey = 40 bushel wine arroba (Spanish wine arroba) = 4.3 gallon wine bottle = 0.800633 quart volume = m^3 (volume; category unit) volume basis calorific value = pressure (pressure) volume basis flow rate = volume flow rate (volume flow rate) volume basis flow rate to area = velocity (velocity) volume basis flow rate to length = kinematic viscosity (kinematic viscosity) volume basis specific fuel consumption = compressibility (compressibility) volume basis throughput = volume flow rate (volume flow rate) volume conductivity = conductivity (conductivity) volume density of charge = electric charge density (electric charge density) Volume Flow Rate cfs (cubic feet per second) = ft^3/s (derived unit) flow rate volume basis = volume flow rate gpm (gallons per minute) = gal/min (derived unit) mgd (megagallons per day) = 1000000 gallon/day miners inch = 1.5 ft^3/minute throughput volume basis = volume flow rate volume basis flow rate = volume flow rate volume basis throughput = volume flow rate volume flow rate = m^3/s (category unit) volume flow rate = m^3/s (volume flow rate; category unit) volume resistivity = resistivity (resistivity) volume to length = area (area) volume to mole concentration = molar volume (molar volume) volume to volume concentration = dimensionless unit Volumetric Heat Capacity volumetric heat capacity = J/m^3*K (category unit) volumetric heat transfer coefficient = volumetric heat capacity volumetric heat capacity = J/m^3*K (volumetric heat capacity; category unit) volumetric heat transfer coefficient = volumetric heat capacity (volumetric heat capacity) W = watt (power) watt = J/s (power; derived unit) Wave Number area to volume = wave number capture unit = wave number curvature of a curve = wave number diopter (lens power) = 1/m (derived unit) kayser = 100 1/m lens power = wave number torsion of a curve = wave number wave number = 1/m (category unit) wave number = 1/m (wave number; category unit) wavelength = length (length) Wb = weber (magnetic flux) weber = V*s (magnetic flux; derived unit) week = 7 day (time) weekend = 2 day (time) weekly = week (time) weight = force (force) Weight Density H20 density (water weight density) = 0.0885472440945 kg/m^2*s^2 Hg weight density = 133322.368421 kg/m^2*s^2 pressure drop to length = weight density weight density = N/m^3 (category unit) weight density = N/m^3 (weight density; category unit) wey = 40 bushel (volume) wey mass = 252 pound (mass) Wh (watt hour) = W*hour (energy; derived unit) width = length (length) wine arroba (Spanish wine arroba) = 4.3 gallon (volume) wine bottle = 0.800633 quart (volume) winter = 89.0416666666667 day*approx (time) wk = week (time) work = energy (energy) y ("y ") = replacement for "ies " Y (myria) = 10000 (prefix) yard = 3 ft (length) yd = yard (length) year = "yr" yield = specific volume (specific volume) yr (year) = 365.24219879 day*approx (time) yr = replacement for "year" ^2 = replacement for "squared" ^3 = replacement for "cubed" ##END_OF_RECORD B B [1] informal abbreviation for "billion," generally meaning the American billion 109. This abbreviation is non-metric: the metric abbreviation for 109 is G, standing for the prefix giga-. The B form has been used in such units as Bcf (billion cubic feet) and BeV (billion electron volts). B [2] a symbol for international standard paper sizes, followed by the size number, as in B4. A table of sizes is provided. bag [1] another name for a sack (3 bushels) used as unit of measure in Britain. bag [2] an old English unit of weight, varying with the contents of the bag but generally in the range of 2-4 hundredweight (100-200 kilograms). bag [3] a unit of weight for cement. Traditionally a bag of portland cement weighs 94 pounds (42.6 kilograms) in the US. and 87.5 pounds (39.7 kilograms) in Canada. However, cement is now being sold also in metric-sized bags of 50 kilograms (110.2 pounds) by many suppliers. baht a traditional weight unit in Thailand, now equal to exactly 15 grams or 1/40 catty (0.5291 ounce). The baht, originally the weight of a silver coin of the same name, is used to measure the weight of precious metals. The unit is pronounced bot. baker's dozen an informal unit of quantity, equal to 13. Bakers often toss in an extra item for each dozen bought, making a total of 13. This custom is very old, dating at least from the thirteenth century, when the weights and prices of loaves of bread were strictly regulated by royal proclamations called assizes, and bakers could be jailed if they failed to provide fair weight at the listed prices. bale (bl) [1] a bundle of merchandise, usually pressed and bound in some way. The word "bale" has been used in many ways to describe standard packages of various commodities. For example, a bale of paper is traditionally equal to 10 reams. In agriculture, a bale of hay is generally a huge round bundle left in the field until needed; these bales can weigh up to 1500 pounds (700 kilograms). In U.S. garden shops, a bale of straw is typically 3 cubic feet (0.085 cubic meter). bale (bl) [2] a commercial unit of weight for shipments of cotton. In the United States, one bale of cotton, formerly equal to 500 pounds (226.80 kg), is now equal to 480 pounds (217.72 kg). The British used the Egyptian bale, formerly equal to 750 pounds (340.19 kg) but now equal to 720 pounds (326.59 kg). Other countries use a variety of cotton bale weights. ball a unit measuring the degree of ice coverage of polar seas. One ball equals 10% coverage. The unit was invented by the Russian naval officer N. N. Zhubov (1895-1960). Balling a unit of density; see degree Plato. balthazar a large wine bottle holding about 12 liters, 16 times the volume of a regular bottle. bank cubic meter (BCM) a traditional unit of volume in coal mining. A bank cubic meter represents the contents of a cubic meter of rock in place, before it is drilled and blasted. bar (b) a metric unit of atmospheric pressure, equal to one million dynes per square centimeter, 100 kilopascals, 750.062 torr, 1.019 72 kilograms of force per square centimeter (kgf/cm2), or about 14.503 78 pounds per square inch (lbf/in2). The word comes from the Greek baros, "weighty." We see the same root in our word, barometer, for an instrument measuring atmospheric pressure. One bar is just a bit less than the average pressure of the Earth's atmosphere, which is 1.013 25 bar. In practice, meteorologists generally record atmospheric pressure in millibars (mb). In English-speaking countries, barometric pressure is also expressed as the height, in inches, of a column of mercury supported by the pressure of the atmosphere. In this unit, one bar equals 29.53 inches of mercury (in Hg). barg symbol for bar gauge, a common unit of pressure in engineering. The term "gauge" means that the pressure has been read from a gauge which actually measures the difference between the pressure of the fluid or gas and the pressure of the atmosphere. barge an informal unit of volume used in the U.S. energy industry. The barges used on American rivers customarily carry about 25 000 barrels of oil (see barrel [2] below). This is equivalent to 1.05 million gallons, roughly 1400 register tons, or about 3975 cubic meters. barleycorn an old English unit of length equal to 1/3 inch or about 8.5 millimeters. The custom of using seeds as units of length or weight is very common in farming societies. In Anglo-Saxon England, where barley was a basic crop, barleycorns played this traditional role. The weight of a barleycorn, later renamed the grain, is the original basis of all English weight systems including the older troy system and the later avoirdupois system. As a length unit, 3 barleycorns were equal to the Saxon ynce (inch). The English foot was actually defined as 12 of these ynces, that is, as 36 barleycorns. barn (b) a slightly humorous unit of area used in nuclear physics. When atoms are bombarded with smaller particles such as electrons, the electrons are scattered as if the nucleus of the atom was a tiny solid object. The barn is used to express the apparent cross-sectional area of this scattering object. One barn is equal to 10-28 square meters. Using this unit, physicists can say that such and such a nucleus is "as big as a barn," or 10 barns, or whatever. The proper SI unit to use for these measurements is the square femtometer (fm2); one barn equals 100 fm2. barrel (bbl or brl or bl) [1] a commercial unit of volume used to measure liquids such as beer and wine. The official U. S. definition of the barrel is 31.5 gallons, which is about 4.211 cubic feet or 119.24 liters. This unit is the same as the traditional British wine barrel. In Britain the barrel is now defined to be 36 imperial gallons, which is substantially larger: about 5.780 cubic feet or 163.66 liters. This unit is slightly larger than the traditional British beer and ale barrel, which held 5.875 cubic feet or 166.36 liters. There are other official barrels, defined in certain U.S. states; most of them fall in the general range of 30-40 gallons. A barrel of beer in the U.S., for example, is usually 31 U.S. gallons (117.35 liters). The origin of the standard symbol bbl is not clear. The "b" may have been doubled originally to indicate the plural (1 bl, 2 bbl), or possibly it was doubled to eliminate any confusion with bl as a symbol for the bale (see above). barrel (bbl or bo) [2] a commercial unit of volume used to measure petroleum. By international agreement a barrel of petroleum equals 42 U. S. gallons, which is about 158.987 liters. The symbol bo (barrel of oil) is used for this unit in the petroleum industry. The petroleum barrel originated in the Pennsylvania oilfields (the first commercial oilfields) in the late nineteenth century. Apparently, 40-gallon barrels were increased to 42 gallons to provide insurance aganst any spillage or underfilling. By coincidence (it seems), this unit is the same size as the traditional tierce, a wine barrel. barrel (bbl or brl or bl) [3] a commercial unit of volume used to measure dry commodities such as apples. The U. S. dry barrel, established by Conress in 1912, is 105 dry quarts, which is about 4.083 cubic feet or 115.63 liters. (This is the only case in the United States customary system where a dry volume is less than the corresponding fluid volume.) For certain commodities, other sizes are traditional in the U.S.; for example, a barrel of sugar was traditionally 5 cubic feet (about 141.58 liters). barrel (bbl) [4] a commercial unit of weight, varying with the commodity being measured. In the U.S., for example, a barrel of flour traditionally holds 196 pounds (88.90 kg) and a barrel of beef, fish, or pork 200 pounds (90.72 kg). A barrel of cement is traditionally equal to 4 bags, which is 376 pounds (170.55 kg) in the U.S. and 350 pounds (158.76 kg) in Canada. barrel bulk a commercial unit of volume equal to exactly 5 cubic feet or 0.141 584 cubic meters. There are exactly 8 barrels bulk in a freight ton and 20 in a register ton. barrer or Barrer a CGS unit of gas permeability for membranes, contact lenses, and similar thin materials. Permeability is defined to be the gas flow rate multiplied by the thickness of the material, divided by the area and by the pressure difference across the material. To measure this quantity, the barrer is the permeability represented by a flow rate of 10-10 cubic centimeters per second (volume at standard temperature and pressure, 0 °C and 1 atmosphere), times 1 centimeter of thickness, per square centimeter of area and centimeter of mercury difference in pressure. That is, 1 barrer = 10-10 cm2·s-1·cmHg-1, or, in SI units, 7.5005 x 10-18 m2·s-1·Pa-1. The unit, often capitalized, honors the New Zealand chemist Richard M. Barrer (1910-1996), who was a leader in research on the diffusion of gases. barrique a large French wine barrel holding 225 liters (about 59.44 U.S. gallons). barye (ba) the CGS unit of pressure, equal to 1 dyne per square centimeter. Pronounced "bar-ee," the barye derives from the Greek word for weight, barys. It's a very small unit; 1 barye is equal to 0.1 pascal (Pa), 1 microbar (µb), or about 14.5 x 10-6 pound per square inch. basis point (bp) a unit of proportion equal to 0.01% or 10-4. The basis point is used in finance to measure small fluctuations in interest rates and the rates of return on investments. Prior to the introduction of the basis point, these fluctuations were measured clumsily in 64ths of a percent. basis weight a unit used in the paper industry to express the weight (really, the thickness) of paper. The basis weight is the weight in pounds of one ream (500 sheets) of a basic size sheet. For details, see pound weight. baud (Bd) a unit used in engineering for measuring the rate of data transmission over telegraph or telephone lines. The baud rate is the number of times per second the signal carrying the communication varies in strength or frequency. In the days of the telegraph, the signal increased at the start of a pulse and decreased at the end, so the baud rate was 2 times the number of dots that could be transmitted per second. In recent years, when the baud has been used to represent digital data transmission, the definition has varied according to the technique used. If the signal has only two states (on or off) then the baud rate is the same as the transmission rate in bits per second. Since this was the case in early (slow) modems, in common language the baud is often used to mean the same thing as 1 bit per second. However, faster modems use signals having 4 possible states (corresponding to 2 bits per second) or 16 possible states (corresponding to 4 bits per second). For those of us who are not communications engineers, the best way to represent data transmission rates is directly in bits per second (b/s or bps), avoiding the use of the baud altogether. The baud is named for the French telegraph engineer J. M. E. Baudot (1845-1903), the inventor of the first teleprinter. Baumé a measure of relative density; see degree Baumé. BC, BCE abbreviations for "before Christ" and "before common era," respectively. These are standard designations for years before the beginning of the common or Christian era (see CE). Oddly, there is no year designated 0 in the common system for numbering years; the year 1 CE (or 1 AD) was preceded by 1 BCE (or 1 BC). The lack of a year 0 means that the number of years elapsing between the year n BCE and the year m CE (or n BC and m AD) is n + m - 1. bcm [1] a symbol for "billion cubic microns per square inch," a unit used in flexographic printing. In this process, a roller engraved with a large number of tiny cells is used to transfer ink to the printing plate. The unit measures the total volume of these cells per unit area of the roller. One billion (109) cubic microns per square inch is the same as one cubic millimeter per square inch (mm3/in2) or one microliter per square inch (µL/in2). The appropriate SI unit for this volume per area measure is the micrometer or micron (µm); 1 bcm is equal to 1.5500 µm. bcm [2] abbreviation for "bank cubic meter" (see above). B/D symbol for barrels of oil per day (see barrel [2] above), a unit used in the energy industry to measure the rate at which oil is pumped from a well. bead sizes see aught. beat a unit of time equal to 0.001 day or 86.4 seconds. "Metric time," meaning decimalized time, is an idea dating back at least to the French Revolution of the 1790's. In most metric time proposals, the day is divided into 10 metric hours, each metric hour into 100 metric minutes (or beats), and each metric minute into 100 metric seconds (sometimes called blinks). In 1998 the Swatch Corporation repackaged metric time in a very attractive way as Internet time. In their proposal, time is counted in beats from midnight Central European Standard (winter) time (2300 Universal Time of the previous day, or 6:00 pm U.S. Eastern Standard Time of the previous day). The time at n beats is recorded as @n.beat; thus midnight U.S. Eastern Standard Time is @250.beat. Beaufort scale an empirical scale, first devised by the British admiral Sir Francis Beaufort (1774-1857), for estimating wind speed by observing the effects of the wind. Using the scale, sailors can judge the wind velocity by observing the wind's effects on the waves. There is a corresponding scale for observers on land. For example, a moderate gale (32-38 miles per hour) is described as "force 7" on the Beaufort scale. The Beaufort scale numbers come rather close to being a unit of measurement, because they are equal to the whole number closest to 0.66 times the wind velocity in miles per hour raised to the exponent 2/3 (Beaufort force = 0.66 x (velocity2/3)). becquerel (Bq) the SI derived unit of activity, usually meaning radioactivity. "Radioactivity" is caused when atoms disintegrate, ejecting energetic particles. One becquerel is the radiation caused by one disintegration per second; this is equivalent to about 27.0270 picocuries (pCi). The unit is named for a French physicist, Antoine-Henri Becquerel (1852-1908), the discoverer of radioactivity. See also curie. Note: both the becquerel and the hertz are basically defined as one event per second, yet they measure different things. The hertz is used to measure the rates of events that happen periodically in a fixed and definite cycle. The becquerel is used to measure the rates of events that happen sporadically and unpredictably, not in a definite cycle. beer gallon, beer and ale gallon see ale gallon. bee space an informal unit of distance used in beekeeping. In a hive, bees seal up an opening smaller than a bee space, and they fill a larger opening with new honeycomb. If an opening is equal to a bee space, the bees leave it open as a passageway. A hive can be disassembled to remove the honey if the individual comb frames are carefully spaced one bee space apart. This discovery, made by the British beekeeper Lorenzo Longstroth in 1852, is crucial to modern beekeeping. The exact size of the bee space varies somewhat with the strain of bees being raised, but it is generally very close to 1/4 inch or 6.5 millimeters. bel (B) a unit of sound intensity, invented by engineers of the Bell telephone network in 1923 and named in honor of the inventor of the telephone, Alexander Graham Bell (1847-1922). If one sound is 1 bel louder than another, this means the louder sound is 10 times more intense than the fainter one. A difference of 2 bels corresponds to an increase of 10 x 10 or 100 times in intensity. The beginning of the scale, 0 bels, can be defined in various ways (see decibel) originally intended to represent the faintest sound that can be detected by a person who has good hearing. In practice, sound intensity is almost always stated in decibels. One bel is equal to approximately 1.151 293 nepers. bell a traditional unit of time. On ships at sea, a practical measure of time is the watch, a period of 4 hours. The watch is divided into 8 bells, so one bell equals 1/2 hour or 30 minutes. Every 30 minutes the ship's bell sounds the number of bells elapsed since the start of the watch. BeV a symbol sometimes used in the U.S. for one billion (109) electronvolts. The correct symbol is GeV. bhp abbreviation for brake horsepower. The brake horsepower of an engine is the effective power output, sometimes measured as the resistance the engine provides to a brake attached to the output shaft. See horsepower. bi- a common English prefix meaning 2. In statements of frequency, bi- and semi- have become confused and it isn't always clear what a word like "bimonthly" means. This is how it's supposed to work: in adverbs of frequency, bi- means "every two." Thus a biweekly payroll is paid once every two weeks, a bimonthly magazine is published once every two months, and the U.S. House of Representatives is elected biennially (every two years). For something that happens twice per time unit, use semi-. biannually a confusing expression of frequency. The word is used both for twice a year and for once every two years, so it should be avoided. Twice a year is semiannually and once every two years is biennially. bicron (µµ) an obsolete metric unit of distance, defined as 10-12 meter or 10-6 micron. This distance is now called a picometer (pm). The name is a reference to the double prefix in "micromicron." BID or b.i.d. abbreviation for the Latin phrase bis in die, twice a day, a unit of frequency traditionally used by doctors in writing medical prescriptions. biennium a unit of time equal to two years. Many U.S. states, including North Carolina, elect legislators every two years and adopt budgets for this two-year period, called a biennium. bigha a traditional unit of land area in South Asia. The bigha varies in size from region to another; in India it is generally less than an acre (0.4 hectare). In Bengal (both in Bangladesh and in West Bengal, India) the bigha was standardized under British colonial rule at 1600 square yards (0.1338 hectare or 0.3306 acre); this is often interpreted as being 1/3 acre. In central India bighas were standardized at 3025 square yards or 5/8 acre (0.2529 hectare). In Nepal the bigha equals about 0.677 hectare (1.67 acres). The bigha was divided into 20 katthas, and each kattha contained 20 dhurs. billennia a new word meaning "billions of years," billennia is appearing informally in science writing in phrases such as "countless billennia" or "the billennia of evolutionary time." The word billennium was coined for another purpose, however; it is a registered trademark of the Billennium® Organizing Committee and is used, in the words of a committee spokesperson, "to describe the positive spirit of the millennium, which is an ongoing and enduring movement." billet a single stick of firewood. Traditional English billets had a length of 40 inches (1.016 meters). billiard a unit of quantity equal to 1015, which is one quadrillion in American terminology or 1000 billion in traditional British terminology. The name is coined to parallel milliard, which has long been a name for 1000 million. billion a number equal to 1000 million in the U.S. but one million million in traditional British usage. See Names for Large Numbers for a discussion of the billion problem. bimester a rarely-seen unit of time equal to 2 months. bimillennium a unit of time equal to 2000 years. bind an old English unit of quantity for eels, equal to 250. biot (Bi) another name for the abampere, a unit of electric current equal to 10 amperes. This unit honors the French mathematician and physicist Jean-Baptiste Biot (1774-1862), one of the founders of the theory of electromagnetism. bips informal name for the bit per second (see below). bit (b) [1] the basic unit of information. Each bit records one of the two possible answers to a single question: "0" or "1," "yes" or "no," "on" or "off." Logically, this is the smallest quantity of information that can exist. The word "bit", coined by the American statistician and computer scientist John Tukey (b. 1915) in 1946, is an acronym for binary digit. bit (b) [2] a logarithmic unit of storage capacity, equal to the base-2 logarithm of the number of possible states of the storage device or location. If data is stored as binary digits, this reduces to definition [1]: an 8-bit storage location, for example, has 28 = 256 possible states, so its capacity is log2 28 = 8 bits. If, however, a storage location stores one letter, then it has 26 possible states, and its storage capacity is log2 26 = 4.7004 bits. bit (b) [3] a unit of information content, now known properly as the shannon. bit per second (b/s or bps) a common unit of data transmission rate in computer science. The symbol bps is often pronounced "bips." Modem transmission rates are often stated in kilobits per second (Kbps or kb/s). blink a unit of time equal to 10-5 day or exactly 0.864 second. This unit is also called the metric second; in most metric time proposals, the day is divided into 10 hours, each hour into 100 minutes (called beats in "Internet time": see "beat" above), and each minute into 100 seconds or blinks. An actual eye blink takes less than half as much time as this unit. block an informal unit of distance popular in the U.S. A block is the average distance between street intersections in the rectangular street grids common in most American cities. The length of a block varies from about 1/20 mile (80 meters) in New York to about 1/16 mile (100 meters) in many midwestern cities to about 1/10 mile (160 meters) in cities of the South and West. blondel another name for an apostilb. Proposed in 1942, this unit honors the French physicist André Blondel (1863-1938) for his pioneering work in photometry. blood alcohol level (BAL) a legal measurement of alcohol concentration in the bloodstream, determining whether a person is considered legally impaired or intoxicated. The blood alcohol level is usually stated as a percentage, such as 0.10%. In most U.S. states, this is a measurement of "weight by volume" (w/v). 0.10% w/v is equivalent to 100 milligrams of alcohol per deciliter (100 milliliters) of blood, which is the same as 1 gram per liter (g/L). In a few states, the measurement is the percentage mass concentration (w/w, or milligrams of alcohol per milligram of blood). 0.10% w/w is the same alcohol concentration as 0.1055% w/v or 1.055 g/L, a slightly higher concentration than 0.10% w/v. Internationally, blood alcohol levels are often stated in millimoles per liter (mmol/L). 1 millimole per liter is equal to 4.61 mg/dL or 0.00461% w/v. In the other direction, each 0.01% w/v is equal to about 2.169 mmol/L. board foot (bd ft, fbm, or BF) a unit of volume used for measuring lumber. One board foot is the volume of a one-foot length of a "standard board" twelve inches wide by one inch thick. Thus a board foot equals 144 cubic inches, or 1/12 of a cubic foot, or approximately 2.360 liters. If lumber is stacked neatly, we can compute the number of board feet from the dimensions of the stack, no matter how wide or thick the boards are. For example, a stack of two-by-fours 4 ft high, 4 ft wide, and 8 ft long contains 4 x 4 x 8 = 128 cubic feet, equivalent to 128 x 12 = 1536 board feet. The symbol fbm is an abbreviation for "foot, board measure." Board of Trade unit a British name for the kilowatt hour, a unit of electrical energy. body mass index (BMI) a measure of "fatness" used in medicine and health. The BMI is equal to a person's weight W (in kilograms) divided by the square of his or her height H (in meters): BMI = W/H2. If measurements are made in traditional English units, the equivalent formula is BMI = 703.07·W/H2, where W is measured in pounds and H in inches. In the U.S., a person with a BMI less than 20 is considered underweight, a person with a BMI of 25 or more is considered overweight, and a person with a BMI of 30 or more is considered obese. (These figures are for adults. Doctors also keep in mind that women's BMI measures are normally lower than those of men.) boe abbreviation for "barrel of oil equivalent," a commercial unit of energy. One boe is equal to about 6.119 gigajoules (GJ) or 5.800 million Btu. bohr or bohr radius (a0) a unit of distance used in particle physics. The bohr radius represents the mean distance between the proton and the electron in an unexcited hydrogen atom. It equals about 52.918 picometers (pm), or 52.918 x 10-12 meter. The unit is named for the Danish physicist Niels Bohr (1885-1962), who explained the structure of atoms in a famous paper in 1913. boiler horsepower a traditional unit measuring the power delivered by a boiler. The boiler horsepower is defined to be the power required to convert 30 pounds (13. 61 kilograms) of water at 100 °F (37.78 °C) to steam at 212 °F (100 °C) at a pressure of 70 pounds per square inch gauge (482.6 kilopascals gauge). More or less equivalently, it has also been defined to be the power required to evaporate 34.5 pounds (15.65 kilograms) of water to steam at 212 °F (100 °C). This power is about 33 471 Btu per hour or 9809.5 watts, more than 13 times the usual mechanical definition of the horsepower. bolt a commercial unit of length or area used to measure finished cloth. Generally speaking, one bolt represents a strip of cloth 100 yards (91.44 meters) long, but the width varies according to the fabric. Cotton bolts are traditionally 42 inches (1.067 meters) wide and wool bolts are usually 60 inches (1.524 meters) wide. Thus a bolt of cotton is 1160 square yards (969.91 m2) and a bolt of wool is 1660 square yards (1387.97 m2). bone-dry unit (bdu) a unit used in the forest products industry to measure bulk products such as wood chips. One bone-dry unit is a volume of wood chips (or whatever) that would weigh 2400 pounds (1.0886 metric ton) if all the moisture content were removed. The bone-dry ton (bdt) is similar but based on a weight of 2000 pounds (0.9072 metric ton). bore the bore of a gun is the inside diameter of its barrel. As a result, "bore" is sometimes used as a unit, meaning the same as gauge [1] for shotguns and the same as caliber [1] for other guns. bottle (btl) a unit of volume. Like actual bottles, this unit varies according to the nature of the contents. For a long time in the U.S., a bottle of milk was 1 quart (946.36 milliliters) and a bottle of whiskey was 1 fifth (757.1 milliliters). Today wine is customarily sold in bottles containing 750 milliliters (about 25.3605 U.S. fluid ounces or 26.3963 British Imperial fluid ounces). See also quart [2]. Other countries have traditional units of about this size. For example, the Russian boutylka contains 768.95 milliliters. bovate an old English unit of land area equal to 1/8 hide. This was roughly 15 acres or 6 hectares. The word comes from the Latin bovis, an ox, indicating that the bovate was an area that could be farmed with the help of one ox. BP abbreviation for "years before present," a unit of time used in anthropology, geology, and paleontology. bpd abbreviation for barrels per day, a unit of production used in the petroleum industry. bpm abbreviation for beats per minute, the common unit of tempo in music. Technically, 1 bpm is equal to 1/60 hertz. braça see braza, below. braccio a traditional Italian unit of distance. The word means "arm", and a braccio is the length of a man's arm, about 27 or 28 inches (68-71 centimeters). In modern times, the braccio has become a metric unit of exactly 70 centimeters (about 27.56 inches). brace another name for a pair. The word is used mostly by hunters, who may speak of a brace of partridges or a brace of shotguns. Derived from the Latin word bracchia for both arms, it literally means "one for each arm." braza a traditional unit of distance in Spain and Latin America. The braza is comparable to the English fathom. In Spain it equals 2 varas, 8 palmos, or about 1.67 meters (5.48 feet or 65.75 inches). In Latin America the braza tends to be larger. The Argentine braza is 1.73 meters (5.68 feet or 68.16 inches), and under the Texas definition of the vara the braza would be 1.693 meters (5.556 feet or 66.67 inches). The Portuguese braça is similar, but it equals 10 palmos or about 2.20 meters (7.22 feet or 86.6 inches). breadth another name for a span (9 inches). This unit is traditionally used to measure the dimensions of flags. breakfast cup a unit of liquid volume, used in food recipes in Britain. The breakfast cup corresponds to the cup used by American cooks, except that it is based on British Imperial units. Thus it equals 1/2 Imperial pint. This is equivalent to 10 Imperial fluid ounces, 17.339 cubic inches, 1.20 U.S. cup, or about 284 milliliters. This unit is also called a tumblerful. breve the standard unit of relative time in music, equal to the time length of 2 whole notes. Although this is the longest interval in musical notation, the word comes from the Latin brevis, brief. The breve is equal to 2 semibreves, 4 minims, 8 crotchets, or 16 quavers. brewster (B) a unit used in optics to measure stress-induced birefringence. A material exhibits birefringence if it has different indices of refraction along different axes. Many crystals exhibit birefringence. In addition, certain substances, including many plastics, exhibit birefringence if they are subjected to compression or stress in one direction. The brewster, which measures this effect, has dimensions reciprocal to those of stress. One brewster is defined to equal 10-12 square meters per newton (m2/N) or 10-13 square centimeters per dyne (cm2/dyn). The unit is named for the British physicist David Brewster (1781-1868), who discovered stress-induced birefringence in 1816. bright, brightness a unit describing surface brightness, or reflectivity, especially of paper. The fraction of light reflected by a surface is called its albedo, and its brightness is the albedo expressed as a percentage. Thus "88 bright" paper reflects 0.88, or 88%, of the light falling on the paper. bril a unit used to express the "brilliance" or subjective brightness of a source of light. The scale used is logarithmic: an increase of 1 bril means doubling the luminance (and thus the actual amount of light energy) emitted by the source. A luminance of 1 lambert is defined to have a brilliance of 100 brils. Mathematically, the brilliance in brils equals (log L)/log 2 + 100, where L is the luminance in lamberts. Brinell hardness (HB or BHN) a measure of the hardness of a metal introduced by J. A. Brinell in 1900. In the Brinell test (generally used for metals of uniform hardness), a hard object such as a steel ball is pressed into the material being tested. The ball is of a specified diameter, usually 1 centimeter. The Brinell hardness is the amount of force applied to the ball divided by the area of the indentation the ball makes in the material. The result is measured in kilograms of force per square millimeter but should be stated as an empirical reading, without units. For readings up to about HB 500, Brinell hardness is about 0.96 times the Vickers hardness. British thermal unit (Btu) a unit of heat energy defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. In America the British thermal unit is sometimes called the heat unit. In defining the Btu, it is necessary to specify the temperature of the water; thus there have been several definitions over the years. However, one Btu is equal to about 251.996 (small) calories, or 0.251 996 of the (kilo-)calories counted by dieters. Using the current definition of the calorie (the IT calorie), one Btu equals approximately 778.169 foot pounds, 1.055 056 kilojoules or 0.293 071 watt hour. The symbol BThU has also been used, especially in Britain. brix a unit of proportion, equal to percent, used in measuring the concentration of sugar in unfermented grape juice, fruit syrups, maple syrup, and similar solutions. Thus one brix equals 1%. The unit is named for the Austrian scientist Adolf Brix (d. 1870), who invented a hydrometer that reads directly the percentage of sugar in the juice, provided the reading is taken at a specified temperature. Also called degree Brix (°Bx). In winemaking, the alcohol concentration in the finished wine is estimated to be 0.55 times the sugar concentration in brix of the unfermented juice. Btuh a non-standard symbol used in the U.S. heating and airconditioning industry for British thermal units per hour, a unit of power. 1 Btuh equals 0.293 071 watt. The proper symbol is Btu/h. bucket a unit of volume, generally informal. In the U.S., many commodities (both wet and dry) are sold in plastic buckets holding 5 U.S. liquid gallons (about 18.927 liters). In Britain, a bucket is often understood to be 4 gallons, or 18.182 liters based on the British Imperial gallon. bunder a traditional Dutch unit of land area. Since the adoption of the metric system in the Netherlands in 1809 the bunder has been considered equal to the hectare (2.471 acres). Historically, the unit varied with locality, generally in the range 0.85-1.3 hectare. The unit was also used in Belgium, being called the bonnier in French-speaking provinces. bundle (bdl) [1] a unit of quantity for paper, equal to 2 reams or 40 quires. This would be 960 sheets using the old definition of 24 sheets per quire, or 1000 sheets using the newer quire of 25 sheets. bundle (bdl) [2] a traditional unit of length for yarn, equal to 20 hanks. For cotton yarn, a bundle contains 16 800 yards (about 15.362 kilometers). For wool, a bundle contains 11 200 yards (10.241 kilometers). bundle (bdl) [3] in the construction trades, a bundle is a package of shingles. Shingles are usually packed so that exactly 3, 4, or 5 bundles are needed to cover a square (100 square feet or 9.29 square meters). Asphalt shingles are often sold in bundles of 27, with 3 bundles per square, but the heavier cedar shingles generally require 5 bundles per square. bushel (bu) [1] a traditional unit of volume used for measuring dry commodities such as grains and fruits. In the United States, the customary bushel is based on an old British unit known as the Winchester bushel. This unit dates to the early fourteenth century, at least: King Edward I defined the bushel to be 8 gallons in 1303. The form used in the U.S. was legalized by Parliament in 1696. One U.S. or Winchester bushel equals 4 pecks or 32 (dry) quarts; this is a volume of 2150.42 cubic inches or about 1.2445 cubic feet, and represents the volume of a cylindrical container 18.5 inches (47.0 cm) in diameter and 8 inches (20.3 cm) deep. The U. S. bushel holds about 35.239 07 liters. Traditionally, there is also a heaped bushel, which is 27.8% (sometimes 25%) larger than a regular bushel. The regular bushel is also called struck measure to indicate that the bushels have been struck, or leveled, rather then heaped. The origin of the word "bushel" is unclear; some scholars believe it derives from an ancient Celtic unit, but most believe it is of medieval French origin, probably a slang name for a wooden crate (the French word for wood is bois). bushel (bu) [2] a unit of volume in the British imperial system (see gallon [3]) equal to 8 imperial gallons, or 2219.36 cubic inches (1.2844 cubic feet). The imperial bushel holds about 36.369 liters. bushel (bu) [3] a commercial unit of weight for grains and other bulk commodities. Agricultural commodities such as wheat are traditionally sold by the bushel, but because commodities tend to settle and compact in shipping, disputes over the volume delivered arise easily. To avoid these disputes, traders in a market or a country generally agree on a standard weight for one bushel of the commodity. Often this standard weight is set by law. Although the bill of lading still shows "bushels," it is really the weight rather than the volume that is sold and guaranteed. For example, in the United States a bushel of wheat equals 60 pounds (27.216 kg), a bushel of barley 48 pounds (21.772 kg), a bushel of oats 32 pounds (14.515 kg), and a bushel of rye 56 pounds (25.401 kg). A more complete table is included. butt a traditional unit of volume used for wines and other alcoholic beverages. A butt is generally defined to be two hogsheads, but the size of hogsheads varies according to the contents. In the United States a hogshead is typically 63 gallons and a butt is 126 gallons: about 16.844 cubic feet or 476.96 liters. In Britain, a butt of beer is 108 imperial gallons: about 17.339 cubic feet or 490.98 liters. The word comes from the Roman buttis, a large cask for wine. button measure a unit of distance equal to 1/40 inch (0.635 millimeter), used, as you might guess, for measuring the thickness of buttons. This unit is also called the line, a bad idea since the line is generally equal to 1/12 inch. Bya or bya a common abbreviation (in English speaking countries) for "billion years ago." byte (B) a unit of information used in computer engineering. One byte is generally the amount of storage used for a single character. Typically this is 8 bits, which means that a byte has 28 = 256 possible states. The unit was named by IBM engineer Werner Buchholz in 1956, and the 8-bit size was popularized shortly thereafter by IBM's System 360, a top-selling early mainframe computer. Although various machines have used other byte sizes, such as 6 or 9 bits, 8 bits is now well established as the norm. The spelling "byte" is used instead of "bite" in order to avoid confusion with the bit. ##END_OF_RECORD A A an international symbol for year, taken from the Latin word annus. Although English-speaking countries continue to use the traditional symbol yr for most purposes, scientists use the a symbol in papers and textbooks. The symbol is often seen in combinations such as Ma (million years) or Ga (billion years). A a symbol for international standard paper sizes, followed by the size number, as in A4 for a standard business-letter sheet. A table of sizes is provided. A* see "angstrom star" below. Aas see "as" below. Ab- a prefix indicating that an electrical unit is part of the CGS absolute electromagnetic system. These units are also indicated by the notation emu (as in "volt emu"). Although these units are defined naturally as part of the CGS system, all of them except the abampere and abcoulomb are either much too large or much too small for most applications. They have been replaced almost completely by the corresponding SI units. Following are the SI equivalents for each of the "ab" units: Electric current: 1 abampere = 10 amperes Electric charge: 1 abcoulomb = 10 coulombs Capacitance: 1 abfarad = 109 farads = 1 gigafarad Inductance: 1 abhenry = 10-9 henry = 1 nanohenry Resistance: 1 abohm = 10-9 ohm = 1 nanohm Conductance: 1 abmho = 109 siemens Magnetic flux density: 1 abtesla = 10-4 tesla = 1 gauss Potential: 1 abvolt = 10-8 volt = 10 nanovolts Power: 1 abwatt = 10-7 watt = 0.1 microwatt Magnetic flux: 1 abweber = 10-8 weber = 1 maxwell. Abv, abw symbols for alcohol by volume and alcohol by weight. 1% abv = 1% v/v and 1% abw = 1% w/v. Academic year a unit of time in U.S. schools, generally equal to 9 months. Acfm abbreviation for "actual cubic feet per minute." See under sccm. Acoustic ohm any one of several units measuring sound resistance. These units got their name by analogy with electric resistance, which is measured in ohms. The sound resistance across a surface in a given medium is defined to be the pressure of the sound wave at the surface divided by the "volume velocity," that is, the rate (volume per time) at which the medium is pushed perpendicular to the surface. Unfortunately, the result is often stated in "acoustic ohms" no matter what units are used to measure pressure and time. In the CGS system, the acoustic ohm equals 1 microbar second per cubic centimeter (µbar·s/cm3), which is the same as 1 dyn·s/cm5. In the MKS system, the acoustic ohm is equal to the SI unit, 1 pascal second per cubic meter (Pa·s/m3). The CGS acoustic ohm equals 105 MKS acoustic ohms. See also rayl. Acre (ac or A) a unit of area used for measuring real estate in English-speaking countries. "Acre" is an Old English word meaning a field. The acre was originally defined as the area that could be plowed in a day by a yoke of oxen. It was in use in England at least as early as the eighth century, and by the end of the ninth century it was generally understood to be the area of a field one furlong (40 rods or 10 chains) long by 4 rods (or 1 chain) wide. Thus an acre is 10 square chains, 160 square rods, 43 560 square feet or 4840 square yards. There are exactly 640 acres in a square mile. In metric countries the unit corresponding to the acre is the hectare, which is 10,000 square meters (the area of a square 100 meters on each side). One acre is equal to 0.404 687 3 hectare. Among traditional European land area units, the acre is typical in being defined as a day's work but unusual in not being visualized as the area of a square. Similar units include the French journal, north German and Dutch morgen, south German and Swiss juchart, Austrian joch, and Czech jitro. Acre foot (ac ft or af) a unit of volume used to measure the capacity of reservoirs. One acre foot is a volume one foot deep covering an area of one acre. Thus an acre foot contains 43 560 cubic feet or about 1233.482 cubic meters (0.123 348 hectare meter). The symbol af is widely used in reservoir management in the U.S., often in combinations such as kaf (1000 acre feet) or maf (million acre feet; this symbol should be Maf). Acre inch (ac in) a unit of volume equal (of course) to 1/12 acre foot. An acre inch contains 3630 cubic feet, or about 102.7902 cubic meters. AD abbreviation for the Latin anno domini, "year of the Lord," the traditional designation for years of the common or Christian era. This abbreviation is often replaced by CE (common era), especially in countries where Christianity is not a dominant religion. -ad a suffix added to a number to create a unit of quantity equal to that number: for example, a 24ad is a unit of quantity equal to 24. Units of quantity equal to 1 through 8 are known, respectively, as the monad, dyad, triad, tetrad, pentad, hexad, heptad, and octad, terms coined by adding -ad to the Greek numbers 1-8. Admiralty mile see nautical mile. Aeon a unit of time equal to one billion years (1 Ga). Proposed in 1957 for use in geology, the aeon is not approved by the SI and hasn't found much favor. AFUE an abbreviation for annual fuel utilization efficiency, a measure of the efficiency of a gas furnace. The rating is designed to represent the percentage of the fuel energy actually delivered as heat energy, averaged over the course of a typical heating season. The actual calculation is quite complex, taking many properties of the furnace into account. Older furnaces have ratings of 60% AFUE or even lower; the newest high-efficiency furnaces are rated in the 90%-95% AFUE range. The U.S. Department of Energy requires new furnaces to operate at 78% AFUE or better. Agate, agate line a traditional unit of distance used in printing and advertising. The agate is equal to 1/14 inch (1.814 millimeters). Its name comes from the traditional type size called agate, which set 14 lines to the inch (very small print!). AH abbreviation for the Latin anno hegirae, traditionally used in the West to designate years of the Islamic calendar. The Islamic calendar is lunar, counting twelve lunar months to the year (see month [2]). Thus its years are shorter than ordinary solar years. Years are counted from the day of the Hijri (Hegira in Latin), the flight of Mohammed from Mecca to Medina, which is fixed at 16 July 622 CE in the Julian calendar. The Islamic year AH 1422 began at sunset of 25 March 2001 in our current (Gregorian) calendar. Air watt an engineering unit used to express the effective cleaning power of a vacuum cleaner or central vacuum system. The air watt is practically the same as the ordinary watt. Measurements of vacuum power, however, are computed from English units using the following formula established by the American Society for Testing and Materials (ASTM): power in air watts equals 8.5·F·S, where F is the air flow in the system in cubic feet per minute (CFM) and S is the suction pressure in inches of water column (in WC). This definition makes the air watt equal to 0.9992 watt. Ale gallon a traditional unit of liquid volume in Britain and the U.S., the ale gallon was equal to 282 cubic inches (4.6212 liters) or about 1.2208 U.S.liquid gallon (1.0165 British imperial gallon). Standardized in the sixteenth century under Queen Elizabeth I, the ale gallon remained in use well into the nineeenth century but is obsolete today. It is also called the beer and ale gallon. Almude a traditional unit of volume in Spain and Portugal. The two countries used the name for units of quite different sizes. The Spanish almude is comparable to the British gallon; it holds about 4.625 liters, equivalent to 1.017 imperial gallon or 1.222 U.S. liquid gallon. The Portuguese almude is much larger; it holds about 16.7 liters, which is 3.67 imperial gallons or 4.41 U.S. liquid gallons. Aln or alen a traditional Scandinavian unit of distance very similar to the north German elle: roughly 2 feet or 60 centimeters. The Danish alen, also used in Norway, was equal to 62.77 centimeters (24.71 inches). The Swedish aln was 2 fot or 59.38 centimeters (23.38 inches). Alpha TE an abbreviation for "alpha tocopherol equivalent," a measure of vitamin E used in nutrition. Vitamin E is actually a group of related chemical compounds called tocopherols. The activity, or potency, of vitamin E in a food or food supplement is measured by the quantity (in milligrams) of alpha tocopherol (the most active of the forms of the vitamin) which would be equivalent to the compounds present in the food or supplement. One milligram alpha TE is equal to 1.5 international units (IU). Alt h traditional abbreviation in pharmacy for alternis horis, every other hour, a unit of frequency sometimes used in medical prescriptions. AM or am [1] abbreviation for the Latin ante meridiem, "before noon," used after a time to indicate that the time is before 12:00 (noon). AM [2] abbreviation for the Latin anno mundi, "year of the world." This abbreviation is traditionally used to designate years in the Jewish calendar, which counts years from the creation of the world as described in the Hebrew scriptures. The Jewish calendar is lunisolar; its years correspond to ordinary solar years (see month [2]). The first day of AM 1 is equivalent to 6 October 3761 BCE in the Julian calendar, and AM 5762 began at sunset of 17 September 2001 in our current (Gregorian) calendar. Amagat units used by physicists to express the relative volume and density of gases. The amagat volume unit is about 22.414 liters per mole (L/mol) or 0.022 414 m3/mol, the volume occupied by a gas at standard temperature (0.01°C) and standard pressure (1 atmosphere). The amagat density unit represents the corresponding relative density, which is equal to one kilomole per standard volume, or 44.615 moles per cubic meter (mol/m3), or 0.044 615 mole per liter (mol/L), again provided the measurement is made at standard temperature and pressure. In general, the ideal gas law shows that the relative density is equal to P/RT, where P is the pressure on the gas, T is the absolute temperature (in kelvins) and R is the universal gas constant, R = 8.314 joules per mole per kelvin. The unit honors the Dutch physicist E. H. Amagat (1841-1915), whose work included the study of gases under pressure. Amber an old English unit of volume, used for both liquids and dry goods. The amber was equal to about 4 bushels or roughly 140 liters. American run see run. Ampere (A or amp) the SI base unit of electric current, named for the French physicist André-Marie Ampère (1775-1836), one of the pioneers in studying electricity. The official definition of the ampere goes like this: suppose we have two parallel conductors, infinitely long and having negligible cross section. Place these conductors one meter apart in a perfect vacuum. One ampere is the current which, if it's flowing in these conductors, creates between them a force of 0.2 micronewtons per meter of length. (You're welcome to object that no one can make an infinitely long conductor, nor a perfect vacuum. But scientists can use the idealized definition to construct appropriate real-world equipment in their laboratories.) The other electrical units are all defined in terms of the ampere. For example, one ampere represents a current flow of one coulomb of charge per second. One ampere of current results from a potential distribution of one volt per ohm of resistance, or from a power production rate of one watt per volt of potential. The unit is known informally as the amp, but A is its official symbol. Ampere hour (A·h or amp hr ) a commercial unit of electric charge often used to state the capacity of a battery. One ampere hour is the charge accumulated by a steady flow of one ampere for one hour. This is equivalent to exactly 3600 coulombs. Ampere per meter (A/m) or ampere-turn per meter the SI unit of magnetic field strength. One ampere per meter is equal to pi/250 oersteds (12.566 371 millioersteds) in CGS units. The ampere per meter is also the SI unit of "magnetization" in the sense of magnetic dipole moment per unit volume; in this context 1 A/m = 0.001 emu per cubic centimeter. Ampere turn (At) the MKS unit of "magnetomotive force." Electric current passing through a coil of wire generates a magnetic field. This field-generating ability is called magnetomotive force; it is equal to the product of the current, in amperes, and the number of turns of wire in the coil. One ampere turn equals 4pi/10 = 1.256 637 gilberts (Gb). Amphora a historic unit of volume. An amphora is the volume of an urn or jar of the same name. These urns were tall, with handles near the top on both sides (the word amphora comes from two Greek words meaning "on both sides" and "carry"). Amphoras were the containers of choice for shipping wine and many other commodities in the ancient world. Archaeologists report that the Greek amphora held about 38.8 liters (10.25 U.S. liquid gallons, or 8.54 British imperial gallons). The Roman amphora was smaller, about 25.5 liters (6.74 U.S. gallons or 5.61 British imperial gallons). Anchor another name for the anker (see below). Angstrom (Å or A) a metric unit of length, equal to 0.1 nanometer or 10-10 meter. Angstroms are used most often to measure the wave length of light waves. There is a technique called spectroscopy for identifying chemical substances by the wave lengths of light which they absorb (or emit, depending on the circumstances). Light shining through or from the substance is passed through a prism, which separates the various wave lengths to form a colorful spectrum. The spectrum often includes bright bands corresponding to wave lengths at which the substance emits light, or dark bands if the substance is absorbing light. The pattern of bands identifies the substance in somewhat the same way a fingerprint identifies a person. One of the pioneers of spectroscopy was the Swedish physicist Anders Jon Ångström (1814-1874), and in his honor the wave lengths of light waves are customarily stated in angstroms. Although English speakers usually pronounce the word as if it were English, the Swedish pronunciation is closer to "ong-strerm." Angstrom star (Å* or A*) a unit used to measure the wavelength of X-rays. Because it's easier to measure the ratio between two X-ray wavelengths than it is to measure the wavelengths themselves, the wavelengths are usually stated as multiples of a standard wavelength. The X unit and the angstrom star are the units used for this purpose. Å* was defined by J.A. Bearden in 1965 to provide a unit approximately equal to the angstrom (10-10 meter or 0.1 nanometer). Later measurements have shown that in fact A* is equal to approximately 1.000 0015 x 10-10 meter or 100.000 15 picometers. Angular mil see mil [2]. Animal unit (AU) a unit of feed consumption used in U.S. dairying and ranching. One animal unit is the feed or grazing requirement of a mature cow weighing 1000 pounds (453.59 kilograms). Total feed requirements are often figured by the animal unit month (AUM), the feed required to sustain one animal unit of livestock for one month. Anker or anchor a small wine barrel used in Britain and elsewhere in northern Europe. In England an anker usually held 10 wine (U.S. liquid) gallons (37.85 liters); the Scottish anker held 20 Scots pints (about 34 liters). The word anker is of Dutch origin. Annual percentage rate (% APR) a unit used in the U.S. for stating interest rates and rates of return on investment. By federal regulation, these rates can be stated however a financial institution wishes, but they must be stated also in % APR so that consumers can compare rates of different loans and investment opportunities. Mathematically, the natural rate r of return on money is the "instantaneous" rate, the rate that allows for compounding of interest continuously. The APR is the percentage growth rate a of the money over a period of one year, as if interest were compounded annually. The two rates are related by the formulas a = er - 1 and r = ln (1 + a), where er is the natural exponential function and ln is the natural logarithmic function. Apgar score a numerical measure of the health of a newborn baby. One minute after birth (and at regular intervals thereafter through the first moments of life) newborns are rated 0, 1, or 2 on five indicators of health (respiratory effort, heart rate, skin color, muscle tone, and reflexive response to smell). Possible scores therefore range from 0 to 10. The unit is named for its inventor, the American anesthetist Virginia Apgar (1909-1974). Apostilb (asb) an MKS unit of luminance, representing the brightness of a surface uniformly radiating 1 lumen per square meter. This is the brightness produced by 1/pi candela or 1 lux of light. Since the apostilb basically measures the same situation as the lux, it is rarely used. The name of the unit is pseudo-Greek for luminance; it combines the ancient Greek stilbein, to "glitter" or "shine," with the prefix apo-, "away from." Apothecary weights a version of the troy weight system formerly used by apothecaries (pharmacists). The troy pound, equal to 373.242 grams or 13.165 72 avoirdupois ounces, is always divided into 12 ounces, each ounce being equal to 480 grains. In the apothecary system, the ounce is divided into 8 drams (60 grains) each containing 3 scruples (20 grains). There is a parallel system of liquid volume measure in which the fluid ounce is divided into 8 fluid drams (or fluidrams) each containing 3 fluid scruples. The apothecary system continued in use into the early twentieth century, but it has been replaced everywhere in pharmacy by the use of metric units. Arcminute (' or min) a unit of angular measure equal to 60 arcseconds and to 1/60 degree. There are 21 600 arcminutes in a circle. Arcsecond (" or as or sec or s) a unit of angular measure equal to 1/60 arcminute. One arcsecond is a very small angle indeed: there are 1 296 000 seconds in a circle. The SI defines s as the symbol for the time unit and recommends " as the symbol for the arcsecond. The symbol as has become common in astronomy, where very small angles are stated in milliarcseconds (mas). Are (a) a unit of area equal to 100 square meters. The word is pronounced the same as "air." Being the area of a square 10 meters on each side, the are is a little large for measuring areas indoors and a little small for measuring areas outdoors. As a result, the are is not used as often as its multiple, the hectare (ha). One are is approximately 1076.3910 square feet, 119.6000 square yards, or 0.02471 acre. Arm's-length in English the term "arm's-length" is used figuretively to mean "a distance discouraging familiarity or conflict." In many cultures, however, the length of a human arm is standardized as a unit of distance equal to about 70 centimeters or 28 inches. Examples include the Italian braccio, the Russian sadzhen, and the Turkish pik. Arpent [1] a traditional unit of distance in French-speaking countries. The arpent equals 30 toises or 10 perches; this is about 191.8 feet or 58.47 meters. The unit was used to measure land; in fact, arpentage is the French word for surveying. In Canada the arpent has an official definition of 191.835 English feet (58.471 308 meters). Arpent [2] a traditional unit of area in French North America (Québec and Louisiana), equal to one square arpent [1]. The arpent of area equals 900 square toises, 100 (square) perches, approximately 0.8445 acre or 0.3419 hectare. By the official Canadian definition, the arpent of area contains 36 800.667 23 English square feet or about 0.844 827 acre (0.341 889 hectare). Arratel or artel versions of a traditional Arab unit of weight; see rotl. The spelling "arratel" is used in Portugal as an alternate name for the libra. Arroba (@) a traditional unit of weight in Spain and Portugal, equal to 1/4 quintal. However, the Spanish and Portuguese quintals are of different sizes. In Spain, the arroba equals 25.36 pounds (11.50 kilograms); arrobas of very similar sizes were established in the Spanish speaking countries of Latin America. In Portugal and Brazil, the arroba equals 32.38 pounds (14.69 kilograms). The arroba has also been used as a metric unit equal to exactly 15 kilograms. The name of the unit comes from ar rub', Arabic for "the quarter." The @ sign has been used in Spanish as a symbol for the arroba since the sixteenth century at least. Arshin a traditional Russian unit of distance. Peter the Great standardized the arshin at exactly 28 English inches, or 71.12 centimeters, early in the 1700s. The arshin was also used in several other countries adjacent to Russia. The arshin is also used as a unit of area equal to one square arshin; this would be equal to 5.4445 square feet or 0.5058 square meter. Artaba a historic unit of volume, used for both liquid and dry measurement throughout the Middle East. In ancient times the artaba varied in size between about 35 and 55 liters. In recent centuries the Arab artaba, equal to about 66 liters, was a common unit in both Arab and non-Arab parts of the area. As a symbol used in astronomy for the arcsecond (see above). As, ass, or aas a traditional unit of mass for gold and silver, used in most of northern Europe but not in England. The unit varied in size from about 48 to 58 milligrams (0.75 to 0.90 English grain). ASA number for many years, the initials of the American Standards Association appeared on film packages in the United States as a measure of the speed of the photographic emulsion (the stuff on the film that "develops" to form the picture. The scale is arbitrary, but the important thing to know is that the speed at which the image registers on the film is proportional to the ASA number. Thus ASA 400 film registers an image twice as fast as ASA 200 film and four times as fast as ASA 100 film. The ASA number is now combined with the European DIN rating as a composite ISO rating. For example, ASA 400 film is now marked ISO 400/27°, because 27 is the DIN rating corresponding to ASA 400. Assay ton (AT) a specialized unit of mass used by minerologists in assaying (testing) ores for the presence of gold, silver, platinum, uranium, or other valuable metals. One assay ton equals 29.167 grams: just a little over an an ounce! The assay ton is actually a sample size. Since there are 29 167 troy ounces in a short ton (2000 lb), the number of milligrams of a precious metal in a sample of one assay ton is numerically equal to the number of troy ounces of that metal in one ton of raw ore. In Britain, the assay ton is based on the long ton and thus equals 32.667 grams. Astronomical unit (ua or au) a unit of distance used by astronomers to measure distances in the Solar System. One astronomical unit equals the average distance from the center of the Earth to the center of the Sun. The currently accepted value, adopted in 1996, is 149 597 870 691 meters (1.495 978 706 91 x 108 kilometers or about 92 955 807 miles), with an uncertainty of about 30 meters. The astronomical unit is a convenient yardstick for measuring the distances between objects in the Solar System. Astronomers find it particularly convenient to use astronomical units in solving the equations of planetary motion. Because these equations are the same regardless of the unit used for distance, the predictions they generate will remain correct even if future astronomers determine a slightly different length for the distance between the Earth and Sun. The official symbol for the unit is ua, but the symbol au is common in English-speaking countries. This unit is accepted for use with SI units. Atmo-meter (atmo-m) a unit used in atmospheric physics to compare the "depth" or total volume of atmospheres, or components of atmospheres. The depth (in atmo-meters) is equal to the depth (in meters) the atmosphere, or one gas component of the atmosphere, would have if it formed a uniform layer at standard temperature (0 °C) and pressure (1 atmosphere). One atmo-meter represents 2.686 99 x 1025 molecules of gas per square meter of planetary surface. At symbol for the technical atmosphere. Atmosphere (atm or atmos) a unit of pressure designed to equal the average pressure of the Earth's atmosphere at sea level. In other pressure units, one atmosphere equals exactly 1013.25 millibars (mb), 101.325 kilopascals (kPa), approximately 29.92 inches of mercury (in Hg), or 14.6959 pounds of force per square inch (lb/in2). This is the standard atmosphere; it equals 1.0332 technical atmosphere. Atom a medieval unit of time. The Greek word atomos means something "uncuttable," thus indivisible. In medieval times the Latin form atomus was also used to mean "a twinkling of the eye," the smallest amount of time imaginable. This was sometimes defined in a precise way equivalent to exactly 1/376 minute or about 160 milliseconds. Atomic mass unit (u or amu) the unit of mass used by chemists and physicists for measuring the masses of atoms and molecules. Early in the nineteenth century, scientists discovered that each chemical element is composed of atoms, and that each chemical compound is composed of molecules in which atoms are combined in a fixed way. No one knew then just how small atoms and molecules really are, but as long as the relative weights of the different atoms were known, the outcome of chemical reactions could be predicted. These relative masses were determined by careful study of various reactions. The general idea was that atoms of hydrogen, known to be the lightest element, should have a mass of 1 amu, and all the other atoms should have masses which are whole-number multiples of this (then unknown) mass of the hydrogen atom. For a long time, physicists and chemists disagreed on the details of this definition. In 1960 they agreed on the definition of the unified atomic mass unit as 1/12 the mass of the most common atoms of carbon, known as carbon-12 atoms. (Most elements are mixtures of atoms which have different masses because they contain different numbers of neutrons; these varieties are called isotopes.) Careful experiments have measured the size of this unit; the currently accepted value (1998) is 1.660 538 73 x 10-24 grams. (This number equals 1 divided by Avogadro's number; see mole.) In addition, 1 amu equals approximately 931.494 MeV (see electron volt). In biochemistry, the atomic mass unit is called the dalton (Da). Atomic number The atomic number was originally defined (about 1865) simply as an index describing the position of an element in the periodic table. Not until 1913 was it known that the atomic number is actually a unit of measurement, equal to the number of electrons surrounding a neutral (uncharged) atom, and also to the number of protons in the nucleus. Atto- (a-) a metric prefix denoting 10-18 (one quintillionth). For example, one electron volt equals 0.1602 attojoule. The root of the prefix is atten, the word for 18 in Danish and Norwegian. AUC abbreviation for the Latin phrase ab urbe condita, "from the founding of the city." The Romans counted years from the legendary founding of Rome by Romulus and Remus, an event placed in 753 BC (or BCE) in our present calendar. The Christian Church continued to count years AUC for centuries after the fall of Rome. Our year 2000 is 2753 AUC. Aught (/0) a unit used by jewelers and craftspeople to measure the size of small beads (often called seed beads). The measurement scale is inverted: larger numbers of aughts correspond to smaller beads. Beads of size 11/0, a common size, average a little less than 2 millimeters in diameter, and other sizes are more-or-less inversely proportional. The measure may have originated as the number of beads that could comfortably be strung on one inch of cord; with present sizes a string of n beads of size n/0 occupies about 0.8 inch (20 mm). The word aught, meaning zero, is a fairly recent corruption of the old English word naught, meaning nothing; apparently the phrase a naught, meaning a zero, came to be misspelled as an aught. Aume an old English wine measure equal to about 40 gallons (roughly 150 liters). The aume is the English version of a German unit, the ohm [2]. Aune a traditional French unit of distance, varying from region to region but similar in length to the English ell. Av or avdp abbreviations for avoirdupois (see below). Avogadro's number, Avogadro constant (NA) a unit of relative quantity equal to the number of atoms or molecules per mole of a substance. The currently accepted value is 6.022 141 99 x 1023 per mole with an uncertainty of 0.000 000 47 x 1023 per mole (about 80 parts per billion). The atomic mass unit (see above), in grams, is equal to one divided by this number. The unit is named for the Italian chemist and physicist Amadeo Avogadro (1776-1856). Avogadro was the first to conclude from Dalton's atomic theory that equal volumes of gases (at the same temperature and pressure) must contain equal number of molecules. Avoirdupois weights the common traditional system of weights in all the English-speaking countries. Until quite recently, almost all weights were stated in avoirdupois units, with only precious metals and pharmaceuticals being measured by troy weights. The name of the system comes from the French phrase avoir du poids, "goods of weight," indicating simply that the goods were being sold by weight rather than by volume or by the piece. The system is based on the avoirdupois pound [1] of 7000 grains. The pound is divided into 16 ounces [1], each divided further into 16 drams [1]. The avoirdupois system was introduced in England around 1300, replacing an older commercial system based on a "mercantile pound" (libra mercatoria) of 7200 grains divided into exactly 15 troy ounces [2]. Scholars believe the avoirdupois pound was invented by wool merchants and modeled on a pound of 16 ounces used in Florence, Italy, which was an important buyer of English wool at the time. The avoirdupois weights quickly became the standard weights of trade and commerce. They continue to be used for most items of retail trade in the United States, and they remain in some use in Britain despite the introduction of metric units there. AWG abbreviation for American Wire Gauge. See gauge [3] and the table of wire gauge equivalents. ##END_OF_RECORD C C a symbol for the velocity (or "speed") of light. One of the fundamental principles of physics is that light always travels at the same velocity in a vacuum, exactly 299 792 458 meters per second or about 670 617 300 miles per hour. Another fundamental principle is that no object can travel faster than light. At velocities that are large fractions of the speed of light, the theory of relativity predicts a variety of strange physical effects. In calculations involving relativity, speeds are customarily expressed as fractions of the speed of light, such as 0.95 c. C [1] the Roman numeral 100, sometimes used as a unit of quantity or as a prefix meaning 100, as in Cwt (hundredweight) or CCF (100 cubic feet). C [2] a symbol for international standard paper sizes, followed by the size number, as in C4. The C series of sizes is used primarily for envelopes. A table of sizes is provided. C [3] a unit of relative current for batteries. For a particular battery, a current of 1C is a current in amperes numerically equal to the rated capacity of the battery in ampere hours. In other words, a 1C current will completely charge or discharge the battery in one hour. CA, CCA abbreviations for "cranking amps" and "cold cranking amps," respectively. These units are often seen on motor vehicle batteries in the U.S. The amps involved are ordinary amperes of electric current. "Cranking amps" measure the current supplied by the battery when starting the vehicle at a temperature of 32 °F (0 °C), while "cold cranking amps" measure the current supplied at 0 °F (-17.8 °C). Caballeria a traditional unit of land area in Spanish speaking countries. In Spain and Peru the caballeria is equal to 60 fanegas, which is roughly 40 hectares (100 acres). In Central America it equals 60 manzanas, which is roughly 45 hectares (110 acres). In Cuba, the caballeria is a smaller unit equal to 33.162 acres or 13.420 hectares. Cable a unit of distance formerly used at sea. The traditional mariner's cable was 120 fathoms long. This is equal to 720 feet, or 0.1185 nautical mile, or about 219.4 meters. A different version of the cable, used in the British Navy, was equal to exactly 0.1 nautical (Admiralty) mile, which is 608 feet or about 185.3 meters. Cade an old name for a cask, sometimes used as a unit of measure for fish. A cade of herring, for example, was 720 fish. Calendar year (cal yr) a civil unit of time, equal to 365 days or (in leap years) 366 days. See year [2]. In archaeology, climatology, and other sciences studying the earth over the last 40 000 years or so, a careful distinction must be made between calendar years (cal yr) and radiocarbon years (14C yr). Caliber (cal) [1] a unit used to express the diameter of the bore of a gun. (The bore is the inside diameter of the gun barrel.) Traditionally, the diameter was stated in inches, so that ".22 caliber" referred to a pistol having a bore of 0.22 inches (5.588 mm). This usage is declining, because bore diameters of many guns are now stated directly in millimeters. "Caliber" is the American spelling; elsewhere the unit is often spelled "calibre." Caliber (cal) [2] a measure of the relative length of a gun barrel, defined as the length divided by the diameter of the bore. Thus a 50-caliber gun on a warship has a barrel 50 times longer than its bore. Confining the shell within the barrel for a longer time increases the velocity, so guns with a higher caliber usually have a longer range. Caliper the thickness of a sheet of paper or card stock. Traditionally measured in points [7] (thousandths of an inch), caliper is now measured in microns (micrometers). The word "caliper" is sometimes used in place of the proper unit, as in ".004 caliper" (4 points) or "120 caliper" (120 microns). Callipic cycle a unit of time equal to 76 years or 4 Metonic cycles, formerly used in astronomy in predicting the phases of the Moon. After the passage of one Callipic cycle, the phases of the Moon repeat essentially on the same calendar dates as in the preceding cycle. The cycle is named for the Greek astronomer Callipus, who discovered it in 330 BCE. Calorie (cal) the CGS unit of heat energy. This calorie (also called a gram calorie or small calorie) is the amount of heat required at a pressure of one atmosphere to raise the temperature of one gram of water by one degree Celsius. Unfortunately, this varies with the temperature of the water, so it is necessary to specify which degree Celsius is meant. A traditional choice was the degree from 14.5°C to 15.5°C; raising the temperature of water through this range requires 4.1858 joules, a quantity called the 15° calorie. Another choice produces the thermochemical calorie, equal to exactly 4.184 joules. More common today is the international steam table calorie, or IT calorie for short, defined by an international conference in 1956 to equal exactly 4.1868 joules or about 0.003 968 32 British thermal units (Btu). The name of the unit comes from the Latin calor, heat. Calorie (kcal or Cal) a common name for the MKS unit of heat energy. This unit is properly called the kilocalorie; it is also called the kilogram calorie or large calorie. It is often (but certainly not always!) distinguished from the small calorie by capitalizing its name and symbol. The large calorie, or rather kilocalorie, is the amount of heat required at a pressure of one atmosphere to raise the temperature of one kilogram of water by one degree Celsius. Since this is 1000 times as much water as mentioned in the definition of the small calorie, the kilocalorie equals 1000 small calories, 4.1868 kilojoules, or 3.9683 Btu. These are the "calories" that joggers are trying to get rid of, the ones we gain by eating. The use of the same term "calorie" for two different-size units is endlessly confusing, but we seem to be stuck with it. Cana, canna, canne traditional units of distance in Spain, Italy, and southern France, respectively. The cana varied in size, but it was most often defined as 8 palmos, which makes it the Mediterranean version of the fathom, equal to roughly 2 meters (6.5 feet). In Italy a measuring stick is still called a canna metrica. The unit is sometimes translated "rod" in English, but "fathom" is the proper choice. Candela (cd) the SI base unit for measuring the intensity of light. Candela is the Latin word for "candle." The unit has a long and complicated history. Originally, it represented the intensity of an actual candle, assumed to be burning whale tallow at a specified rate in grains per hour. Later this definition was replaced with a definition in terms of the light produced by the filament of an incandescent light bulb. Still later a standard was adopted that defined the candela as the intensity of 1/600 000 square meter of a "black body" (a perfect radiator of energy) at the temperature of freezing platinum (2042 K) and a pressure of 1 atmosphere. This definition has also been discarded, and the candela is now defined to be the luminous intensity of a light source producing single-frequency light at a frequency of 540 terahertz (THz) with a power of 1/683 watt per steradian, or 18.3988 milliwatts over a complete sphere centered at the light source. The frequency of 540 THz corresponds to a wave length of approximately 555.17 nanometers (nm); normal human eyes are more sensitive to the yellow-green light of this wavelength than to any other. In order to produce 1 candela of single-frequency light of wavelength l, a lamp would have to radiate 1/(683V(l)) watts per steradian, where V(l) is the relative sensitivity of the eye at wavelength l. Values of V(l), defined by the International Commission on Illumination (CIE), are available online from the Color and Vision Research Laboratories of the University of California at San Diego and the University of Tübingen, Germany. Candle (cd) an older name of the candela (see above), or of the candlepower (see below). Candlepower (cp) a unit formerly used for measuring the light-radiating capacity of a lamp or other light source. One candlepower represents the radiating capacity of a light with the intensity of one "international candle," or about 0.981 candela as now defined. Since 1948 the candela has been the official SI unit of light intensity, and the term "candlepower" now means a measurement of light intensity in candelas, just as "voltage" means a measurement of electric potential in volts.. Candy a traditional weight unit of South Asia. The candy was quite variable, generally within the range 500 to 800 pounds (225 to 365 kilograms). In the international cotton trade, the candy was generally equal to exactly 7 (British) hundredweight, which is 784 pounds or 355.62 kilograms. Can sizes Food cans are identified by their nominal dimensions, diameter × height. (The "nominal" dimensions are somewhat larger than the actual dimensions, as is the case for lumber and some other products.) In the metric world the dimensions are 2- or 3-digit numbers representing dimensions in millimeters. In traditional U.S. nomenclature, the dimensions are stated as 3-digit numbers, with the first digit representing inches and the remaining two digits representing 16ths of an inch. A common can for fruits and vegetables, for example, is designated 83 × 116 in metric terminology, or 307 × 409 (3-7/16 × 4-9/16) in traditional terminology. If only one number is mentioned it is the diameter; thus a "404" can has a nominal diameter of 4-4/16 = 4.25 inches and a "65" can has a nominal diameter of 65 millimeters. Link: chart of standard can sizes with metric and traditional designations, from Dantraco Associates. Cantar an English spelling for the Arab form of the quintal. In recent years, the cantar has been interpreted as a metric unit equal to 50 kilograms (110.23 pounds); traditional cantars tended to be a few percent larger than this. Cape foot (cf) a traditional unit of distance in South Africa. The Cape foot equals 12.396 English inches, 1.0330 English foot, or 31.4858 centimeters. This unit is not the traditional Dutch foot, but it is similar in length to the "Rhine foot" of northern Germany. The Cape foot was widely used for land measurement and appears on many deeds in South Africa. Europeans often referred to South Africa as "The Cape," meaning the Cape of Good Hope. Cape rood a traditional unit of distance in South Africa, equal to 12 Cape feet or 12.396 English feet (3.7783 meters). See also rood. Carat (ct or c) [1] a unit of mass used for diamonds and other precious stones. Some say the word carat comes from the Arabic qirrat, meaning the seed of a coral tree, and some say it comes from the Greek keration, a carob bean. Probably both seeds were used as standards for weighing precious stones. Traditionally the carat was equal to 4 grains. The definition of the grain differed from one country to another, but typically it was about 50 milligrams and thus the carat was about 200 milligrams. In the U. S. and Britian, the diamond carat was formerly defined by law to be 3.2 troy grains, which is about 207 milligrams. Jewelers everywhere now use a metric carat defined in 1907 to be exactly 200 milligrams. Carat (ct or c) [2] in Britain, the spelling "carat" is also used for the unit of gold purity known in America as the karat. Carcel a former French unit for measuring the intensity of light. The unit was defined as the intensity of a standard Carcel lamp, which burnt colza oil in a precisely defined way. One carcel equals about 9.74 candelas (see above). Carga a traditional unit in Spanish and Portuguese speaking countries. The word means "load". It was often used as a unit of mass or weight equal to 3 quintals or as a unit of volume equal to the volume holding 3 quintals of the commodity being shipped. Carnegie unit a unit of academic credit used in college admissions decisions in the U.S. The unit was introduced by the Carnegie Foundation for the Advancement of Teaching in 1914 to provide colleges with a standard measure of students' course work in high schools. A Carnegie unit represents the equivalent of one academic year of study in a subject in a class meeting 4 or 5 times a week for 40 to 60 minutes per meeting, a minimum of 120 hours of total class time. Carreau a traditional unit of land area in Haiti equal to approximately 1.29 hectares (3.18 acres). The unit originated as the area of a square 100 pas (Haitian paces) on a side, with the pas being equal to 3.5 pieds (French feet). Foreigners are not allowed to own more than 1 carreau of urban land or 5 carreaux of rural land in Haiti. Carucate another name for the hide, an old English unit of land area. The name comes from a Latin word meaning "plowland." Case a conventional unit of sales for many items, varying with the item and over time. A case of wine, for example, is traditionally twelve 750-milliliter bottles. The word comes from the Latin capsa, a chest. Category the ranking of a hurricane on the Saffir-Simpson scale, used by the U.S. National Weather Service. A somewhat different scale of categories is used for tropical cyclones by the Australian Bureau of Meterology. Catty a weight unit of the colonial period in East and Southeast Asia, originating as the kati in Malaya. The catty varied a little from market to market. Typically it was equal to about 4/3 pound avoirdupois (604.79 grams), and it is still equal to that weight in Malaysia. In Thailand, the catty is used now as a metric unit equal to exactly 600 grams (1.3228 pounds). In China, the catty was identified with the jin, a traditional Chinese unit. Cawny or cawney a traditional unit of land area in southern India, equal to about 4/3 acre (0.54 hectare). The name is an English tranliteration of a Tamil word for the unit. Cb- abbreviation for "cubic," seen in combinations such as cbm (cubic meter) or cbft (cubic foot). The proper symbol for cubic meters is m3, not cbm. Cc [1] an alternate symbol for the cubic centimeter (cm3). This symbol is obsolete and should not be used; cm3 should be used in its place. The cubic centimeter is the same volume as the milliliter (mL or ml). Cc or cc [2] a symbol for the centesimal second (see below). CCF an abbreviation for 100 cubic feet. Local water and sewer utilities often sell water in CCF units; for this purpose one CCF equals about 748.05 gallons (U.S.) or about 2831.7 liters. Utilities sometimes sell natural gas in CCF units; for this purpose the CCF is really a unit of energy roughly equivalent to the therm (1 CCF of natural gas provides about 1.034 therm). CCS a unit of telecommunications traffic equal to 100 call-seconds. One CCS could represent a single call 100 seconds long, or 10 calls each 10 seconds long, etc. One CCS equals 1/36 erlang. CE [1] abbreviation for "common era." This abbreviation is a non-religious designation used in place of the traditional AD for years of the common or Christian era. Years of the common era are supposed to be counted from the birth of Jesus of Nazareth, founder of the Christian religion. However, the year-numbering system was not established until more than 500 years later. It is based on calculations of the priest and scholar Dionysius Exiguus placing Jesus's birth in the Roman year 753 AUC. Dionysius had incomplete information, and there is evidence that he picked this particular date to simplify the calculation of the date of Easter. According to the Biblical account, Jesus was born several years before the death of Herod the Great, who died, we now know, in 749 AUC (4 BC). Thus the calculation of the common era is off by 6 or 7 years at least. In the conventional use of the common era system, there is no year 0 and the year prior to 1 CE is designated 1 BCE (or 1 BC). In astronomy, however, it simplifies calculations to define the year 0 CE = 1 BC and to apply negative numbers to earlier years. Thus Herod died in -3 CE, and, in general, -n CE is the year more commonly called n + 1 BC. CE [2] French abbreviation for colonne d'eau, water column, seen in pressure measurements. See centimeter of water (below) or millimeter of water. Celsius see degree Celsius. The word "degree" is often omitted in informal statements of temperature, as in "we expect a high of about 23 Celsius today." Celsius heat unit (Chu) a unit of heat energy equal to the energy required to raise the temperature of one pound of water by 1°C at standard atmospheric pressure. 1 Chu is equal to exactly 1.8 Btu, approximately 453.59 IT calories (see above), or 1.8991 kilojoules. The unit is also called the centigrade heat unit. Cent [1] an old English unit of quantity, usually equal to 100 but sometimes 120 (the great or long hundred) or some other figure of similar size. The Latin number 100, centum, is also used in English works for this quantity. Cent [2] an informal name for 1/100 of almost any unit, for example, a centiliter (0.01 liter). Phrases such as "15 cents of an inch" were formerly common in English. Cent [3] a unit used in music when it is necessary to specify the ratio in frequency between two tones with great precision. There are 100 cents in a semitone, or 1200 in an octave. If two notes differ by 1 cent, the ratio between their frequencies is 21/1200 or approximately 1.000 5778. Cent [4] a unit used in nuclear engineering to describe the "reactivity" of a nuclear reactor, equal to 0.01 dollar. For a discussion of reactivity, see inhour. Cental (cH or ctl) an alternate name in Britain for the U.S. hundredweight, which is equal to exactly 100 pounds (the British hundredweight is 112 pounds). Introduced by British merchants around 1850, the name was apparently coined after the model of the quintal. The cental has sometimes been confused with the centner (see below). Centesimal minute (c), centesimal second (cc) units of angle measure sometimes used in surveying. In the centesimal system, the right angle is divided into 100 grads or gons. Each gon is divided into 100 centesimal minutes (or centigons) and each centesimal minute into 100 centesimal seconds. Thus the centesimal minute equals 0.009° or 0.54 arcminute, and the centesimal second equals 0.324 arcseconds. Centi- (c-) a metric prefix meaning one hundredth, or 0.01. The prefix comes from the Latin word centum for one hundred. Centiare (ca) a metric unit of area. The centiare equals 0.01 are, which is exactly 1 square meter (about 10.7639 square feet). Centibar (cbar or cb) a metric unit of pressure identical with the kilopascal (kPa). One centibar equals 0.01 bar, 7.5006 torr, or 0.1450 pounds per square inch (lbf/in2 or psi). The centibar is traditionally used in agriculture as a unit of soil water tension (the water pressure on the roots of plants) as measured by devices called tensiometers. Centigrade a temperature scale; see degree centigrade. Centigram (cg) a metric unit of mass equal to 10 milligrams or about 0.154 grain. Centihg an informal unit of pressure equal to 1 centimeter of mercury. This is equivalent to 10 millimeters of mercury, approximately 0.3937 inHg, 0.1933 lb/in2, 13.33 millibars, or 1333 pascals. The word is pronounced "sentig". Centiliter (cl or cL) a common metric unit of volume. One centiliter equals 10 cubic centimeters; this is about 0.610 24 cubic inch, 0.3318 U.S.fluid ounce or 0.3519 British fluid ounce. In the kitchen, a centiliter is roughly equal to 2 U.S. teaspoons (or 0.704 British tablespoonful). Centimeter (cm) [1] the basic unit of distance in the former CGS version of the metric system, equal to 0.01 meter. One centimeter is about 0.393 700 787 inch. Centimeter (cm) [2] an obsolete name for the statfarad (approximately 1.11 picofarad), the CGS electrostatic unit of capacitance. Centimeter (cm) [3] an obsolete name for the abhenry, the CGS electromagnetic unit of inductance. The abhenry is the same as the nanohenry. Centimeter of mercury (cmHg) a traditional unit of pressure equal to 10 mmHg, 1.333 22 kilopascal, or about 0.193 pounds per square inch. Centimeter of water (cmH2O, cm WC, cm CE, cm WS) a unit of pressure equal to the pressure exerted at the Earth's surface by a water column (WC) 1 centimeter high. This is about 98.067 pascals, 0.980 67 millibars, 0.3937 inch of water, or 2.04 pounds per square foot. The unit is used in respiratory medicine and elsewhere to measure air pressures. The French symbol is cm CE (colonne d'eau), and the German symbol is cm WS (Wassersäule). Centimorgan (cM) a unit of genetic separation used in genetics and biotechnology. If two locations on a chromosome have a 1% probability of being separated during recombination in a single generation, then the distance between those locations is one centimorgan. In humans, the centimorgan is approximately equal to one million base pairs. The unit honors the pioneering American geneticist Thomas Hunt Morgan (1866-1945), who received the 1933 Nobel Prize in Medicine for his discoveries concerning the role played by the chromosome in heredity. Centinewton (cN) a metric unit of force equal to 0.01 newton. This unit has some popularity in engineering as a substitute for the gram of force (gf), since it equals about 1.019 72 gf (about 0.0360 ounces of force in the English system). In the textile industry, the breaking strength of fibers is commonly expressed in centinewtons per tex. Centipoise (cP, cPs, or cPo) a common metric unit of dynamic viscosity equal to 0.01 poise or 1 millipascal second (mPa·s). The dynamic viscosity of water at 20 °C (68 °F) is about 1 centipoise. The correct symbol for the unit is cP, but cPs, cPo, and even cps are sometimes used. Centiradian a unit of angle measure equal to 0.01 radian or about 0.572958° (34' 22.65"). Centisecond (cs or csec) a unit of time equal to 0.01 second or 10 milliseconds. Centiseconds are frequently used in the study of human speech to measure precisely the length of sounds. Centistokes (cSt) a common metric unit of kinematic viscosity equal to 0.01 stokes or 1 mm2/s. The viscosity of lubricating oils and many other liquids is frequently stated in centistokes. Although the centistokes is not an SI unit it is likely to remain in use, since it provides a convenient and traditional name for an SI-appropriate quantity (1 mm2/s). The older symbol cS should not be used for this unit, since S is now the symbol for the siemens. Centner [1] the English name for a German weight or mass unit, the zentner, equal to 50 kilograms or about 110.231 pounds. The name centner should not be used for the cental (see above). Centner [2] a Russian weight or mass unit equal to 100 kilograms (approximately 220.4623 pounds). This centner, also used in Ukraine and the other former Soviet republics, is equal to the decitonne and to the metric quintal; it is twice the size of the centner [1] used in western Europe. Centrad an informal name for the centiradian (see above). Century [1] a unit of quantity equal to 100. In ancient Rome, a "century" was originally a company of about 100 soldiers led by an officer called a centurion. Century [2] a traditional unit of time equal to 100 years. In naming centuries, historians recall that there was no year 0 in the conventional year numbering system. Thus the First Century included the years 1-100 and the Twentieth Century includes the years 1901-2000. (As an example in the other direction, the Fifth Century BC includes the years 500-401 BC.) With this convention, 2001 is the first year of the Twenty-first Century. Centum the Latin number 100, sometimes used in works in English. Cetane number a measure of the ability of diesel fuel to reduce engine knocking. The cetane number plays the same role in diesel engine technology that the octane number plays in conventional automobile engine technology. It is the percentage by volume of cetane which must be added to methylnaphthalene to give the mixture the same resistance to knocking as the diesel fuel sample being tested. Cetane is the name of a hydrocarbon compound whose molecules contain 16 carbon atoms and 34 hydrogen atoms, the 16 carbons being arranged in a long chain. Adding one oxygen atom to cetane produces cetyl alcohol, a waxy compound found in whale oil. The words "cetyl" and "cetane" are both derived from the Latin word cetus for a whale. Cfm, cfs traditional abbreviations for cubic feet per minute and cubic feet per second, respectively. 1 cfm = 28.3169 liters per minute (L/min) and 1 cfs = 28.3169 liters per second (L/s). CFU abbreviation for "colony forming units," a count of the number of active bacterial cells in preparations of Lactobacillus acidophilus and other "friendly" organisms of the digestive system. Counts as high as one billion CFU per gram are not uncommon. Chain (ch) a unit of distance formerly used by surveyors. The traditional British surveyor's chain, also called Gunter's chain because it was introduced by the English mathematician Edmund Gunter (1581-1626) in 1620, is 4 rods long: that's equal to exactly 1/10 furlong, 22 yards, or 66 feet (20.1168 meters). The traditional length of a cricket pitch is 1 chain. Gunter's chain has the useful property that an acre is exactly 10 square chains. The chain was divided into 100 links. American surveyors sometimes used a longer chain of 100 feet, known as the engineer's chain or Ramsden's chain. (However, Gunter's chain is also used in the U.S.; in fact, it is an important unit in the Public Lands Survey System.) Chain number a size designation for roller chains, such as the drive chains of bicycles or motorcycles. These chains are traditionally designated by a three-digit number. The first digit specifies the pitch, the distance between pins, in eighths of an inch; the second and third digits specify the width of the rollers in 80ths of an inch. Thus a 425 chain has a pitch of 4/8 (= 0.5) inch and a roller width of 25/80 (= 0.3125) inch. Chalder or chaldron (chd) a traditional British unit of volume or weight used for dry commodities such as coal or lime. As a volume measure, the chaldron is equal to 36 bushels, or 288 British Imperial gallons; this is equivalent to 46.237 cubic feet or 1.3091 cubic meters. As a measure for coal, the chalder equals 1/8 keel or 53 hundredweight (5936 pounds or 2692.52 kilograms). The words "chalder," "chaldron," and "cauldron" are English spellings of the same old French word, which originally meant a large kettle. Champagne quart see quart [2]. Character (char) a unit of information used in computer science and telecommunications. One character is usually equal to 8 bits or one byte. There is, however, a newer coding system designed to include all the characters used in all the world's languages; this system, Unicode, assigns 16 bits or 2 bytes to each character code. Charka a traditional Russian unit of volume containing about 123.0 milliliters, 4.159 U.S. fluid ounces or 4.329 imperial fluid ounces. There are 6.25 charki in a boutylka (bottle) and 10 in a schtoff. The word charka means a cup or glass. Charrière, Charrière gauge (Ch) a unit of distance used for measuring the diameters of small tubes such as catheters, fiber optic bundles, etc. The gauge number is the diameter of the tube in units of 1/3 millimeter. In English-speaking countries, the scale is usually called the French gauge and the unit is simply called "French." Charrière was a nineteenth century French instrument manufacturer. Cheval vapeur (cv) French name for the metric horsepower. Ch'ih a unit of distance used in China during the colonial period. The ch'ih equals 10 t'sun, 35.814 centimeters, or 14.1 inches. There are 1800 ch'ih in a li. Chiliad [1] a unit of quantity equal to 1000. The word comes from the Greek numeral 1000, chilioi, which is also the origin of the metric prefix kilo-. Pronounced "killiad," the chiliad was once fairly common in learned writing, but it has nearly disppeared from use today. Chiliad [2] another name for a millennium (1000 years). Chilo- Italian spelling of the metric prefix kilo- (1000). Chin one of several spellings in English for the jin, a traditional Chinese weight unit. Chopine a traditional French unit of volume. The unit varied regionally, but by the 18th century it was more or less standardized as 23.475 cubic pouces (465.7 milliliters). The chopine is obsolete in France today, but the word survives (especially in Canada) as a French name for the English pint units. Choppin a traditional Scottish unit of volume equal to 2 mutchkins or 1/2 Scots pint. The choppin is equivalent to about 51.702 cubic inches, 0.8953 U.S. liquid quart, 1.491 British imperial pints, or 847.3 milliliters. Cicero a unit of distance used by typesetters and printers in continental Europe, equal to 12 Didot points. This is approximately 0.1780 inch or 4.52 millimeters. The cicero corresponds to the British and American pica. Presumably, this unit got its name because it was used in printing the works of classical authors such as the Roman stateman and orator Marcus Tullius Cicero (106-43 BCE). Cinque an old English word for the number 5, pronounced "sink" and derived from the French number 5, cinq. In English history, the original Cinque Ports were Sandwich, Dover, Hythe, Romney and Hastings. The word survives today as the name for a 5-spot showing in dice, or for a 5-card in card games. Circle (cir) the traditional unit of angle measure, divided into 2 pi radians or 360 degrees. Circular inch, circular mil informal units of area. A circular inch [mil] is the area of a circle one inch [mil] in diameter. A circle of diameter d has an area of pi·d2/4, so the circular inch is equal to approximately 0.785 398 square inches or 5.067 07 square centimeters, and the circular mil is equal to approximately 0.785 398 square mils or 506.707 square micrometers. City block see block. Civil year a year as measured by the conventional (Gregorian) calendar, equal to 365 days in most years but 366 days in a leap year. See year [2]. Clark degree see degree [4]. Clausius a unit of entropy. Entropy is a measure of the extent to which heat or energy in a physical system is not available for performing work. It is computed in units of energy per kelvin. One clausius is equal to 1 kilocalorie per kelvin (kcal/K) or 4.1868 kilojoules per kelvin (kJ/K). The unit honors the German physicist Rudolf Clausius (1822-1888), who introduced and named the concept of entropy in 1850. Click U.S. military slang for the kilometer (about 0.621 mile). Also spelled klick or klik. This unit became popular during the Vietnam War, but it was invented by U.S. troops in Germany during the 1950s. Occasionally it was used as a non-metric unit equal to 1000 yards (0.9144 kilometer). Clo a unit of thermal insulation used for clothing. One clo is intended to represent the insulation required to keep a resting person warm in an indoor room at 70 °F (21.1°C). The rate of a person's heat loss is measured in watts per square meter of skin area per kelvin of temperature difference across the clothing; the value of insulation is measured by the reciprocal of this rate, in square meter kelvins per watt (m2K/W). One clo is equal to 0.155 m2K/W or 1.550 togs. Clove an old English unit of weight. A clove is usually considered equal to 1/2 stone or 1/16 hundredweight; that's 7 pounds (3.175 kilograms) by the modern definition of the stone, but in the past the clove varied from 6.25 to 8 pounds. Clusec a unit of power used to express the performance or leakage of vacuum pumps. One clusec represents a flow of 10 milliliters per second at a pressure of one micrometer (or micron) of mercury. This is equivalent to 0.01 lusec or 1.333 microwatts. The name of the unit is an acronym for "centi-lusec." Coffee measure a flat-bottomed scoop or spoon used to measure coffee in U.S. homes. The coffee measure holds 2 U.S. tablespoons (about 29.57 milliliters). Coffeespoon a unit of volume formerly used in U.S. food recipes. A coffeespoon is 1/2 teaspoon, 1/12 fluid ounce, or about 2.5 milliliters. Collothun an ancient Persian unit of liquid volume, equal to 1/8 artaba or (in recent centuries) about 8.25 liters. Color units several systems have been devised to measure colors. The subject was mostly academic until recently, but now computers require precise methods for describing the colors to be displayed or printed. Computer monitors use the RGB system, which specifies colors with three variables measuring the intensity of the three primary colors red, blue, and green in the color. Each variable is specified by one byte and therefore takes values in the range 0 to 255. If all three are 0, the resulting color is black; if all three are 255 the resulting color is white. The RGB settings for the Carolina blue background of this page are R=153, G=204, and B=255. Since it is difficult to estimate the relative amounts of red, green, and blue needed to create a particular color, many graphics design programs use the HSV color system, which describes colors using three variables called hue, saturation, and value. Once again, all three variables are assigned values from 0 to 255. Hue, which is what we call "color" in ordinary language, is described on a circular scale. Hue values begin with red at 0 and run through yellow, green, blue, and purple before returning to red at 255. Saturation is the purity of the color, the extent to which it is not watered down with gray. The pure color has saturation 255. As saturation is reduced, the color becomes grayer, until at saturation 0 the color is replaced by a neutral gray of the same intensity as the original color. The value (or intensity) of the color is its brightness. The pure or most natural form of the color has value in the middle of the scale, at 127. As value is increased the color becomes brighter, becoming pure white at value 255. In the opposite direction, the color becomes less bright, becoming black at value 0. This page's background has hue 140, saturation 240, and value 192 in the HSV system. See also: Lovibond color units (used for beer and honey). Colpa, colp, or collop a traditional Irish unit. The colpa was originally a unit of livestock equal to one cow or horse or to 6 sheep. Later it was used as a unit of pastureland equal to the pasturage supporting one colpa of livestock. This varied according to the quality of the land, but it was roughly equal to the Irish acre (0.6555 hectare). "Collop" is an English version of the Irish word "colpa." Column inch (col in) a unit of relative area used in journalism. A column inch is an area one column wide and one inch deep. The width of a column varies; a standard size in the U.S. is 2-1/16 inch. At this width a column inch is 2.0625 square inches or about 13.31 square centimeters. Commercial acre a unit of area used in U.S. real estate, equal to exactly 36 000 square feet or about 0.826 45 ordinary acre (0.334 45 hectare). The unit was invented by commercial realtors to express the approximate portion of an acre of subdivided land that remains to be sold in the retail market after portions are set aside (dedicated) for necessary streets and other utilities. Buyer beware! It is legal to sell land by the commercial acre in many U.S. states, although most consumers are not aware of the smaller size of the unit. Cooling degree day (CDD) see degree day. Cong a metric unit of area used in Vietnam. One cong equals 1000 square meters, which is 0.1 hectare, 0.24177 acres, or 1196.00 square yards. Congius a historic unit of liquid volume. The Roman congius was equal to about 3.2 liters (3.4 U.S. quarts or 2.8 British Imperial quarts); it was divided into 6 sextarii (sixths) which corresponded closely to modern pints. In the nineteenth century, the congius was used in British medicine and pharmacology as a name for the British Imperial gallon (4.546 09 liters). Coomb or coom a traditional British unit of volume used mostly for dry commodities. A coomb is 4 imperial bushels; this is equivalent to 5.1374 cubic feet or about 145.48 liters. COP an abbreviation for coefficient of performance, a measure of the efficiency of heat pumps, air conditioners, refrigerators, and freezers. The COP is the ratio of the useful energy output of the system divided by the electric energy input when the unit is operating in a steady-state test condition. Typical values are in the range 2-4. (The energy output exceeds the input, because the system takes advantage of the heat released or absorbed by the refrigerant when it condenses or evaporates.) A heat pump that delivers two units of cooling for each unit of electricity also rejects three units of heat; thus it has a COP of 2.0 for cooling or 3.0 for heat. For air conditioners, the COP is considered to equal the energy efficiency ratio (EER) divided by 3.412. Cord (cd) [1] a traditional unit of volume used to measure stacked firewood. Like most traditional units of trade, the cord has varied somewhat according to local custom. In the United States, the cord is defined legally as the volume of a stack of firewood 4 feet wide, 8 feet long, and 4 feet high. (In Maryland, the law specifies that the wood be stacked "tight enough that a chipmunk cannot run through it." Presumably it is up to the buyer to provide the chipmunk.) One cord is a volume of 128 cubic feet, about 3.6247 cubic meters, or 3.6247 steres. The name apparently comes from an old method of measuring a stack of firewood using a cord or string. Cord (cd) [2] in the U.S. timber industry, the cord is also used as a unit of weight for pulpwood. The weight varies with tree species, ranging from about 5200 pounds (2.36 metric tons) for pine to about 5800 pounds (2.63 metric tons) for hardwood. Cord foot (cd ft) a traditional unit of volume used to measure stacked firewood. A cord foot is the volume of a stack of firewood 4 feet wide, 1 foot long, and 4 feet high. Thus the cord foot is 1/8 cord, or 16 cubic feet, or about 0.4531 cubic meter. Coulomb (C) the SI unit of electric charge. One coulomb is the amount of charge accumulated in one second by a current of one ampere. Electricity is actually a flow of particles called electrons, and one coulomb represents the charge on approximately 6.241 506 x 1018 electrons. The coulomb is named for a French physicist, Charles-Augustin de Coulomb (1736-1806), who was the first to measure accurately the forces exerted between electric charges. Count (ct) [1] a unit of quantity equal to 1. This unit is used in commerce to specify that the quantity stated represents a reliable count. For example, a carton marked "oranges 24 ct" contains exactly 24 oranges. Count (ct) [2] a traditional unit measuring the texture of a fabric, equal to the number of threads per inch. A 100 count fabric has 39.37 threads per centimeter. Count (ct) [3] an informal unit of volume in bartending. Bartenders usually fit bottles with pourers designed to restrict the flow to 0.5 fluid ounce per second (14.8 mL/s). They can then measure a quantity of liquid by counting, "one thousand one, one thousand two, ..." while pouring. This is much faster than using a measuring glass, and just about as accurate. Cousins English, like most languages, has a procedure for stating the precise relationship between persons of common descent; a typical designation is "second cousins, once removed." First cousins are persons sharing a common grandparent; second cousins are persons sharing a common great-grandparent, and, generally, for n > 1, n-th cousins are persons sharing a common (n - 1)-times-great-grandparent. This means n-th cousins have n + 1 generations in each of their descents from the common ancestor. The "removed" phrase is used when the number of generations in the descent from the common ancestor is not the same for both cousins: "r times removed" means the difference in the number of generations is r. Thus, for n-th cousins r times removed, the common ancestor is an (n - 1)-times-great-grandparent of one cousin and an (n + r - 1)-times-great-grandparent of the other cousin. (In the case of first cousins r times removed, the grandparent of one cousin is also an r-times-great-grandparent of the other cousin.) For n-th cousins r times removed, there are n + 1 generations in the descent from a common ancestor for one cousin, and n + r + 1 generations for the other. The number of degrees of consanguinity between n-th cousins r times removed is 2n + r + 2. Covado, covido Portuguese and Arabic names, respectively, for the cubit (see below). The Portuguese covado is equal to 3 palmos (66 centimeters, or 20.12 inches), while the Arabic covido is about 48 centimeters or 19 inches. Cover a traditional Welsh unit of area, standardized in the British system to be exactly 2/3 acre (about 0.2698 hectare). The word is an Anglicized version of the Welsh name cyfair for the unit. Cps a traditional abbreviation for cycles per second; also an incorrect symbol for the centipoise (see above). Cran a traditional unit of volume formerly used by fishermen. The cran, originally intended to represent the volume of fish held by a barrel, was standardized at 37.5 Imperial gallons, or about 170.46 liters. This is somewhat larger than the usual Imperial barrel. The name of the unit is an old Gaelic word meaning a quantity or measure of something. Crannock an old English unit of volume for grains. Never standardized, the crannock was roughly 10 bushels (350 liters). Crith a unit of mass sometimes used in the physics and chemistry of gases. The crith is equal to the mass of a liter of hydrogen at standard temperature (0.01°C) and pressure (1 atmosphere); this is about 89.885 milligrams. The name comes from an ancient Greek word for a barleycorn. Crore a traditional unit of quantity in India, equal to 107 or 10 million. Large numbers are usually described in India using the crore and the lakh (105); for example, the number 25 600 000 is called 2 crore 56 lakh and written "2,56,00,000". Crotchet a unit of relative time in music equal to 1/4 whole note or 1/8 breve. The word, pronounced crotch-it, comes from the old Norse word krok for a hook; in this context it refers to the traditional hooked symbol for a quarter note. Cu abbreviation for "cubic," as in cubic feet or cubic meters. This symbol is forbidden by the SI, but it remains in common use in ordinary English text. The proper symbol for cubic feet is ft3, not cu. ft. Cuadra [1] a traditional Latin American unit of distance. The cuadra is generally equal to 100 varas (about 84 meters or 275 feet) in Central America and northern South America. In Argentina and Chile, the cuadra is equal to 150 varas (roughly 130 meters or 410 feet). Cuadra [2] a traditional Latin American unit of area equal to one square cuadra [1]. Except in Argentina and Chile, this is 10 000 square varas, generally in the range 1.75-1.85 acres (0.71-0.75 hectare). In Argentina and Chile, the cuadra was 22 500 square varas (4.18 acres or 1.69 hectare). Cuartillo a traditional Spanish unit of volume comparable to the liter or the English quart. The cuartillo equals 4 octavillos or 1/4 almude and contains 1.156 25 liters, which is about 1.222 U.S. liquid quart or 1.017 British imperial quart. Cubic centimeter (cm3 or cc) the CGS unit of volume, equal to 10-6 cubic meter, 1 milliliter, or about 0.061 023 7 cubic inch. Cubic foot (ft3, cu ft, or cf) a traditional unit of volume in English speaking countries. One cubic foot equals 1728 in3, 1/27 yd3, 0.028 316 85 m3, or 28.316 85 liters. The cubic foot also holds about 7.4805 U.S. liquid gallons or about 6.2288 British Imperial gallons. Cubic inch (in3 or cu in) a traditional unit of volume in English speaking countries. One cubic inch equals 1/1728 = 5.787 037 x 10-4 ft3, 16.3871 cm3, 16.3871 milliliters, 0.5541 U.S. fluid ounce, or 0.5767 British Imperial fluid ounce. Cubic meter (m3) the SI unit of volume, equal to 106 cm3, 1000 liters, 35.3147 ft3, or 1.307 95 yd3. A cubic meter holds about 264.17 U.S. liquid gallons or 219.99 British Imperial gallons. Cubic yard (yd3) a traditional unit of volume in English speaking countries. One cubic yard equals 27 ft3, 46 656 in3, 0.764 555 m3, or 764.555 liters. A cubic yard holds about 201.97 U.S. liquid gallons or about 168.20 British Imperial gallons. Cubit a historic unit of distance frequently mentioned in the Bible. The word comes from the Latin cubitum, "elbow," because the unit represents the length of a man's forearm from his elbow to the tip of his outstretched middle finger. This distance tends to be about 18 inches or roughly 45 centimeters. In ancient times, the cubit was usually defined to equal 24 digits or 6 palms. The Egyptian royal or "long" cubit, however, was equal to 28 digits or 7 palms. In the English system, the digit is conventionally identified as 3/4 inch; this makes the ordinary cubit exactly 18 inches (45.72 centimeters). The Roman cubit was shorter, about 44.4 centimeters (17.5 inches). The ordinary Egyptian cubit was just under 45 centimeters, and most authorities estimate the royal cubit at about 52.35 centimeters (20.61 inches). Cuerda (cda) [1] a traditional unit of land area in Puerto Rico. The cuerda is equal to about 3930 square meters, 4700 square yards, 0.393 hectare, or 0.971 acre. Because the cuerda and the acre are so close to being equal, they are often treated informally as being equal. Mainlanders sometimes call the unit the "Spanish acre." Cuerda (cda) [2] a traditional unit of distance in Guatemala, equal to 25 varas or about 21 meters (roughly 69 feet). Since cuerda means a cord or rope in Spanish, this unit probably arose as the length of a measuring rope. The cuerda is also used as an area measure equal to 1 square cuerda or 625 square varas; this is about 440 square meters or 527 square yards. Cuerda (cda) [3] a traditional unit of volume for firewood in Cuba, analogous to the U.S. cord (see above). A cuerda of firewood is equal to 128 cubic pies, 2.87 cubic meters, or 0.79 cord. Cumec an informal unit of flow equal to 1 cubic meter per second or about 35.3147 cubic feet per second. The name was coined after the model of the cusec (see below). Cunit a measure of wood volume used in forestry. One cunit (pronounced cue-nit) is a volume of timber containing 100 cubic feet (2.8317 cubic meters) of actual wood (excluding bark and air between the logs). The unit is used mostly for wood intended as pulpwood or firewood. Cup (c) a traditional unit of volume used in recipes in the United States. One cup equals 1/2 pint, or 8 fluid ounces. Technically, one cup equals exactly 14.4375 cubic inches or approximately 236.6 milliliters, not that anyone measures quite so precisely in the kitchen. American cooks use the same size cup for measuring both liquid and dry substances. In Britain, cooks sometimes use a similar unit called the breakfast cup. Curie (Ci) a unit of radioactivity. One curie was originally defined as the radioactivity of one gram of pure radium. In 1953 scientists agreed that the curie would represent exactly 3.7 x 1010 atomic disintegrations per second, or 37 gigabecquerels (GBq), this being the best estimate of the activity of a gram of radium. See also becquerel. The unit is named for Pierre and Marie Curie, the discoverers of radium. Cusec a traditional unit of flow equal to 1 cubic foot per second or about 28.317 liters per second. Cut [1] a traditional unit of length for yarn in Scotland and northern England. One cut equals 1/12 hank, a unit varying with the material. A cut of cotton yarn is 70 yards; a cut of wool is 46 2/3 yards. Cut [2] a unit of concentration for shellac; see pound cut. Cwt or cwt traditional symbol for the hundredweight. Cy a traditional symbol for the cubic yard. Cycle (c) an informal name for "cycle per second." The frequencies of radio signals and of alternating electric current were previously stated in cycles; thus the alternating current in American homes is often described as "60-cycle" and a radio station might describe its signal frequency as "1040 kilocycles", really meaning 1040 kilocycles per second. Cycles per second (cps) a traditional unit of frequency equal to one per second, or one hertz. Almost all measurements of frequency are now stated in hertz, the SI unit. ##END_OF_RECORD D- an incorrect symbol for the metric prefix deka- or deca-, seen in combinations such as DL (dekaliter) or DTH (dekatherm). The correct prefix is da-. Dag symbol for the dekagram (10 grams; see entry below), a common metric unit of mass. Daily value (DV) a unit of nutrition used in the United States. The U.S. Food and Drug Administration establishes recommended daily amounts of various nutrients, both "good" ones like vitamins and "bad" ones like fat and sodium. These so-called daily values are based on a hypothetical person, male or female, who requires a diet of 2000 Calories per day. The results are approximate at best, since nutritional needs vary with age, sex, and other factors. Food packages generally carry nutritional lables specifying the amount of each nutrient contained in a standard serving, expressed as a percentage of the daily value (%DV). A table of the official daily values for each nutrient is provided. Dal or daL symbol for the dekaliter (10 liters; see entry below), a common metric unit of volume. Dalton (Da or D) an alternate name for the unified atomic mass unit (u or amu). The dalton is often used in microbiology and biochemistry to state the masses of large organic molecules; these measurements are typically in kilodaltons (kDa). It seems necessary to have such a unit, since "kilo-amu" would be such a clumsy name. The SI accepts the dalton as an alternate name for the unified atomic mass unit and specifies Da as its proper symbol. The unit honors the English chemist John Dalton (1766-1844), who proposed the atomic theory of matter in 1803. Dam symbol for the dekameter (10 meters; see entry below), a metric unit of distance. Dan a traditional Chinese weight unit, previously spelled tan in many English works. During the European colonial era the unit was equal to 100 catties or 133.333 pounds. In modern China the dan is equal to 100 jin, which is exactly 50 kilograms (110.231 pounds). The dan is the Chinese equivalent of the European quintal or hundredweight. DaN symbol for the decanewton or dekanewton (see below under dekanewton), a common metric unit of force. Daraf a unit of electrical elastance, which is the ability of an electric potential to charge a capacitor. The daraf is equal to one volt of potential per coulomb of charge (V/C). The name of the unit is "farad" spelled backwards, because the elastance, in darafs, is 1 divided by the capacitance in farads. This unit is not recognized as part of the SI. Darcy a CGS unit of permeability. Permeability is the extent to which a solid allows the flow of a fluid. This flow depends on the properties of the solid and also on the dynamic viscosity of the fluid and the difference in pressure driving the flow. One darcy is the permeability of a solid through which one cubic centimeter of fluid, having a viscosity of one centipoise, will flow in one second through a section one centimeter thick and one square centimeter in cross section, if the pressure difference between the two sides of the solid is one atmosphere. It turns out that permeability has the same units as area; since there is no SI unit of permeability, square meters are used. One darcy is equal to about 0.98692 x 10-12 square meter. The unit is named for a French scientist, H. Darcy (1803-1858), who did pioneering work in the study of permeability. Dash (ds) [1] an informal unit of volume used in food recipes. American cookbook editors usually figure 2 dashes to the pinch and 8 pinches to the teaspoon. This makes the dash equal to roughly 0.01 fluid ounce or 0.3 milliliter. Dash (ds) [2] an informal unit of volume used in bartending. The bartender's dash (or splash) is usually taken to be 1/32 fluid ounce or about 0.92 milliliter. Date dates of the year are described in many ways in different languages. To avoid confusion in international communications, the International Organization for Standardization (ISO) adopted in 1986 an international standard (ISO 8601) for representation of dates and times. The ISO 8601 format for dates is YYYY-MM-DD, where YYYY is the year number, MM is the month (01 through 12) and DD the day number. Hyphens, not slashes, must be used as the separators in the representation. Thus 2001-04-05 is the representation for the date called April 5, 2001, in traditional U.S. notation. The international standard covers numeric representations only; it doesn't prohibit traditional statements of the date, using words, in whatever the local style is. Thus "April 5, 2001" remains perfectly acceptable in U.S. usage. However, non-standard numeric representations, such as 04/05/01, should not be used, because these representations are interpreted differently in different countries. Day (d or da) a traditional unit of time marked by one rotation of the Earth on its axis. As viewed from outside the Solar System, Earth rotates once in approximately 23 hours 56 minutes 4 seconds, a period astronomers call the sidereal day. The day seems a little longer than that to us, because by the time 23:56:04 has passed, the Earth has moved 1/365 of the way in its orbit around the Sun. This causes the Sun to appear a little farther east than it did at the start of the day. It takes the Earth, on the average, about 3 minutes 56 seconds to turn a bit more and put the Sun back in the same apparent position it occupied at the start of the day. There's more. The speed of the Earth in its orbit varies, because the Earth moves faster when it is closer to the Sun. This causes the length of an actual day to vary slightly over the course of the year. Days are longer during the Northern Hemisphere winter and shorter during the Northern Hemisphere summer, because, despite what Northern Hemisphere residents might think, the Earth is actually closer to the Sun during the Northern Hemisphere winter. Because of this irregularity in the length of actual days, what our clocks measure is the average period of time between two successive noons, that is, between two successive appearances of the Sun on the meridian (the imaginary line joining due north to due south across the sky). Astronomers call this period of time the mean solar day. By ancient custom we divide the day into 24 hours, or 1440 minutes, or 86 400 seconds. But there is also a small problem with doing this. The second has its own scientific definition based on the frequency of a certain radiation from cesium atoms and having nothing to do with the Earth's rotation. The reason for making this scientific definition of the second is that the length of the mean solar day is not constant from one year to the next. The gravitational attraction of the Moon is very gradually slowing the Earth's rotation, so that each day is a tiny bit longer, about 40 nanoseconds on the average, than the previous day. This lengthening adds one second to the length of the day about every 60 000 years. During the first decades of the 21st century the actual length of the mean solar day will be about 86 400.002 seconds. For this reason, time as kept by our best clocks runs a bit faster than time as kept by the Earth. To keep clock time and sun time in step (within 0.9 seconds), every so often a leap second must be added at the end of a day. This is done either on June 30 or on December 31. Because the rate of slowing is erratic, the need for these leap seconds cannot be predicted more than a few months in advance. Link: explanation of leap seconds from the U.S. Naval Observatory. DB - a symbol indicating that a measurement is made using a logarithmic scale similar to that of the decibel (see below) in that a difference of 10 dB- corresponds to a factor of 10. In each case, the actual measurement a is compared to a fixed reference level r and the "decibel" value is defined to be 10 log10(a/r). Many units of this kind have been used and only a few of the more common ones are mentioned in the next entries. In each case the dB symbol is followed by a second symbol identifying the specific measurement. Often the two symbols are not separated (as in "dBA"), but the Audio Engineering Society recommends that a space be used (as in "dB A"). DB A, dB C units of sound intensity, exactly like the decibel except that before the measurement is made sounds of high and low frequencies, heard poorly or not at all by the human ear, have been filtered out. The letters A and C refer to two filtering methods. DB c a unit of signal strength used in electronics, especially in measuring noise levels. The signals are measured relative to the strength of the carrier signal, which is the desired signal. A typical statement might be that a certain noise level is -50 dB c, meaning that the noise is 50 "decibels below carrier" or 10-5 times the carrier signal strength. DB FS abbreviation for "decibels full scale," a unit of power as measured by a digital device. A digital measurement has a maximum value M depending on the number of bits used. If the actual power measurement is p, the dB FS value displayed is 20·log10(p/M) dB FS. Since p cannot exceed M, this reading is always negative. DB i a unit measuring the gain of an antenna. The reference level is the strength of the signal that would be transmitted by an isotropic antenna: one radiating equally in all directions. For example, an antenna rated 20 dB i transmits a signal in the desired direction 102 = 100 times stronger than an isotropic antenna. DB m, dB W logarithmic units of power used in electronics. These units measure power in decibels above the reference level of 1 milliwatt in the case of dB m and 1 watt in the case of dB W. A power of n watts equals 10 log n dB W; conversely, a power of p dB W equals 10(p/10) watts. The same formulas link dB m to milliwatts. An increase of 10 dB m or 10 dB W represents a 10-fold increase in power. Since 1 watt = 1000 milliwatts, 0 dB W = 30 dB m. DB rn a symbol for "decibels above reference noise," a unit measuring noise levels in telecommunications. The usual reference level is -90 dB m, which is equivalent to a power of 1 picowatt (1 pW). For example, 50 dB rn equals -40 dB m. DB spl a logarithmic unit of sound intensity as computed from the sound pressure level. The reference level is a pressure of 20 micropascals. If sound waves exert a pressure of P pascals, the sound intensity is 100 + 20·log10(P/2) dB spl. DB u a logarithmic unit of power, similar to dB m but computed from voltage measurements. The reference level is 0.775 volts, the voltage which generates a power of 1 milliwatt across a circuit having an impedance of 600 ohms. A voltage of V volts corresponds to a power of 20·log10(V/0.775) dB u. DB V a logarithmic unit of power, similar to dB m but computed from voltage measurements. The reference level is 1 volt. A voltage of V volts corresponds to a power of 20·log10(V) dB V. DB W see dB m above. DB Z a unit of radar reflectivity used in meteorology. The unit measures the amount of energy returned to a weather radar site as a function of the amount transmitted. The scale is logarithmic, a difference of 10 dB Z indicating a 10-fold increase in energy returned. For display purposes, dB Z values are grouped as follows: (Level 1, 18-30 dBZ) - Light precipitation (Level 2, 30-38 dBZ) - Light to moderate rain (Level 3, 38-44 dBZ) - Moderate to heavy rain (Level 4, 44-50 dBZ) - Heavy rain (Level 5, 50-57 dBZ) - Very heavy rain; hail possible (Level 6, >57 dBZ) - Very heavy rain and hail; large hail possible The colorful "radar images" shown on television are actually plots of these levels. Deadweight ton (dwt) a traditional unit of weight or mass used in the shipping industry. The deadweight tonnage of a ship is the difference between its weight when completely empty and its weight when fully loaded. This includes the weight of everything portable carried by the ship: the cargo, fuel, supplies, crew, and passengers. The deadweight ton is traditionally equal to the British ("long") ton of 2240 pounds (1016.047 kilograms). However, more and more often it is being taken to equal the metric ton (exactly 1000 kilograms, or 2204.623 pounds). Debye (D) a CGS unit of electric dipole moment used in chemistry and physics. A charged molecule can be regarded as a tiny bar with opposite charges at the ends. Such a bar is called a dipole, and its dipole moment is the amount of the charge multiplied by half the length of the bar. Thus the appropriate unit of dipole moment in the SI would be the coulomb meter (C·m). Since this is much too large for molecules, the debye is defined as an electric dipole moment of 10-18 statcoulomb centimeter or 3.33564 x 10-30 coulomb meter. The unit honors the Dutch physicist P.J.W. Debye (1884-1966), who was famous for his research on polar molecules. Deca- (da-) a metric prefix meaning 10, derived from the Latin word for ten, decem. The Greek form deka- is also used, and it has the advantage of avoiding confusion with deci-, one tenth. The International Bureau of Weights and Measures (BIPM) specifies deca- for the prefix and da- for the symbol. In its interpretation of the International System (SI) for U.S. use, the U.S. National Institute of Standards and Technology (NIST) recommends deka- for the spelling of the prefix. Following NIST, this dictionary uses the spelling deka-. Decade [1] a traditional unit of quantity equal to 10. Also called the decad. In medieval English this unit was anglicized as the dicker (see below). Decade [2] a traditional unit of time equal to 10 years. Decade [3] a unit of time equal to 10 days or 1/3 month. The revolutionary governments in France (1790s) and the Soviet Union (1920s) tried to decimalize and secularize the calendar by eliminating the week (with its traditional day of religious observance) and substituting the decade. Both efforts failed. However, the decade survived as a unit of civil time in Russia and as a 10-day unit in some scientific contexts (for example, a 10-day cycle in meteorology is called a decade). Where the decade equals 1/3 month, the three decades of a month are the 1st-10th, 11th-20th, and 21st through the last day of the month. Decade [4] another name for the bel or the order of magnitude: a logarithmic unit used to compare the sizes of quantities. Two quantities differ by one decade if one is 10 times the other, by two decades if one is 10·10 = 100 times the other, and so on. Decatherm see dekatherm, below. Decay time [1] a unit of relative time used in physics. Decay time is similar to half life, but shorter and less familiar. It is the time required for an exponentially decaying process (such as radioactivity) to decrease to 1/e = 36.7879% of its original value. The fraction of activity remaining at time T, if T is measured in decay time units, is simply e-T. The decay time equals 0.693 147 half life. Decay time [2] a unit of relative time used in various engineering applications. In many cases, it is the time required for a decaying process to decrease to 10% of its original value. However, a variety of definitions are used in different fields. Deci- (d-) a metric prefix meaning one tenth, or 0.1. The prefix is derived from the Latin word decimus for a tenth. Decibar (dbar) a metric unit of pressure equal to 0.1 bar or 10 kilopascals. One decibar equals 75.006 torr, or 1.450 pounds per square inch (lbf/in2 or psi). The decibar is often used to measure the pressure of seawater, because an increase of 1 decibar in pressure corresponds closely to an increase of 1 meter in depth. Decibel (dB) a customary logarithmic unit most commonly used (in various ways) for measuring sound. The human ear is capable of detecting an enormous range of sound intensities. Furthermore, our perception is not linear. Experiment shows that when humans perceive one sound to be twice as loud as another, in fact the louder sound is about ten times as intense as the fainter one. For this reason, sound is measured on logarithmic scales. Informally, if one sound is 1 bel (10 decibels) "louder" than another, this means the louder sound is 10 times louder than the fainter one. A difference of 20 decibels corresponds to an increase of 10 x 10 or 100 times in intensity. The beginning of the scale, 0 decibels, can be set in different ways, depending on exactly which aspect of sound is being measured. For sound intensity (the power of the sound waves per unit of area) 0 decibels is equal 1 picowatt per square meter; this corresponds approximately to the faintest sound that can be detected by a person who has good hearing. A quiet room has a normal sound intensity of around 40 decibels, ten thousand times louder than the faintest perceptible sound, and a thunderclap may have an intensity of 120 decibels, a trillion times louder than the faintest sound. For sound pressure (the pressure exerted by the sound waves) 0 decibels equals 20 micropascals (µPa) RMS, and for sound power 0 decibels sometimes equals 1 picowatt. In all cases, one decibel equals about 0.115 129 neper and d decibels equal d(ln 10)/20 nepers. See also dB- (above). Decigram (dg) a metric unit of mass equal to 100 milligrams or about 1.5432 grains. Decile a statistical unit equal to 10 percentiles, or 1/10 of a ranked sample. See percentile. The word is pronounced with a soft "c": des-ile. Deciliter (dl or dL) a common metric unit of volume equal to 0.1 liter or 100 cubic centimeters. A deciliter contains 6.10237 cubic inches, 3.38140 U.S. fluid ounces, or 3.519 British fluid ounces. The deciliter is similar in size to the gill, an old English unit of volume. Decimeter (dm) a fairly common metric unit of distance equal to 10 centimeters or 3.9370 inches. The decimeter is very close to the hand, a traditional English unit. Decipol an empirical unit of indoor air pollution introduced by the Danish environmental scientist P.O. Fanger in 1988. One olf is defined as the air pollution produced by one "standard person", and one decipol is the perceived air pollution level in a space having a pollution source of strength one olf and ventilation at the rate of 10 liters/second with unpolluted air. Measurements are recorded by human observers using protocols laid out by Fanger and his colleagues. Decitex (dtex) a common metric unit of yarn density equal to 0.1 tex, 0.9 denier (see below), or 0.1 milligram per meter. This unit was previously called the drex. Decitonne (dt or dtn) a metric unit of mass or weight equal to 100 kilograms (approximately 220.4623 pounds). This unit is becoming common in international trade; it is the same as the Russian centner, the German doppelzentner (see below), and the French metric quintal. Degree (° or deg) [1] the standard unit of angle measure, equal to 1/360 circle, 60 minutes, 3600 seconds, or about 0.017 453 293 radian. So far as we know, this unit was introduced by the Greek geometer Hipparchus of Nicaea (ca. 180-ca. 125 BC), who developed the first trigonometric tables. Degree (° or deg) [2] a unit of distance sometimes used at sea, equal to 60 nautical miles, approximately 69.05 statute miles, or 111.12 kilometers. This distance is the average length of one degree of latitude (that is, the average distance between two lines of latitude 1° degree apart). Because the Earth is not exactly spherical, a degree of latitude actually varies from about 68.7 miles at the Equator to 69.4 miles at the poles. One degree of longitude is about 69.17 miles at the Equator (shrinking to nothing at the poles!). Degree (° or deg) [3] any of the various units of temperature (see below). In the U.S., the unmodified unit "degree" generally means the degree Fahrenheit; in other countries, it means the degree Celsius. Degree (° or deg) [4] a unit measuring the hardness of water. Water is called "hard" if it contains a high concentration of mineral salts, especially calcium carbonate. This concentration can be expressed clearly in parts per million (ppm) or in milligrams per liter (mg/L). But starting in the mid-19th century, several uses of "degree" for water hardness became well established. In the U.S. and Britain, hardness is measured in Clark degrees, named for the scientist who devised the first reliable test for water hardness. The Clark degree is defined as 1 part of calcium carbonate per 70 000 parts of water; this is about 14.3 parts per million (ppm), 17.1 mg/L, or 1 grain per gallon (gpg, another popular measure of hardness). The French, properly decimal as always, used a degree equal to exactly 10 ppm, while the German degree was equivalent to 17.8 ppm. Degree (° or deg) [5] various scientific quantities, especially specific gravity and viscosity, have been measured in "degrees" based on the readings of particular instruments. The degree scales of Baumé (see below) and Beck, and many others, have been used in measuring specific gravity, and the scales of Engler and MacMichael are common in the measurement of viscosity. Often these degree scales can't be converted in a simple way to the quantities being measured; one needs the conversion formula or table for the particular instrument. Some of this data can be found in scientific and engineering handbooks, especially older ones. Degree (° or deg) [6] the percentage of alcohol, by volume, present in a mixture. In winemaking, for example, a 13° wine is 13% alcohol by volume (13% v/v). This unit is also called the degree Gay-Lussac (° GL) after the French chemist Joseph-Louis Gay-Lussac (1778-1850). Degree (deg) [7] a unit used in mathematics to describe algebraic expressions. The degree of an expression having a single variable is the highest exponent (or power) to which that variable appears. If the expression has more than one variable, then the degree is the highest total exponent of the individual terms, where in each term the exponents of the different variables are added. Thus x2y4z3 is a term of degree 9. Degree (°) [8] the DIN speed rating for film; see DIN below. Degree (deg) [9] a unit measuring consanguinity: the extent to which two persons are related to one another by common descent ("by blood," as the usual saying is). Essentially all cultures, societies, and nations have prohibited marriage between closely related individuals, and the determining factor is often the number of degrees of consanguinity between the two persons. If X is an ancestor of Y, the number of degrees of consanguinity between X and Y is the number of descents; thus parent and child are separated by 1 degree, grandparent and grandchild by 2 degrees, and so on. If persons X and Y are not linked by direct descent but do share a common ancestor A, the usual definition is that the number of degrees of consanguinity between X and Y is the total number of descents, counting up from X to A and then back down from A to Y. For example, siblings (brother and sister) are separated by 2 degrees, aunt and nephew by 3 degrees, and first cousins by 4 degrees. This calculation is backed by modern science, because we have learned that persons separated by n degrees of consanguinity share about 1/2n of their genes. See also cousins. Degree API (°API) a unit invented by the American Petroleum Institute (API) to measure the specific gravity, or one might say the "lightness," of petroleum. Lighter oils are more valuable, so the API scale gives them higher readings than heavier oils. If S is the specific gravity of the petroleum, the API degree rating is equal to (141.5/S) - 131.5 degrees. Degree Balling see degree Plato, below. Degree Baumé (°B or °Bé) a unit of relative density, as read on a type of hydrometer invented by the French chemist Antoine Baumé (1728-1804). Two scales are used, depending on whether the liquid is lighter than water or heavier than water. For lighter liquids, the relative density d in degrees Baumé is related to specific gravity S by the formula d = (144.3 / S) - 144.3; for heavier liquids the formula is d = 144.3 - (144.3 / S). Degree Brix (°Bx) see brix. Degree Celsius (°C) a metric unit of temperature. The Celsius temperature scale is named for the Swedish astronomer and physicist Anders Celsius (1701-1744), who used a similar scale. The freezing point of water (at one atmosphere of pressure) was originally defined to be 0 °C, while the boiling point is 100 °C. Thus the Celsius degree is 1/100 of the difference between these two temperatures. (The scale actually used by Celsius was inverted, so that 0 °C was the boiling point of water and 100 °C the freezing point.) In the SI system, the Celsius scale is defined so that the temperature of the triple point of water (the temperature at which water can exist simultaneously in the gaseous, liquid, and solid states) is exactly 0.01 °C, and the size of the degree is 1/273.16 of the difference between this temperature and absolute zero (the temperature at which all molecular motion ceases). For practical purposes this is equivalent to the original definition. See also degree Fahrenheit, kelvin. Degree centigrade (°C) an older name for the degree Celsius. The Celsius scale is called "centigrade" because it has 100 (centi-) gradations between the freezing point and boiling point of water. About 1850 scientists began calling the unit "degree Celsius" in honor of its (presumed) inventor. Degree day (deg da) a unit used in meteorology and engineering to measure the demand for heating or cooling. In the United States, it's agreed that 65 °F (18.3 °C) is the critical outside temperature; below this tempeature heating is needed and above it cooling is needed. The number of degree days recorded on a given date is equal to the difference between 65 and the mean temperature for that date (figured as the average of the highest and lowest temperatures). If the mean temperature is above 65, cooling degree days (CDD) are recorded; if it is below 65, heating degree days (HDD) are recorded. The total number of heating or cooling degree days recorded in any period of time measures rather accurately the demand during that time for fuel used in heating and cooling buildings. Degree EBC (°EBC) a unit used in Europe to measure the color of beer. EBC is an abbreviation for European Brewing Convention. EBC degrees are related to the Lovibond degrees used in the U.S. (see below) by the approximate formula °EBC = (°L * 2.65) - 1.2. Degree Fahrenheit (°F) a traditional unit of temperature still used customarily in the United States. The unit was defined by the German physicist Daniel G. Fahrenheit (1686-1736), who also invented the mercury thermometer. Fahrenheit set 0° at the coldest temperature he could conveniently achieve using an ice and salt mixture, and he intended to set 100° at the temperature of the human body. (He was off a little there; normal temperature for humans is between 98 °F and 99 °F.) On this scale, the freezing point of water (at normal sea level atmospheric pressure) turned out to be about 32 °F and the boiling point about 212 °F. Eventually the scale was precisely defined by these two temperatures. 1°F equals 5/9 °C, but in converting between scales we have to be careful to adjust the zero points as well. To convert a temperature in °F to the Celsius scale, we must first subtract 32° and then multiply by 5/9. In the other direction, to convert a temperature in °C to the Fahrenheit scale, we must first multiply by 9/5 and then add 32°. The Celsius scale is now used everywhere outside the United States, so only Americans need to remember these formulas. Degree Gay-Lussac (°GL) see degree [6] above. Degree Kelvin (°K) an obsolete name for the kelvin. In the International System, temperatures on the absolute temperature scale are stated in kelvins, not in degrees Kelvin. Degree KMW (°KMW) a unit used in Austria to measure the sugar content of must, the unfermented liquour from which wine is made. One degree KMW is roughly equivalent to 1% sugar by weight or 5° Oe; for the exact conversion see below under degree Oeschle. KMW is an abbreviation for Klosterneuburger Mostwaage (Klosterneuburg Must Scale). Degree Lovibond (°L) a unit used in the U.S. to measure the color (really the darkness) of beer and honey. The scale is open-ended, but most readings fall between 1 (a very light gold, or yellow) and 25 (a very dark brown). A good description of the scale, with comments on other units in use, has been posted by pdlab.com. Degree MacMichael (°McM) a unit used to measure the viscosity, or thickness, of chocolate. Typical values range from around 60 °McM (very thin chocolates suitable for pouring into molds) to around 190 °McM (very thick chocolates suitable for hand dipping or forming around a center). A MacMichael viscometer is used to make the measurement. Degree Oechsle (°Oe) a unit used in Germany and Switzerland to measure the sugar content of must, the unfermented liquour from which wine is made. One degree Oechsle (or Öchsle) is roughly equivalent to 0.2% sugar by weight. This unit is related legally to °KMW by the formula °Oe = °KMW * ([.022 * °KMW] + 4.54). Degree Plato (°P) a unit measuring sugar content, especially of the wort, the unfermented liquour from which beer is made. Named for a German chemist, one degree Plato represents a sugar content equivalent to 1% sucrose by weight. Not all the sugar in a wort is sucrose; the unit standardizes the measurement to the sucrose equivalent. The reading is made with a device called a saccharometer. The degree Balling is a somewhat older unit equivalent (approximately) to the degree Plato. Degree Quevenne (°Q) a unit measuring the density of milk. 1 °Quevenne represents a difference in specific gravity of 0.001, so, for example, 20 °Q milk has specific density 1.020. Degree Rankine (°R) a traditional unit of absolute temperature. 1 °Rankine represents the same temperature difference as 1 °Fahrenheit, but the zero point of the scale is set at absolute zero. This means the Rankine temperature is 459.67° plus the Fahrenheit temperature. 1 °Rankine is equal to exactly 5/9 kelvin. The unit is named for the British physicist and engineer William Rankine (1820-1872). Degree Réaumur (°r) a unit of temperature formerly used in continental Europe. The Réaumur temperature scale is named for the French scientist René-Antoine Ferchault de Réaumur (1683-1757). It is similar to the Celsius scale in having its zero point at the freezing temperature of water, but the interval between the freezing and boiling temperatures of water equals 80 °r instead of 100 °C. Therefore, 1 °r equals 1.25 °C or 2.25 °F. Degree Twaddle (°Tw) a unit measuring the specific gravity of liquids denser than water. 1 °Twaddle represents a difference in specific gravity of 0.005 or 1/200, so a liquid of specific gravity S is measured at 200(S - 1) °Tw. For milk, 1 °Twaddle equals 5° Quevenne (see above). Deka- (da-) a metric prefix meaning 10, taken directly from the Greek word for ten, deka. The Latin spelling deca- is also used (see deca-, above). Although the U.S. National Institute for Standards and Technology recommends deka-, deca- is seen frequently in American publications. The spelling deca- is also used in French and in other languages where "k" rarely occurs in the middle of a word. Regardless of spelling, da- is the official symbol. (The symbol dk- for deka- is incorrect, but it is seen fairly often.) Dekagram or decagram (dag) a common metric unit of mass, the dekagram is frequently used in European food recipes. One dekagram is equal to 10 grams, 0.01 kilogram, or 0.352 739 66 ounce. The symbol dkg sometimes used for this unit is incorrect. Dekaliter or decaliter (daL or dal) a metric unit of volume equal to 10 liters and comparable to the English peck. The dekaliter is equal to about 2.641 72 U.S. liquid gallons, 1.135 10 U.S. pecks, or 2.199 69 British imperial gallons (1.099 85 British pecks). The symbol dkL sometimes used for this unit is incorrect. Dekameter or decameter (dam) a common metric unit of distance equal to 10 meters (about 32.8084 feet). The symbol dkm sometimes used for this unit is incorrect. Dekanewton or decanewton (daN) a fairly common metric unit of force equal to 10 newtons. The dekanewton is equal to 1 megadyne, to 1.019 716 kilograms of force (kgf) or kiloponds (kp), to 2.248 09 pounds of force (lbf), and to 72.3301 poundals. In engineering, the dekanewton is a convenient substitute for the kilogram of force or kilopond, since it is nearly equal to those units. Dekan a unit of angle measure equal to 10° or 1/36 circle. The ancient Egyptians divided the circle of the Zodiac into 36 divisions, which the Greeks called dekans. The unit is still used occasionally in astrology, where one dekan equals 1/3 sign. Dekare or decare a metric unit of area equal to 10 ares, that is, 1000 square meters or 0.1 hectare. In English units, the dekare equals approximately 10 763.91 square feet, 1195.99 square yards, or 0.247 105 acres. Various traditional units of land area have been identified with the dekare, including the Middle Eastern dunum (see below), the Norwegian mål, the Greek stremma, and the Vietnamese cong. Dekatherm or decatherm (DTH) a unit of energy equal to 10 therms, 1 million Btu, or about 1.055 057 gigajoules (GJ). This unit is used in the energy industry as a synonym for the million-Btu (MM Btu). Demal (D) an obsolete unit of concentration in chemistry. From 1901 to 1964, the liter was officially defined to be exactly 1.000 028 cubic decimeter. During this period, there was a small difference between measuring concentration in moles per liter and in moles per cubic decimeter. Concentration in moles per liter is called molar, while concentration in moles per cubic decimeter was called demal. This distinction has now disappeared along with the awkward definition of the liter. The former conversion was 1 demal = 1.000 028 molar. Demisemiquaver a unit of relative time in music equal to 1/32 whole note or 1/64 breve. Denaro a traditional Italian weight unit equal to 24 grani; its size varied from about 1.1 grams to 1.25 grams. Denier [1] a traditional unit of yarn density. One denier is the density of a thread having a mass of 1 gram per 9 kilometers of length. The metric unit of yarn density is the tex; 1 denier equals 1/9 tex or 10/9 decitex. Denier [2] a traditional French weight unit, comparable to the English pennyweight. Like the pennyweight, the denier is equal to 24 grains (1.275 grams). This unit was also called the scrupule. Dessertspoon or dessertspoonful (dsp or dssp) a unit of volume sometimes used in food recipes. The dessertspoon is equal to 2 teaspoons; this is roughly equivalent to 10 milliliters in the U.S. and 7 milliliters in Britain. Dessiatina a traditional unit of land area in Russia equal to 2400 square sadzhens. By coincidence, this makes the dessiatina very nearly the same as a hectare: it equals about 1.0925 hectare or 2.6996 acres. Deuce an old English word for two, derived from the old French deus (now spelled deux). The word survives as the name for a two-spot showing in dice or a two card in card games. In tennis, "deuce" describes a tie situation in which a player must win the next two points in order to win the game. Devil's dozen an alternate name for the baker's dozen, a traditional unit of quantity equal to 13. This name reflects the long-standing association of the number 13 with bad luck or evil. Dex a logarithmic unit being used in astronomy. Originally, dex was a convenient function defined by dex (x) = 10x. But the notation is now being used after the exponent in expressions such as -.043 dex, meaning 10-.043. Thus 1 dex equals a factor of 10, making the dex identical to the bel. The name "dex" is a contraction of "decimal exponent." Dhur a traditional unit of land area in South Asia, equal to 1/20 kattha or 1/400 bigha. Like the bigha, the dhur varied in size from region to another. In Nepal, where the unit is still in use, the dhur equals about 16.9 square meters or 66 square yards. Dialog unit (dlu) a unit of relative distance used in computer graphics. Actually there are two units: the horizontal dialog unit equals 1/4 the average width of the font being used, and the vertical dialog unit equals 1/8 the average height of the font. If the font's aspect ratio (the ratio of height to width) is 2:1, these two units will be the same. This is often the case. The unit is used particularly in the design of dialog boxes. Diameter (dia) a unit of magnification equivalent to power. "Power" tends to be used in adjective phrases, such as "120-power eyepiece" or "7-power binoculars." "Diameters" often appear in declarative statements of magnification, such as, "the eyepiece lens magnifies 120 diameters." Diamond a mysterious marking on many tape measures in the U.S. The diamonds mark a distance unit equal to exactly 8/5 feet (19.2 inches or 48.768 centimeters). This is useful to carpenters if they wish to place 5 studs, floor joists, etc., in a distance of 8 feet. Dicker a traditional unit of quantity equal to 10. The word is an English version of the Latin word for ten, decem. After centuries of use, especially by traders in furs and skins, the dicker has nearly disappeared from the English language. Didot point a unit of distance used in typography; see point [2]. Dieb. alt., dieb. tert. abbreviations for the Latin diebus alternis, every other day, and diebus tertius, every third day, units of frequency sometimes used in medical prescriptions. Digit a historic unit of distance equal to the width of a person's finger. Used in all the ancient civilizations of the Middle East and Mediterranean, the digit was equal to 0.75 inch or 19 millimeters with only the smallest variations. Typically, there were 4 digits in a palm, 16 in a foot, and 24 (sometimes 28) in a cubit. The word digit is from the Latin word for a finger or toe, digitus. Dimension (dim) a mathematical unit measuring the number of independent directions in a set or space. Traditionally, a space has as many dimensions as there are mutually perpendicular directions at each point in the space: thus a line has 1 dimensions, a plane has 2 dimensions, and the ordinary space we live in has 3 dimensions. The theory of relativity is set in a "space-time" having 4 dimensions, and higher dimensional spaces are frequently used in science and economics. In addition, mathematicians have developed several methods for assigning fractional dimensions to certain complex sets. DIN the initials of Deutsches Institut für Normung, the German standards agency. Outside the U.S., film was generally marked with a DIN speed rating such as DIN 24 (corresponding to ASA 200). A difference of 3 in the DIN rating corresponds to a doubling of the film speed; that is, DIN 27 film (ASA 400) is twice as fast as DIN 24. The ASA and DIN ratings are now combined, with a degree symbol on the DIN number, and marked with ISO (the initials of the International Organization for Standardization). Thus DIN 27 film is now marked ISO 400/27°. Diopter (dpt or D) a metric unit used in optics to measure the refractive power of a lens. Each lens has a focal length, defined to be the distance from the center of the lens to the point at which the lens focuses light. The shorter the focal length, the greater the refractive power of the lens. The refractive power of the lens, in diopters, equals 1 divided by the focal length of the lens, in meters, so 1 diopter = 1 m-1. The unit is often spelled dioptre outside the United States. The lower case Greek letter delta is also used as a symbol for the diopter. See also prism diopter. Diraa a traditional Egyptian unit of distance equal to about 58 centimeters (22.8 inches). Djerib see jerib. Dk- an incorrect symbol for the metric prefix deka- (10), sometimes seen in combinations such as dkg for the dekagram or dkm for the dekameter. The correct symbol is da-. DN a symbol for "nominal diameter," a size measure for piping, valves, fittings, etc. Nominal diameter is essentially the inside diameter of the piping in millimeters. Industrial standards organizations, such as the American National Standards Institute (ANSI), set standards for pipes and fittings based on DN ratings; these standards specify in detail the size, composition, and strength of each component. Dobson unit (DU) a unit used in geophysics to measure the ozone in the atmosphere. One Dobson unit represents the amount of atmospheric ozone that would form a uniform layer 0.01 millimeter (10 micrometers) thick at standard temperature (0 °C) and pressure (1 atmosphere or 1013.25 millibars). The Dobson unit equals 10-5 atmo-meter. Under normal conditions, the atmosphere contains about 300 Dobson units of ozone, but this falls to 100 Dobson units or less in the "ozone holes" over the Earth's poles. The unit is named for the British physicist G.M.B. Dobson; in 1920 he invented a spectrometer to measure ozone concentrations from the ground. Dog watch a unit of time on ships at sea equal to 2 hours, one half the usual length of a watch. Dog year an informal unit of time equal to 1/7 of a normal, or "human" year. According to folklore, dogs age 7 times faster than humans. Dol a unit proposed for the measurement of pain. James Hardy, Herbert Wolff, and Helen Goodell, all of Cornell University, proposed the unit based on their studies of pain during the 1940s and 1950s; they defined one dol to equal 2 "just noticeable differences" (jnd's) in pain. However, the unit did not come into widespread use and other methods are now used to assess the level of pain experienced by patients. The name of the unit is from the Latin word for pain, dolor. Dollar ($) a unit used in nuclear engineering to describe the "reactivity" of a nuclear reactor. For a particular reactor, one dollar is the reactivity at which the chain reaction is just self-sustaining. The size of the unit varies with the design of the reactor; a typical size is approximately 10-5 = 0.001%. For a discussion of reactivity, see inhour. Donkey power a somewhat light-hearted metric unit of power equal to 250 watts or about 0.3353 horsepower. Donum see dunum (below). Doppelzentner (Dz) a name often used in Germany for the decitonne or metric quintal. Since a zentner is 50 kilograms, a doppelzentner or "double" zentner equals 100 kilograms (about 220.462 pounds). Double a traditional unit of volume for liquor equal to 2 shots. In the U.S., a double equals 2 fluid ounces or about 59.15 milliliters. Double magnum a large wine bottle holding about 3 liters, 4 times the volume of a regular bottle. A double magnum is also called a jeroboam. Doublet another name for a pair. Double word or doubleword a unit of information equal to 2 shortwords, 4 bytes or 32 bits. See also word [2]. Douzième a traditional unit of distance used in watchmaking. The word is French for "twelfth," and a douzième is equal to 1/12 Swiss ligne. This is about 188 micrometers (microns) or 7.4 mils. Dozen (doz or dz) a familiar unit of quantity equal to 12. Division into units of 12 rather than 10 has the advantage that 12 can be evenly divided into halves, thirds, or quarters. For this reason, units of 12 have been common since the earliest civilizations of the Middle East. "Dozen" comes from an old French word dozaine related to the Latin word duodecem, "twelve." Dpa abbreviation for "displacements per atom," a measure of the damage to a crystalline material caused by bombarding the material with energetic particles. Each displacement represents an atom dislodged from its place in the crystal by the radiation. Dpi a unit used to measure the resolution, or sharpness, of a photograph or video image. The unit, an abbreviation for "dots per inch," remains in use even though individual picture elements are now called pixels rather than dots. Dpm, dps abbreviations for "decay per minute" and "decay per second," respectively, units used in the measurement of rates of radioactivity. 1 dps is properly called 1 becquerel (Bq). 1 dpm is equal to 1/60 becquerel or 0.45045 picocuries. Drachm traditional British spelling of dram (see below). Drachma a traditional unit of weight in Greece and the eastern Mediterranean. The drachma was a Greek coin, and the unit was originally the weight of the coin. This is about 3.2 grams, 49.3 grains, or 0.113 ounce avoirdupois. There is also a traditional Dutch weight unit of the same name, but it is somewhat larger: 3.906 grams, 60.28 grains, or 0.1374 ounce avoirdupois. Compared to a similar English unit, the Greek drachma is about 2.5 pennyweight and the Dutch is almost exactly 3 pennyweight. Drachme a traditional unit of weight in France and Germany, generally between 3.7 and 3.9 grams. Dram (dr) [1] a unit of weight in the traditional avoirdupois system (see pound [1]), equal to 1/16 ounce or 1/256 pound. One dram equals about 1.7718 gram. The word dram comes from a Latin weight unit, the dragma, and derives ultimately from the Greek drachme, meaning a handful. The word is usually spelled "drachm" in Britain and "dram" in the United States, but both spellings are pronounced "dram." The avoirdupois dram is sometimes abbreviated dr. av. to distinguish it from the apothecaries' dram (next entry). Dram (dr) [2] a unit of weight in the traditional system of English apothecaries, equal to 60 grains, 3 scruples, 1/8 troy ounce, or approximately 3.8879 gram. See troy weights for additional information. The apothecaries' dram is sometimes abbreviated dr. ap. to distinguish it from the avoirdupois dram. It is equivalent to about 2.1943 avoirdupois drams. There is a similar Italian unit, the dramma, equal to 72 grani or roughly 3.5 grams. Dram (dr) [3] a traditional unit of volume. See fluid dram. Drex a unit traditionally used in the textile industry to measure the density of a single fiber of yarn. One drex equals a density of one gram per 10 kilometers of length, or 1 microgram per centimeter (µg/cm). Since 1 drex equals 0.1 tex, the unit is now called the decitex (dtex) . Drill sizes traditional drill sizes are numbers 1-80, with larger numbers indicating smaller drills. Number 1 has diameter 0.2280 inch and number 80 has diameter 0.0135 inch. Larger sizes are designated by letters or by specifying the diameter directly in 64ths of an inch. The metric drill size is the diameter in millimeters. MyB2O.com has a table showing the traditional sizes and metric equivalents. Drink a unit measuring the alcohol content of beverages, used in describing the medical effects of alcohol. U.S. physicians generally consider one drink equal to 0.5 U.S. fluid ounces of alcohol; the appropriate metric equivalent would be 15 milliliters. In U.S. fluid units, one drink corresponds to about 4 ounces of wine or 1.25 ounces of whiskey. Drop (gtt) [1] a unit of volume used in pharmacy. Traditionally the drop was another name for a minim, a unit of volume equal to 1/60 fluid dram or 1/480 fluid ounce (about 0.0616 milliliter in the U.S., 0.0592 milliliter in Britain). Now that prescriptions are written in metric units, the pharmacist's drop is equal to exactly 0.05 milliliter (20 drops/ml). In hospitals, intravenous tubing is used to deliver medication in drops of various sizes ranging from 10 drops/ml to 60 drops/ml. The traditional abbreviation is from the Latin gutta, drop. Originally, gt was the symbol for a single drop, with gtt being the plural. Drop [2] a informal unit of volume used in recipes. According to some older kitchen references, 24 drops = 1/4 teaspoon; with U.S. definitions this makes the drop equal to 1/576 fluid ounce or about 0.051 milliliter, comparable to the pharmacist's drop (previous entry). Drop [3] a traditional unit of weight in Scotland, equal to 1/16 troy ounce or about 1.944 grams. Drought severity category (D) a measure of drought severity developed by the U.S. National Drought Mitigation Center and used widely by other agencies in the U.S. Categories are denoted D0-D5, with higher numbers indicating more severe drought. A table is provided. Drum a unit of volume sometimes used in the oil trade. The traditional standard drum of oil contains 55 U.S. gallons or about 208.198 liters. One 55-gallon drum equals 1.3095 barrels [2]. In the metric world, drums of 200 liters, 205 liters, and 208 liters are in use as substitutes for the traditional size. Dry pint, dry quart common names for the U.S. pint and quart units used for dry commodities, used to distinguish these units from the pint and quart used for liquids. The dry units are equal to about 1.163 647 times the corresponding liquid units. Dry ton see wet ton. Dsp, dssp symbols for the dessertspoon (2 teaspoons). See "dessertspoon" above. Duet or duo a traditional unit of quantity equal to 2. The word duet is a French version of the Italian duo, two. Dunum or donum a traditional unit of land area in the Middle East and the Balkans. The unit is of Turkish origin, but it seems to be obsolete in modern Turkey. As it is commonly used today (in Israel and Palestine and in Croatia and other areas of the former Yugoslavia), the dunum is a metric unit equal to 1000 square meters or 0.1 hectare (about 0.2471 acre). The traditional size seems to have been around 900 square meters, except in Mesopotamia and Arabia, where it ranged from 2500 to 4000 square meters. Duro, durometer a measure of the hardness of plastic, rubber, or similar material, made an instrument called a durometer. These instruments test the material with a steel pin, and the depth of the indentation made by the pin is read by a calibrated spring meter. There are various versions of the durometer, designed for materials of different kinds; to be called durometers, all versions must conform to standards set by standards organizations such as the American Society for Testing and Materials (ASTM). Durometer scales run from 0 to 100, with larger numbers indicating harder material. The different scales are identified by letters, including A, B, C, D, O, DO, and OO. However, since only one type is used for a particular material, the scale designation is often omitted. Typical notations used are "50 duro", "65 durometer A", and "D40 duro". Dwt abbreviation for deadweight ton (see above); also the traditional symbol for pennyweight. Dyad a unit of quantity equal to 2. This unit is frequently used in cell biology and biotechnology. Dyne (dyn) the CGS unit of force. One dyne is the force that accelerates a mass of one gram at the rate of one centimeter per second per second. Expressed in SI units, the dyne equals 10-5 newton. This is quite a small force: it equals about 2.248 x 10-6 pound of force (lbf) in the traditional English system. The word dyne comes from the Greek dynamis, power. ##END_OF_RECORD e [1] a symbol for the electric charge on one electron. Since the charges on other particles in atomic physics are whole-number multiples of this charge, the symbol e is often used as a unit of measure. 1 e is equal to approximately 160.217 764 62 x 10-21 coulomb, or 160.217 764 62 zeptocoulombs (zC). E [2] a mathematical unit used as the base of "natural" logarithms and exponentials. The real number e is irrational, which means that its decimal expansion is infinite and non-repeating. To 25 significant digits e equals 2.718 281 828 459 045 235 360 287. Of the many properties of this number, the most important is that the rate of change in the function ex is equal to the value of the function itself. As a result, the larger the value of this function is, the faster the function grows: the behavior we call "exponential growth." The Swiss mathematician Leonard Euler (1707-1783) introduced the symbol e, probably because it is the first letter of the word "exponential." Other mathematicians continued to use the letter in his honor, sometimes calling it Euler's constant. Earth-rate unit (eru) a unit of angular velocity equal to 15° per hour, the rate at which the earth rotates on its axis. This unit is used to measure the drift rates of gyroscopes and various pointing devices in aerospace engineering. EBC abbreviation for European Brewing Commission, frequently used for a unit of turbidity equal to 4.08 NTU or FNU. Eclipse year a unit of time used in astronomy in the prediction of eclipses. The plane of the moon's orbit around the earth and the plane of the earth's orbit around the sun intersect in a line called the line of nodes. Eclipses of the sun or moon can only occur at times when the earth, moon, and sun all lie very close to this line. However, the line of nodes rotates slowly, so comparable appearances of the sun on the line do not fall exactly a year apart. The actual interval between these crossings, the eclipse year, has a length of 346.62 days. EER an abbreviation for energy efficiency rating, a U.S. measure of the efficiency of an air conditioner. The EER is computed as the cooling capacity of the unit (in Btu per hour) divided by the electric power consumed (in watts) at a temperature of 95 °F (30 °C) and under specified test conditions. Typical values are in the range 8-12. Technically, 1 EER is a number (the ratio between two measurements of power) equal to 0.293 071. See also COP and energy factor (EF, below). Effective dose (ED) a measure used in pharmacology to express the percentage of a population that receives the desired benefit from a dose of the substance being studied. The measurement is usually given as a subscript. For example, the ED50 dose is the amount of the substance that benefits 50% of the test population. Einstein, Einstein unit a unit of light energy concentration sometimes used in physical chemistry. One einstein (or Einstein unit) is the energy per mole of photons carried by a beam of monochromatic light. Suppose a beam of light (in vacuum) has frequency v (in hertz) and wavelength l (in meters). Then one einstein is equal to (3.990 313 x 10-10)v or (0.119 627)/l joules per mole (J/mol). The unit is named for the physicist Albert Einstein (1879-1955), who explained how light carries energy in a famous 1905 paper. El the Dutch ell (see below), a traditional unit of length equal to about 68-70 centimeters (roughly 27 inches). Electric horsepower a unit of power, equal to exactly 746 watts (550.221 foot pounds per second), used in the electric industry. This is slightly larger than the ordinary or mechanical horsepower of exactly 550 foot pounds per second. Electron (me) the mass of the electron, often used as a unit of mass in particle physics. An electron has a mass of about 9.109 382 x 10-31 kilogram, 9.109 382 x 10-28 gram, or 0.510 9989 million electronvolts (MeV, see next two entries). Electronvolt (eV) [1] a unit of work or energy used in physics. One electronvolt is the work required to move an electron through a potential difference of one volt. The size of the electronvolt must be determined experimentally; the currently accepted value (1998) is 160.217 646 2 x 10-21 joule, or 1.602 176 462 x 10-12 erg. This unit is accepted for use with SI units. The official spelling is electronvolt (one word) rather than electron volt. Electronvolt (eV) [2] a unit of mass used in particle physics. Mass and energy are related by Einstein's famous equation, E = mc2. The constant c is the speed of light, 299.79 x 106 m/sec. An energy of 1 electronvolt is therefore equivalent to a mass of about 1.782 662 x 10-33 gram, or about 1.073 544 x 10-9 atomic mass unit. This is such a small unit that most particle masses are measured in MeV or GeV. Ell a traditional unit of length used primarily for measuring cloth. In the English system, one ell equals 20 nails, 45 inches, or 1.25 yards; in metric terms, an English ell equals exactly 1.143 meters. The word comes from the Latin ulna, which originally meant the elbow and is now the name of the bone on the outside of the forearm. Unfortunately, the same word ulna was also used for the yard, creating frequent confusion between the two units in medieval documents. Probably the ell originated through a custom of measuring lengths of cloth using two forearms, with the hands touching or overlapping. The ell was used with a similar length in France (where it was called the aune). In Scotland, the ell was practically the same as the yard, being equal to 37 Scots inches or 37.2 English inches (94.5 centimeters). This Scottish length appears to reflect an old practice of cloth merchants in giving an extra inch with each yard, to allow for any irregular cutting at the ends of the piece. In eastern Europe, the ell was a shorter distance: see next entry. Elle a traditional unit of distance in German speaking countries. The elle varied considerably, but it was always shorter than the English ell or French aune. A typical value in northern Germany was exactly 2 fuss (German feet), which would be close to 24 inches or 60 centimeters. In the south, the elle was usually longer, about 2.5 fuss. In Vienna, the elle was eventually standardized at 30.68 inches (77.93 centimeters). Although the German word elle is often translated "yard" in English, this is not a very good equivalent. Em a printer's unit of relative distance. One em is the height of the type size (in points) being used. If 12 point type is being set, then one em is 12 points, and so on. See point [2]. Emu [1] an abbreviation for "electromagnetic unit." See ab-. Emu [2] a CGS unit of magnetic dipole moment equal to 4pi micro-oersteds (1.256 637 x 10-5 Oe). In SI units, one emu equals 0.001 A·m2. Emu/cm3 or emu/cc a CGS unit of magnetization. In SI units, one emu/cm3 can be interpreted either as 4pi/10 milliteslas (1.256 637 mT) as a unit of magnetic polarization or excess magnetic induction, or as 1000 amperes per meter as a unit of magnetic dipole moment per unit volume. En a printer's unit of relative distance, equal to 1/2 em. If 12 point type is being set, then one en is 6 points, and so on. Energy factor (EF) a measure of the energy efficiency of an appliance. In the U.S., the Department of Energy has defined energy factors for a variety of appliances. For dishwashers, the energy factor is the number of cycles per kilowatt hour of power input. For clothes washers, it is the capacity of the washer in cubic feet divided by the number of kilowatt hours of power input per washing cycle. For clothes dryers, it is the number of pounds of clothes dried per kilowatt hour of power consumed. Engineer's chain see chain. Engler degree a unit of kinematic viscosity given by readings on an Engler viscometer. The reading is the time (in seconds) required for 200 milliliters of the liquid being tested to flow through the device. The conversion of Engler degrees to absolute units requires an appropriate table, but for liquids having a viscosity of 100 centistokes or more the Engler degree is roughly equal to 7.6 centistokes. Ennead a unit of quantity equal to 9, coined from the Greek word for nine, ennea. Enzyme unit (U or EU) a unit used by biochemists to measure the activity of enzymes, which are proteins produced by living cells to cause or facilitate necessary chemical reactions within the cell. One enzyme unit is the quantity of enzyme needed to cause a reaction to process 1 micromole of substance per minute under specified conditions. Thus, one enzyme unit has a catalytic activity of 1/60 microkatal (µkt) or 16.667 nanokatals (nkt). Eon alternate spelling for aeon, a unit of time equal to 1 billion years. Eotvos unit (E) a unit used in geophysics to measure the change in the acceleration of gravity with horizontal distance. One Eotvos unit equals 10-9 Gal per centimeter or 10-4 Gal per kilometer. In proper SI terms, the Eotvos unit equals 10-9 per second squared (s-2). The unit honors the Hungarian physicist Roland von Eötvös (1848-1919). Ephah a ancient unit of volume for grains and dry commodities, used in the Bible. The ephah is believed to equal about 40.32 liters or 1.4239 cubic feet. This is equivalent to about 1.144 U.S. bushel. Epoch [1] a measure of time used in astronomy. In an epoch system, times are specified as years and fractions of years (such as epoch 1998.5). To set a starting point for the system, a specific epoch time must be fixed as a particular clock time of a particular date. In 1984, the International Astronomical Union agreed that epoch times should be fixed by requiring that epoch 2000.0 equals 12 hours Universal Time of the day 2000 January 1. Other epoch conventions were used in the past. See also Julian epoch. The name comes from a Greek word epoche meaning a stopping point or fixed point. Epoch [2] a unit of time equal to 19 years, used in predictions of the tides. In this use, an epoch is another name for a Metonic cycle. All possible alignments of the sun and moon occur in this 1-year cycle, so tidal heights and other tidal phenomena are averaged over this period. Equivalent or equivalent weight (Eq) a unit of relative amount of substance used in chemistry. One equivalent weight of an element, compound, or ion is the weight in grams of that substance which would react with or replace one gram of hydrogen. Since one gram of hydrogen is very nearly equal to one mole and since hydrogen has one electron free to react with other substances, 1 Eq of a substance is effectively equal to one mole divided by the valence of the substance (the number of electrons the substance would engage in participating in the reaction). In practice, this is a large unit and measurements are more likely to be in milliequivalents (mEq or meq). Erg the unit of work or energy in the CGS system, equal to the work done by a force of one dyne acting through a distance of one centimeter. Equivalently, one erg is the kinetic energy of a mass of 2 grams moving at a velocity of 1 cm/sec. This is equal to 0.1 microjoule, or about 7.375 x 10-8 foot-pound. The name of the unit is from the Greek word ergon, work. There is no symbol; the word is spelled out in full. Adding prefixes is a problem: 1000 ergs is usually called a kiloerg, but one million ergs is a megalerg, the "l" being added to ease pronunciation. Erlang (E) a unit of telecommunications traffic. The traffic in erlangs is equal to the number of calls multiplied by the average length of a call in hours or fractions of an hour. Thus one erlang could represent a single hour-long call, or 60 calls each one minute long, etc. One erlang equals 3600 call-seconds, often written 36 CCS. The unit honors A. K. Erlang (1878-1929), a Danish mathematician who studied the mathematics of telephone networks. Estadio a traditional unit of distance in Spain and Portugal. The estadio, like the English furlong, is equal to 1/8 mile. The Spanish estadio is equal to 1/8 milla or 625 pies; this is about 571 feet or 174 meters. The Portuguese unit is 1/8 milha, which is much longer: about 856 feet or 261 meters. Esu an abbreviation for "electrostatic unit." See stat-. Ett- or etto- (h-) Italian spelling for the metric prefix hecto- (100). The hectare, for example, is ettaro in Italian. The International System allows national variations in spelling of the names of units, but not in the symbols used for them; thus the symbol for etto- is h-. Etto (hg) an informal Italian name for the hectogram, a unit of mass equal to 100 grams or about 3.5274 ounces. EU a symbol sometimes used for the enzyme unit (see above). Exa- (E-) a metric prefix denoting 1018 (one quintillion in U.S. nomenclature). The Latin and Greek prefix ex- means "out of," and is often used to indicate "a long way," as in the words "expanse" or "extreme." In addition, the prefix suggests the Greek hexa, meaning 6, this being the sixth prefix (n = 6 in 103n) in the SI system of metric prefixes. Exajoule (EJ) a metric unit of energy. One exajoule equals 947.817 (U.S.) trillion Btu, 277.7778 petawatt hours, or about 9480 megatherms. The unit is often used in discussing global energy production, which is measured in hundreds of exajoules per year. Exameter (Em) a metric unit of distance equal to 1015 kilometers. This is equivalent to about 621.371 trillion miles, 105.7 light years, or 32.408 parsecs. One exameter is approximately the distance from the earth to the Hyades star cluster in Taurus. Exbi- (Ei-) a binary prefix meaning 260 = 1 152 921 504 606 846 976. This prefix, adopted by the International Electrotechnical Commission in 1998, replaces exa- for binary applications in computer science. The prefix is a contraction of "exabinary." Exposure value (EV) a unit used in photography to describe relative exposure. EV 0 is assigned to a specific combination of exposure time and lens aperture, such as 1 second at f/1. The difference between two exposure values is equal to the number of stops separating the two exposure settings. Different combinations of exposure time and lens aperture can have the same exposure value; the unit was invented to simplify the relationship. Regardless of camera settings, EV 6 is one stop "faster" than EV 5, that is, an EV 6 setting exposes the film to half as much light as an EV 5 setting. Eyesight a term often used with a Snellen fraction in phrases such as "20/20 eyesight." ##END_OF_RECORD f or f/ see f ratio, below. F see farad or Fujita-Pearson scale, below. Face cord a unit of volume for firewood. See rick. Faden an alternate name for the klafter, a German unit of distance. Faggot a traditional unit of volume for firewood. A faggot was 3 feet in length and 2 feet in circumference; this is a volume of about 0.955 cubic feet or 27 liters. There are about 134 faggots in a cord. Fahrenheit see degree Fahrenheit. Fall [1] a traditional unit of distance equal to 6 ells. The fall was used in land measurement somewhat like the rod. Measurements in rods were often made with an actual wood pole, while measurements in falls were often made with a rope 6 ells long. The distance falling under the rope was called a fall. The fall was used mostly in Scotland, where its traditional length was 6 Scots ells or about 18.6 English feet (5.67 meters). The Scots furlong was equal to 40 falls (226.8 meters) rather than 40 rods, and the Scots mile was equal to 320 falls (5952 English feet, 1.127 English mile or 1814.2 meters). After the unification of Scotland and England the fall was reinterpreted to equal 6 English ells (22.5 feet or 6.858 meters). Fall [2] a traditional unit of area equal to one square fall [1]. In the traditional Scots system of measurement, a fall of land equals about 346 square feet or 32.15 square meters. A traditional Scots acre was equal to 160 falls or about 6150 square English yards (1.27 English acres or 0.514 hectare). In the English system a fall of land is 506.25 square feet, 56.25 square yards, or about 47.03 square meters. Fanega [1] a Spanish unit of volume, mostly for dry goods. The word is derived from an Arabic word faniqa for a large sack. The fanega equals 12 almudes or 48 cuartillos; this is about 55.50 liters or 1.960 cubic feet (1.575 U.S. bushels). Very similar units were used in Portugal and in most of the Latin American countries. In Chile and Argentina, however, a much larger fanega, roughly 2.5 U.S. bushels, was customary. Fanega [2] or fanegada a traditional unit of land area in Spain and in some Latin American countries. The unit originated as the amount of land that could be planted with a fanega [1] of seed. It varied considerably from one area to another. In 1801 it was standardized in Spain as the area of a square 96 varas (80.2 meters) on a side; this comes to 0.643 hectare (1.59 acres). After the introduction of the metric system it became customary to refer to an area 80 meters square as a fanega. This unit is not legal but is still used informally in some parts of Spain. The Central American manzana is a counterpart of this traditional Spanish unit. Farad (F) the SI unit of electric capacitance. Very early in the study of electricity scientists discovered that a pair of conductors separated by an insulator can store a much larger charge than an isolated conductor can store. The better the insulator, the larger the charge that the conductors can hold. This property of a circuit is called capacitance, and it is measured in farads. One farad is defined as the ability to store one coulomb of charge per volt of potential difference between the two conductors. This is a natural definition, but the unit it defines is very large. In practical circuits, capacitance is often measured in microfarads, nanofarads, or sometimes even in picofarads (10-12 farad, or trillionths of a farad). The unit is named for the British physicist Michael Faraday (1791-1867), who was known for his work in electricity and electrochemistry. Faraday (Fd) a unit of electric charge. In a process called electrolysis, chemists separate the components of a dissolved chemical compound by passing an electric current through the compound. The components are deposited at the electrodes, where the current enters or leaves the solution. The British electrochemist and physicist Michael Faraday (1791-1867) determined that the same amount of charge is needed to deposit one mole of any element or ion of valence one (meaning that each molecule of the ion has either one too many or one too few electrons). This amount of charge, equal to about 96.4853 kilocoulombs or 26.8015 ampere hours, became known as Faraday's constant. Later, it was adopted as a convenient unit for measuring the charges used in electrolysis. One faraday is equal to the product of Avogadro's number (see mole) and the charge (1 e) on a single electron. Fardel an old English word meaning a fourth part, used sometimes as a unit of land area equal to 1/4 virgate, 1/2 nook, or about 8-10 acres. Farsakh, farasang Arabic and Persian names, respectively, for the unit classically known as the parasang. Farthing an old English word for 1/4, later used as the name of a coin equal in value to 1/4 penny. Farthingdale [1] an older name for the rood [2], a unit of land area equal to 1/4 acre (0.1012 hectare). "Farthingdale," like "fardel," means a fourth part. Farthingdale [2] a fardel (see above). Fathom (fth or fath) a traditional unit of distance equal to 2 yards or 6 feet (approximately 1.829 meters). The word comes from the Old English fæthm, meaning "outstretched arms", because a fathom is the distance between a man's outstretched fingertips. This is a generic unit that has been used in many cultures since ancient times. Other versions include the Spanish braza, the French toise, the German klafter, the Danish favn (6.18 feet or 1.88 meters), the Swedish famn (5.84 feet or 1.78 meters) and the Japanese ken. In England, the fathom was a common unit during Saxon times, and it continued to be used for many purposes through the medieval era. In fact, the length of the foot may have been defined, early in the twelfth century, specifically to assure that 1 foot = exactly 1/6 fathom. Today the fathom is used almost exclusively at sea, measuring water depth, the length of ships' cables, etc. FAU abbreviation for formazin attenuation unit, a unit of water turbidity. This unit is used to express turbidity measured by a nephelometer that measures directly the fraction of light transmitted through a water sample as compared to the fraction transmitted through a standard preparation of formazin. The procedure is specified by standard ISO 7027 of the International Organization for Standardization. See NTU for additional information on turbidity. Fatt a traditional unit of volume for grain, generally equal to 9 bushels or 1/4 chalder. This would be about 317 liters based on the traditional grain bushel now used in the U.S. or about 327 liters based on the British imperial bushel. Fbm a symbol sometimes used for the board foot. The "bm" stands for "board measure." Feddan an Egyptian unit of land area, formerly used throughout the Middle East and North Africa. The feddan equals about 0.42 hectare or 1.038 acre. Femto- (f-) a metric prefix standing for 10-15 (one quadrillionth). The prefix was coined from femten, the word for fifteen in Danish and Norwegian. This was an attractive idea because it preserved the established symbol for 10-15 meter (see next entry). Fermi (fm or f) a metric unit of distance formerly used in atomic physics. One fermi equals 10-15 meter, or 1 femtometer. The unit is named for Enrico Fermi (1901-1954), the Italian-American physicist who built the first nuclear reactor. There was a need for this unit before the metric prefixes for very small quantities were defined; now distances formerly measured in fermis are stated in femtometers instead. FEU a unit of cargo capacity, especially for container ships. These ships carry cargo in standard metal boxes, called containers, that can be transferred easily to trains or trucks. FEU is an abbreviation for "forty-foot equivalent unit." One FEU represents the cargo capacity of a standard container 40 feet long, 8 feet wide, and (usually) a little over 8 feet high. One FEU equals roughly 25 register tons (see ton [3]) or 72 cubic meters. Fifth [1] a traditional U.S. unit of liquid volume equal to 1/5 gallon. The fifth contains exactly 46.2 cubic inches, or about 757.084 milliliters. This unit is an American version of the traditional bottle. Fifth [2] a unit used in music to describe the ratio in frequency between notes. Two notes differ by a fifth if the higher note has frequency exactly 3/2 times the frequency of the lower one. On the standard 12-tone scale, the perfect fifth is very closely approximated as 7 half steps, corresponding to a frequency ratio of 27/12 = 1.4983. Fineness (fine) a unit of proportion equal to 1/1000. This unit is used to express the purity of alloys of gold or other precious metals. A statement that a gold bar is "999 fine" means that the bar contains at most 0.1% other metals. See also per mill. Finger [1] a traditional unit of distance equal to 2 nails or 4.5 inches (11.43 centimeters). This unit represents the length of the middle finger, from the tip to the joint where the finger is attached to the hand. Finger [2] a name for the digit, a unit of distance equal to 3/4 inch (19.05 mm). The finger-width was a common unit of measurement in Anglo-Saxon England. After the 12-inch English foot became established, the finger survived as an informal measure. Finsen unit (FU) a unit formerly used in medicine to measure the intensity of the ultraviolet light used in various medical treatments. Ultraviolet radiation of the standard wavelength 296.7 nanometers has intensity 1 Finsen unit if its energy density is 105 watts per square meter. The unit honors the Danish physician Niels Finsen (1860-1904), who received the Nobel prize for medicine in 1903 for his research on the use of light in the treatment of various diseases. Firkin (fir) [1] a traditional unit of volume equal to 1/4 barrel or 1/2 kilderkin. Since barrels are of various sizes, the capacity of a firkin varies. Based on the standard U.S. barrel of 31.5 gallons, a firkin would equal 7.875 gallons, 1.05 cubic feet, or about 29.81 liters. Traditional British barrels and firkins are larger; in the imperial system a firkin holds 1.445 cubic feet or 40.91 liters. The unit is of Dutch origin, and its name is based on the Dutch word veer for four. Firkin (fir) [2] a traditional unit of weight for butter and soap, equal to 4 stone or 56 pounds (about 25.40 kilograms). Fiscal year a unit of one year used for budgeting or accounting. A fiscal year has the same length as an ordinary year, but its starting date may be different. In the U.S., for example, the federal government's fiscal year starts on October 1. Generally, fiscal year n ends in ordinary year n, so the U.S. federal fiscal year 2002 begins October 1, 2001 and ends September 30, 2002. Fist [1] an informal unit of distance equal approximately to the hand (10 centimeters or 4 inches). The unit represents the width of a clenched fist. Fist [2] an informal unit of angle measure equal to 10°, which is the approximate angle subtended by a clenched fist held at arm's length (this works for both adults and children). This unit is used by amateur astronomers to estimate angular distances in the sky. FIT an acronym for "Failure In Time" or "Failure unIT," a unit used to express the expected failure rates of semiconductors or other electronic components. One FIT is equal to a rate of one failure per U.S. billion (109) hours, that is, 1 FIT = 10-9/h. Since 109 hours equals about 114 thousand years, these failure rates are estimated from accelerated test procedures using various statistical techniques. Flagon a traditional unit of liquid volume, generally equal to the wine (or U.S. liquid) gallon (about 3.785 liters). A flagon is a large, narrow-necked pitcher or bottle. Flask a commercial unit of weight formerly used to measure liquid mercury (or quicksilver, as it was called). The flask was equal to 76.5 pounds avoirdupois, which is equivalent to almost exactly 34.7 kilograms. Flock an old English unit of quantity equal to 2 score or 40. Floor another name for a story as a unit of height for buildings. In Britain, the floor above the ground floor is usually called the first floor, so a three-floor building has four stories. In the U.S., the numbering of floors is most often the same as the numbering of stories. Flops a unit of computing power equal to one floating point operation per second. In computer science, there is a distinction between fixed point numbers (which have a fixed number of decimal places) and floating point numbers (which are stored with as many digits as the computer's design allows). A floating point operation is an addition or subtraction of two floating point numbers. The power of supercomputers is being measured in teraflops (Tflops): trillions of floating point operations per second. Fluid dram or fluidram (fl dr) a unit of volume in the traditional apothecary system, equal to 1/8 fluid ounce. This unit is usually called the fluid dram or fluidram to avoid confusion with the weight dram. The U. S. fluid dram contains about 0.225 586 cubic inches or 3.696 691 milliliters. In the British imperial system, the fluid dram is about 0.216 734 cubic inches or 3.551 633 milliliters. Fluid ounce (fl oz) a traditional unit of liquid volume, called the fluid ounce to avoid confusion with the weight ounce. In the U. S. customary system there are 16 fluid ounces in a pint, so each fluid ounce represents 1.804 687 cubic inches or 29.573 531 milliliters. In the British imperial system there are 20 fluid ounces in an imperial pint, so each fluid ounce represents about 1.733 871 cubic inches or 28.413 063 milliliters. A fluid ounce of water weighs just a bit more than one ounce avoirdupois. Fluid scruple a traditional British unit of liquid volume equal to 1/3 fluidram or about 1.1839 milliliters. Flux unit (fu) a former name for the jansky, a unit used in astronomy to measure the strength of a radio signal received from an object in the sky. FNU abbreviation for formazin nephelometric unit, a unit of water turbidity equivalent to the NTU (nephelometric turbidity unit) used in the U.S. The symbol FNU, specified by the International Organization for Standardization in its standard ISO 7027, is widely used outside the U.S. See NTU. Fod the traditional Danish foot, equal to about 12.365 inches or 31.41 centimeters. Folio in traditional legal practice, a unit of quantity for the words in a legal document. In the U.S., a folio is 100 words; in Britain it is 72 or 90 words, depending on the type of document. In the days before mechanical copying, clerks were paid by the folio to copy legal documents. The word is from the Latin folium, leaf, and originally referred to a page; apparently the unit once represented the number of words on a page. Food calorie the large or kilogram Calorie; see calorie [2]. Foot (ft or ') a traditional unit of distance. Almost every culture has used the human foot as a unit of measurement. The natural foot (pes naturalis in Latin), an ancient unit based on the length of actual feet, is about 25 centimeters (9.8 inches). This unit was replaced in early civilizations of the Middle East by a longer foot, roughly 30 centimeters or the size of the modern unit, because this longer length was conveniently expressed in terms of other natural units: 1 foot = 3 hands = 4 palms = 12 inches (thumb widths) = 16 digits (finger widths) This unit was used in both Greece and Rome; the Greek foot is estimated at 30.8 centimeters (12.1 inches) and the Roman foot at 29.6 centimeters (11.7 inches). In northern Europe, however, there was a competing unit known in Latin as the pes manualis or manual foot. This unit was equal to 2 shaftments, and it was measured "by hand," grasping a rod with both hands, thumbs extended and touching. The manual foot is estimated at 33.3 centimeters (13.1 inches). In England, the Roman foot was replaced after the fall of Rome by the natural foot and the Saxon shaftment (16.5 centimeters). The modern foot (1/3 yard or about 30.5 centimeters) did not appear until after the Norman conquest of 1066. It may be an innovation of Henry I, who reigned from 1100 to 1135. Later in the 1100s a foot of modern length, the "foot of St. Paul's," was inscribed on the base of a column of St. Paul's Church in London, so that everyone could see the length of this new foot. From 1300, at least, to the present day there appears be little or no change in the length of the foot. Late in the nineteenth century, after both Britain and the U.S. signed the Treaty of the Meter, the foot was officially defined in terms of the new metric standards. In the U.S., the Metric Act of 1866 defined the foot to equal exactly 1200/3937 meter, or about 30.480 060 96 centimeters. This unit, still used for geodetic surveying in the United States, is now called the survey foot. In 1959, the U.S. National Bureau of Standards redefined the foot to equal exactly 30.48 centimeters (about 0.999 998 survey foot). This definition was also adopted in Britain by the Weights and Measures Act of 1963, so the foot of 30.48 centimeters is now called the international foot. Elsewhere on this page are entries for the Danish fod, Swedish fot, and German fuss. Other foot units include the French pied and the Italian and Spanish pie. Football field a common informal unit of distance in the United States. Americans aren't quite agreed as to whether the unit is exactly 100 yards (91.44 meters), the distance between the goal lines on an American football field, or 120 yards (109.728 meters), the distance including the two end zones. (Canadian football fields are 10 yards longer.) The football field is also used sometimes as an informal unit of area; including the end zones, an American football field represents an area of 1.3223 acre or about 0.535 hectare. Footcandle (fc or ftc) a traditional unit of illuminance or illumination, defined as the illuminance received by a surface at a distance of one foot from a source of intensity one international candle. The "international candle" was the predecessor of the candela as the standard unit of light intensity. Illuminance is now measured in lux; one footcandle equals 10.764 lux or about 1.0764 milliphot. The unit is also spelled foot-candle or foot candle. Footlambert (fL, fl or ftL) a unit of luminance. The only difference between "luminance" and "illuminance" is the source of the light: if it comes from the surface we speak of "luminance," and if it comes from outside we speak of "illuminance." The footlambert describes the luminance of a surface that emits or reflects one lumen per square foot; it is the luminance of a perfectly reflecting surface under an illumination of one footcandle. One footlambert equals 3.426 259 candelas per square meter or about 1.0764 millilambert (mLb). The unit is also spelled foot-lambert or foot lambert. The one-word spelling and symbol fL have been endorsed by the Illuminating Engineering Society of North America (IES) and by the Institute of Electrical and Electronics Engineers (IEEE). (Of course, fL is also the SI symbol for the femtoliter.) Foot of head (ft hd) a traditional unit of water pressure used in plumbing and hydraulics. "Head" is short for "headwaters"; it refers to the depth of the water upstream from the point at which the pressure is measured. One foot of head is equivalent to a pressure of 0.433 lb/in2, 2.989 kilopascals (kPa), 29.89 millibars (mb), or 0.882 inches of mercury (in Hg). Foot per minute (ft/min or fpm) a traditional unit of velocity or flow rate. One foot per minute equals exactly 30.48 cm/min, 5.08 mm/s, or 0.018 288 miles per hour. Foot per second (ft/s or ft/sec or fps) a traditional unit of velocity. One foot per second equals exactly 15/22 mile per hour or exactly 1.097 28 kilometers per hour. Foot pound (ft·lbf or ft·lb) a traditional unit of work, equal to the work done by a force of one pound acting through a distance of one foot. This is equivalent to approximately 1.355 818 joule, 1.285 07 x 10-3 Btu, or 0.323 832 (small) calorie. Foot poundal a unit of work, equal to the work done by a force of one poundal acting through a distance of one foot. One foot poundal is equivalent to about 0.031 081 foot pound, or 0.042 140 joule. Foot pound per second (ft·lbf/s or ft·lb/s) a traditional unit of power equal to about 1.355 818 watt or 0.001 818 horsepower. Foot ton (ft·tn) a traditional unit of work, equal to the work done by a force of one ton acting through a distance of one foot. In the U.S. system, a foot ton equals 2000 foot pounds (2.7116 kilojoules or 2.570 Btu); in the British Imperial system a foot ton equals 2240 foot pounds (3.0370 kilojoules or 2.8786 Btu). Force a measurement of wind velocity based on the Beaufort scale. Fortnight a traditional English unit of time equal to 2 weeks or 14 days. The word, a contraction of "fourteen nights," has been used since at least the 1100s. Fot the traditional Swedish foot, also called the Stockholm foot, equal to 11.689 inches or 29.69 centimeters. The fot was divided into 2 kvarter or into 12 tum. Fother a traditional English unit of weight for lead. The fother, equal to 30 fotmals (next entry), was always a little smaller than a (long) ton. The original version seems to have been equivalent to 2160 avoirdupois pounds, and the version still being used in the nineteenth century was equal to 19.5 hundredweight or 2184 pounds. Fotmal a traditional English unit of weight for lead. In medieval England the fotmal was equal to 70 "mercantile" pounds, which is equivalent to 72 avoirdupois pounds. Fourth a unit used in music to describe the ratio in frequency between notes. Two notes differ by a fourth if the higher note has frequency exactly 4/3 times the frequency of the lower one. On the standard 12-tone scale, the perfect fourth is very closely approximated as 5 half steps, corresponding to a frequency ratio of 25/12 = 1.3348. Fpm, fps common abbreviations for foot per minute (ft/min) and foot per second (ft/s or ft/sec), traditional units of flow (see above). Frail a kind of basket, sometimes used as a commercial unit of weight for fruit. Depending on the item, a frail could equal anywhere from 32 to 75 pounds (15-34 kilograms). In the nineteenth century a frail of raisins was often taken to equal 50 pounds (22.68 kilograms). F ratio (f/) a measure of the light-gathering power of the lenses in cameras and telescopes. The f ratio, expressed as "f/4" or whatever, is the focal length of the lens (the distance from the lens to the point where light is focused) divided by the aperture (the diameter of the lens, which in the case of cameras may be reduced by the shutter mechanism). Thus "f/4," for example, indicates that the focal length is 4 times the aperture. In cameras the f ratio is proportional to the square root of the exposure time, so in comparison to an an f/4 lens an f/8 lens requires a four times ((8/4)2) longer exposure than than the f/4 lens. Some cameras have "zoom" lenses with variable focal length; for other cameras the focal length of the lens cannot be changed, so the shutter is used to reduce the aperture when longer exposure times are needed in order to match the available light to the speed of the film. Because of this connection with exposure times, the f ratio is often said to express the "speed" of a lens. Franklin (Fr) a CGS unit of electric charge equal to 3.3356 x 10-10 coulombs or 1 statcoulomb. An electrostatic charge of one franklin exerts a force of one dyne on an equal charge at a distance of one centimeter. The unit honors Benjamin Franklin (1706-1790), one of the leaders of the American Revolution, who was also an early investigator of electricity. Freight ton originally a unit of volume equal to 40 cubic feet; see ton [5]. More recently, another name for the revenue ton. French (Fg or Ch) a unit of distance used for measuring the diameters of small tubes such as catheters, fiber optic bundles, etc. One French is equal to 1/3 millimeter (about 13.123 mils). The name and the symbol Ch refer to the Charrière gauge scale, which is often called the French scale. Frigorie a unit of refrigeration formerly used in Europe, equal to 1 (kilogram) Calorie per hour (4.1868 kilojoules per hour, or 3.9683 Btu per hour). The name was coined from the calorie by replacing the Latin calor, for heat, with frigor, for cold. Fuder a traditional German unit of liquid volume. A "fuder" is a cartload. In most of the German states the traditional fuder held about 9 hectoliters (roughly 238 U.S. gallons), making the fuder about the same size as the British tun or the French wine tonneau. In the Mosel wine region of Germany, a fuder is now a metric unit equal to 10 hectoliters (1 cubic meter, or 264.17 U.S. gallons). In Austria, the traditional fuder was equal to 18.11 hectoliters (478.42 U.S. gallons), twice the size of the German unit. In Belgium today, the fuder (or foudre) is a metric unit equal to 30 hectoliters (792.52 U.S. gallons). Fujita-Pearson scale (F) an empirical scale for estimating the wind speed of a tornado from the damage it causes. The scale, developed by the American meteorologists Theodore Fujita and Allen Pearson, ranges from F0 to F5. Full step see step [2]. Funt a traditional Russian unit of weight or mass corresponding the German pfund (which is pronounced "funt"). The funt is equal to about 0.9028 pound avoirdupois or 409.5 grams. The plural is funte. Furlong (fur) a traditional unit of distance. Long before the Norman Conquest in 1066, Saxon farmers in England were measuring distance in rods and furlongs and areas in acres. The word "furlong", from the Old English fuhrlang, means "the length of a furrow"; it represents the distance a team of oxen could plow without needing a rest. A furlong equals 40 rods, which is exactly 10 chains, 220 yards, 660 feet, or 1/8 mile. One furlong is exactly 201.168 meters, so a 200-meter dash covers a distance very close to a furlong. The length of horse races is often stated in furlongs. Fuss the German foot (see foot, above). The length of the fuss varied somewhat; the Viennese version was equal to 12.444 inches or 31.608 centimeters, while the Rheinfuss (Rhine foot), used in much of western and northern Germany, was equal to 12.357 inches or 31.387 centimeters. In Bavaria a shorter fuss of about 29 centimeters was used. There's no change in the plural. ##END_OF_RECORD g a symbol for the average acceleration produced by gravity at the Earth's surface (sea level). The actual acceleration of gravity varies from place to place, depending on latitude, altitude, and local geology. The symbol g is often used informally as a unit of acceleration. By agreement among physicists, the standard acceleration of gravity gn is defined to be exactly 9.806 65 meters per second per second (m/s2), or about 32.174 05 feet per second per second. At latitude p, a conventional value of the acceleration of gravity at sea level is given by the International Gravity Formula, g = 978.0495 [1 + 0.0052892 sin2(p) - 0.0000073 sin2 (2p)] centimeters per second per second (cm/s2). The symbol g was used as a unit first in aeronautical and space engineering, where it is important to limit the accelerations experienced by the crew members of aircraft and spaceships: the "g forces," as they are called. This use became familiar through the space programs, and now a variety of accelerations are measured in g's. The name grav is also used for this unit. Note that g is also the symbol for the gram. G informal abbreviation in computer science for 230 = 1 073 741 824. See also giga- [2]. Ga symbol for one billion (109) years. The a stands for the Latin annus, year. Gage alternate spelling for gauge (see below). Galactic year the unit of time in which the Solar System makes one revolution around the Milky Way galaxy. The galactic year is estimated to be about 225 million ordinary years. The age of the Solar System is about 20 galactic years. Galileo (Gal or gal) the CGS unit of acceleration. One galileo is an acceleration of 1 centimeter per second per second (cm/s2). This unit is used by geologists, who make careful measurements of local variations in the acceleration of gravity in order to draw conclusions about the geologic structures underlying an area. These variations are typically measured in milligals (mGal). One Gal is approximately 0.001 019 7 g, so a milligal is a very small acceleration, about 10-6 g. The name of the unit honors the Italian astronomer and natural philosopher Galileo Galilei (1564-1642), who proved that all objects at the Earth's surface experience the same gravitational acceleration. To avoid confusion with the symbol for the gallon, and to conform to the usual metric style, the symbol for this unit should be Gal rather than gal. Gallon (gal) [1] a traditional unit of liquid volume, derived from the Roman galeta, which originally meant a pailful. Gallons of various sizes have been used in Europe ever since Roman times. In the United States, the liquid gallon is legally defined as exactly 231 cubic inches; this is equal to the old English wine gallon, which originated in medieval times but was not standardized until 1707, during the reign of Queen Anne. Some scholars believe the wine gallon was originally designed to hold 8 troy pounds of wine. The U. S. gallon holds 4 liquid quarts or exactly 3.785 411 784 liters; a U.S. gallon of water weighs about 8.33 pounds. American colonists were also familiar with the Elizabethan beer and ale gallon, which held 282 cubic inches (4.621 liters). Gallon (gal) [2] a historic British unit of dry volume still used implicitly in the U.S. In the U. S., the term "gallon" is not used in dry measure, but if it were it would be equal to 1/2 peck, or 4 dry quarts, or 268.8025 cubic inches, or approximately 4.404 884 liters. This unit is the English corn or grain gallon, standardized during the reign of Elizabeth I in the sixteenth century. The earliest official definition of a dry gallon in Britain is a 1303 proclamation of Edward I, where the gallon is defined as the volume of 8 pounds of wheat; the current U.S. "gallon" contains about 7.5 pounds of wheat. Grain gallons have tended to be larger than liquid gallons throughout the history of British units, apparently because they were based on heaped rather than "struck" (leveled) containers. A container in which grain has been heaped above the top will hold about 25% more grain, and the traditional corn gallon is in fact 22.4% larger than the wine gallon. Gallon (gal) [3] currently the British use a larger gallon than either of the American gallons. The Imperial Weights and Measures Act of 1824 established a new unit for all volumes, liquid or dry, replacing all the other gallons in previous use in Britain. The imperial gallon, designed to contain exactly 10 pounds of distilled water under precisely defined conditions, holds exactly 4.546 09 liters or approximately 277.4194 cubic inches. The imperial gallon equals 1.20095 U.S. liquid gallons (British wine gallons) or 1.03206 U.S. dry gallons (British corn gallons). Gamma [1] a unit of magnetic flux density equal to 10-9 tesla (1 nanotesla) or 10-5 oersted (10 µOe). In geophysics, small changes in the Earth's magnetic field are traditionally stated in gammas. The nanotesla (nT) is now recommended for these measurements. Gamma [2] an informal metric unit of mass equal to 1 microgram (µg). Garnets a traditional unit of liquid volume in Russia. The garnets is equivalent to approximately 3.28 liters; this is about 3.47 U.S. quarts or 2.89 British imperial quarts. Gauge (ga) [1] a traditional unit measuring the interior diameter of a shotgun barrel. The gauge of a shotgun was the number of lead balls, each of a size just fitting inside the barrel, that were required to make up a pound. In other words, if a lead ball weighing 1/12 pound just fit in the barrel of a shotgun, then it was a 12-gauge shotgun. Today, the internal diameters for each gauge number are read from a table. Gauge (ga) [2] a unit expressing the fineness of a knitted fabric, equal to the number of loops per 1.5 inches (38.1 millimeters). The same unit is also used to express the size of the knitting needles used to create a fabric of that fineness. Gauge (ga) [3] a traditional unit measuring the diameter (or the cross-sectional area) of a wire. Various wire gauge scales have been used in the U.S. and Britain. In traditional scales, larger gauge numbers represent thinner wires. In the American Wire Gauge (AWG) scale, 0000 gauge represents a wire having a diameter of 0.46 inch and 36 gauge represents a diameter of 0.005 inch (5 mils). Diameters for the other gauges are obtained by geometric interpolation, meaning that the ratio between successive diameters is a constant, except for necessary roundoff. Thus n gauge wire has a diameter of .005·92((36-n)/39) inch (gauges 00, 000, and 0000 are interpreted as -1, -2, and -3, respectively, in this calculation). The metric wire gauge number is equal to 10 times the diameter of the wire, in millimeters; thus a metric 8 gauge wire has diameter 0.8 millimeters. A table of wire gauge equivalents is provided. Gauge (ga) [4] a traditional unit measuring the thickness of sheet metal. Larger gauge numbers represent thinner metal: 10 gauge represents a thickness of 0.1345 inch (3.416 millimeters) and each increase of 1 in the gauge number corresponds to a reduction of about 10% in the thickness. A table is provided. Gauss (G or Gs) [1] the CGS unit of magnetic flux density. A field of one gauss exerts, on a current-carrying conductor placed in the field, a force of 0.1 dyne per ampere of current per centimeter of conductor. One gauss represents a magnetic flux of one maxwell per square centimeter of cross-section perpendicular to the field. In SI units, one gauss equals 10-4 tesla. The unit is named for the German mathematician and astronomer Karl Friedrich Gauss (1777-1855). Gauss (G or Gs) [2] a former name for the CGS unit of magnetic field strength, officially renamed the oersted in 1930. Gauss (G or Gs) [3] the CGS unit of magnetic dipole moment per unit volume, more commonly written emu/cm3 or emu/cc. In this use the gauss equals 1000 amperes per meter in SI units. GDU a symbol for gelatin digesting unit, used for measuring dosage of bromelain, an enzyme used as a digestive aid and for reduction of pain and inflammation. This unit cannot be converted to a weight unit, because different preparations of the enzyme differ in activity. Bromelain is also measured in milk clotting units (MCU); 1 GDU equals approximately 1.5 MCU. Gee pound another name for a slug. Generation (gen) an informal unit of time. Roughly speaking, a generation is the average length of time between the birth of a parent and the birth of the child. This leaves a question, however: should just the father, or just the mother, or both parents be included in the calculation? Various answers to this question, plus a lack of consistent data, have led to a range of estimates for the length of a generation, from about 25 to 35 years. Genealogists tend to use the higher figures, anthropologists the lower ones. There is some research suggesting that the approximate length of the generation in the world today is about 28 years. Geographical mile another name for the nautical mile, especially the Admiralty mile (6080 feet or 1853.184 meters). For a (different) German use of the term geographische meile, see meile. GeV the symbol for one billion (109) electronvolts. Thanks to Einstein's equation E = mc2 equating mass wth energy, the GeV can be regarded either as a unit of energy equal to 160.217 646 2 picojoules, or as a unit of mass equal to 1.782 662 x 10-24 gram or 1.073 544 atomic mass unit. G/g a symbol for "gram per gram", a unit of mass concentration. For example, a concentration of 0.02 g/g, or 2% g/g, means that the substance being measured comprises 2% of the mass of the mixture in which it is found. This symbol is equivalent to the traditional symbol w/w. Gibi- (Gi-) a binary prefix meaning 230 = 1 073 741 824. This prefix, adopted by the International Electrotechnical Commission in 1998, is supposed to replace giga- for binary applications in computer science. (This innovation does not appear to be sucessful.) The prefix is a contraction of "gigabinary." Gig an informal contraction of the gigabyte (GB) common in computer science, especially as a unit of storage capacity. Giga- (G-) a metric prefix denoting 109 or one billion (in the American meaning of the word billion). This prefix was coined from the Greek word gigas, meaning giant. The Greeks pronounced the word with a hard g, as in "gig." However, the prefix is usually pronounced with a soft g, as in "jig." Dictionaries usually list the hard g pronunciation as a second choice. Gigabecquerel (GBq) a unit of radioactivity equal to one billion atomic disintegrations per second or 27.027 millicuries. Gigaflops (Gflops) a unit of computing power equal to one billion (109) floating point operations per second. See flops. Gigagram (Gg) an SI unit of mass equal to 109 grams, one million kilograms, or 1 kilotonne (1000 metric tons). This unit is used in scientific contexts, such as the measurement of atmospheric emissions. One gigagram equals about 2.2046 million pounds or 1102.3 U.S. (short) tons. Gigahertz (GHz) a unit of frequency equal to 109 per second, or 1 per nanosecond. Cellular phones and microwave ovens operate with radio waves having frequencies in the gigahertz range. Gigajoule (GJ) a metric unit of energy commonly used in the energy industry. One gigajoule equals 947 817 Btu, 277.7778 kilowatt hours, or about 9.48 therms. Gigaliter (GL or Gl) a metric unit of volume equal to 109 liters or one million cubic meters. This is equivalent to 810.713 acre feet or 35.315 million cubic feet. Gigameter (Gm) a metric unit of distance equal to one million kilometers or about 621 371.2 miles. Seldom used, this unit would be a good yardstick for the inner Solar System: the distance from the earth to the sun is about 149.60 gigameters. Gigaparsec (Gpc) a non-metric unit of distance equal to one billion parsecs, 3.2616 billion light years, or 30.856 78 zettameters (30.856 78 x 1021 kilometers). The unit is used in astronomy. Gigapascal (GPa) a metric unit of pressure. One gigapascal equals 10 kilobars or approximately 145 038 pounds (72.519 short tons) per square inch. Pressures in this range are common inside the earth and can be produced by various high-energy events. Gigatonne (Gt) a metric unit of mass or weight equal to one billion metric tons (tonnes) or about 2.2046 trillion pounds. This very large unit is used, for example, in discussing the amounts of carbon added to the atmosphere by human activities. Gigawatt (GW) a metric unit of power equal to one billion (109) watts or about 1.341 million horsepower. Gigawatt hour (GW·h) a metric unit of energy equal to one billion watt hours, 3.6 terajoules, or about 3.412 billion Btu. Gigayear (Gyr) a unit of time equal to one billion years. The international symbol Ga is also used, the "a" being taken from the Latin word annus for a year. Gilbert (Gi) the CGS unit of magnetomotive force, equal to 10/4pi = 0.795 775 ampere-turns. The unit is named for the English physicist and physician William Gilbert (1540-1603), who published his discoveries on magnetism in 1600. Gill (gi) a traditional unit of volume for liquids, especially wine and other alcoholic beverages. The gill is 1/4 pint. In the U. S. customary system, one gill is equal to 1/2 cup, 4 fluid ounces, 7.21875 cubic inches, or about 118.3 milliliters. In the British Imperial system, the gill equals 5 fluid ounces, 8.670 cubic inches, or about 137.7 milliliters. The unit is pronounced "jill", with a soft "g" sound. Its name comes a Latin word gillo for a small wine vessel. Gillion an informal alternate name for the number 109, called "billion" in America but often called "milliard" in France and "thousand million" in Britain. The reasoning here is that if mega- means a million and tera- a trillion, then giga- should mean a gillion! Gin one of several spellings used in English for the jin, the Chinese weight unit also called the catty. G/kg a symbol for "gram per kilogram", a unit of mass concentration equal to 1 per mill (1 part per thousand). 1 g/kg also equals 0.1% g/g or 0.1% w/w. Glasgow coma scale (GCS) a scale used in emergency medicine to assess the condition of trauma victims and other partially-conscious patients. The scale range is 3-15; patients with Glasgow scores of 3-8 are usually said to be in a coma. The score is obtained by observing the patient's eye-opening, motor response, and response to commands. A table is provided. Glass [1] a unit of time measured by an hourglass or sandglass. At sea, time was traditionally measured with half-hourglasses, making the glass a nautical unit of time equal to 1/2 hour. In this use the glass is another name for the bell. Glass [2] another name for the U.S. cup (236.6 milliliters). Doctors in the U.S. are fond of saying that everyone should drink 8 glasses of water a day, and this is the amount they have in mind for a glass. Glass [3] an informal unit of volume used in Australian pubs. In the eastern states of Australia a glass of beer is usually 200 milliliters, but in Western Australia it is 285 milliliters. Glean an old English unit of quantity for herrings, equal to 25 fish. Gm an incorrect symbol for the gram (see below). The correct symbol is g. GMT abbreviation for Greenwich Mean Time, the standard time of longitude 0°. This meridian of longitude, called the prime meridian, was fixed as the longitude of the Royal Greenwich Observatory in London by the International Meridian Conference of 1884. GMT is five hours later than U.S. Eastern Standard Time. The abbreviation UT (Universal Time) has largely replaced GMT. Gnat's eye an idiomatic "unit" of distance; it's common to hear that something is "as small as a gnat's eye." In fact, the eyes of typical gnats tend to have diameters similar in size to a hair's breadth: roughly 100-150 micrometers (0.10-0.15 millimeters). Go a traditional Japanese unit of liquid volume. One go is about 180.39 milliliters, 0.3812 U.S. pint, or 0.3174 British imperial pint. Goad a traditional unit of distance sometimes used in measuring cloth. One goad is equal to 54 inches or 1.5 yards (1.3716 meters). A goad was originally a spear; later it was a pointed rod used for prodding animals to get a move on. The unit must have originated as the length of such an instrument. Gon (g) another name for the grad, a unit of angle measurement equal to 1/400 circle, 0.01 right angle, 0.9°, or 54'. This name is used especially in German, Swedish, and other languages in which the word grad means degree. The name is taken from the ancient Greek word gonia, angle. Gong- in the Pinyin system for transliterating Chinese, the word gong (preferably with a macron, or long-vowel sign, over the o) represents a character whose basic meaning is "public" or "official." This meaning has been extended in recent years to include "metric." Thus a gongjin is a metric jin, that is, a kilogram, and a gongli is a metric li, or kilometer. Googol a unit of quantity equal to 10100 (1 followed by 100 zeroes). The googol was invented by the American mathematician Edward Kasner (1878-1955) in 1938. According to the story, Kasner asked his nephew Milton Sirotta, who was then 8 years old, what name he would give to a really large number, and "googol" was Milton's response. Kasner also defined the googolplex, equal to 10googol, that is, 1 followed by a googol of zeroes. These inventions caught the public's fancy and are often mentioned in discussions of very large numbers. In the traditional American system for naming large numbers, the googol is equal to 10 dotrigintillion. Gpg customary symbol for grains per gallon (gr/gal), a traditional unit measuring the hardness of water. Water is "hard" if it contains dissolved minerals such as calcium or magnesium salts. 1 gpg is equivalent to about 17.118 milligrams per liter (mg/L). This unit is also called the Clark degree; see degree [4]. Gpm, gps customary symbols for gallons per minute (gal/min) and gallons per second (gal/s), traditional units for measuring the flow of liquids. 1 gpm equals about 3.785 41 liters per minute (L/min) if U.S. gallons are meant, or exactly 4.546 09 L/min if British imperial gallons are meant. Gr an ambiguous symbol, traditionally used in English both for the grain and the gross. It has also been used, very improperly, for the gram (g). Grad or grade or gon (g or grd) [1] a unit of angle measurement equal to 1/400 circle, 0.01 right angle, 0.9°, or 54'. This unit was introduced in France, where it is called the grade, in the early years of the metric system. The grad is the English version, apparently introduced by engineers around 1900. The name gon is used for this unit in German, Swedish, and other northern European languages in which the word grad means degree. Although many calculators will display angle measurements in grads as well as degrees or radians, it is difficult to find actual applications of the grad today. Grade [2] a measure of the steepness of a slope, such as the slope of a road or a ramp. Usually stated as a percentage, the grade is the same quantity known as the slope in mathematics: the amount of (vertical) change in elevation per unit distance horizontally ("rise over run"). Thus a 5% grade has an elevation gain of 0.05 meter for each meter of horizontal distance, or 0.05 foot for each foot of horizontal distance. The angle of inclination, in grads or grades [1], is not equal to the percentage grade in this sense: for a 5% grade the angle of inclination is about 2.86° or 3.18 grads. Grade [3] a measure of quality for ball bearings. A bearing of grade g is required to be spherical to an accuracy of g parts per million (g 10-6). Thus lower grade numbers represent better bearings; a 25 grade bearing is spherical to within 25 ppm, but a 1000 grade bearing is only spherical to within 1000 ppm, or 0.1%. Grade point (gp) a unit of academic recognition used in U.S. schools and colleges. To compute the number of grade points awarded to a student for a course, the student's grade (often assigned as a letter or a percentage) is converted to a standard scale (traditionally 0.0-4.0, with 4.0 being the highest score). This scale number is multiplied by the number of semester hours or quarter hours assigned to the course. Thus a student who has a grade of A (4.0) on a course carrying 3 semester hours of credit receives 12 grade points. Grain (gr) [1] a traditional unit of weight. The grain, equal to 1/480 troy ounce (see also pound [2]), or exactly 64.798 91 milligrams, was the legal foundation of traditional English weight systems, with various pounds being defined as a specified number of grains: 5760 grains in a troy pound and 7000 grains in an avoirdupois pound, for example. In the version of the troy system used by jewelers, there are 24 grains in a pennyweight and 20 pennyweight in an ounce. In the version used by apothecaries, there are 20 grains in a scruple, 3 scruples in a dram, and 8 drams in an ounce. Originally the grain was defined in England as the weight of a barleycorn. This made the English grain larger the corresponding grain units of France and other nations of the Continent, because those units were based on the weight of the smaller wheat grain. Grain (gr) [2] a unit of weight formerly used by jewelers in measuring diamonds and other precious stones. The jeweler's grain is exactly 1/4 carat. Now that the carat has been standardized at 200 milligrams, the jeweler's grain is exactly 50 milligrams, or approximately 0.7716 troy grain. Grain (gr) [3] a traditional French unit of weight equal to 53.115 milligrams. Grain per gallon (gr/gal or gpg) see gpg above. Gram (g) a unit of mass in the metric system. The name comes the Latin gramma, which was a small weight similar to the English grain. The gram was originally defined to be the mass of one cubic centimeter of pure water, but to provide precise standards it was necessary to construct physical objects of specified mass. One gram is now defined to be 1/1000 of the mass of the standard kilogram, a platinum-iridium bar carefully guarded by the International Bureau of Weights and Measures in Paris for more than a century. (The kilogram, rather than the gram, is considered the base unit of mass in the SI.) The gram is a small mass, equal to about 15.432 grains or 0.035 273 966 ounce. The original French spelling gramme is sometimes used. Note: The only correct symbol for the gram is g. The abbreviations "gm" and (worse) "gr" should never be used. Gram atom, gram atomic weight older names for the atomic mass unit. Gram calorie the CGS unit of heat energy; see calorie. Gram equivalent (gEq), gram equivalent mass (GEM), gram equivalent weight (GEW) various names for the mass in grams of a substance that would react with or replace one gram of hydrogen. See equivalent, the shorter name now used in many contexts for this unit. Gram mole, gram molecule, gram molecular weight (gmol or gmole) older names for the mole. Gram rad a name sometimes used for the rad, a unit of radiation dose. Gram weight or gram force (gf) an informal unit of weight equal to the force exerted on a mass of one gram by gravity at the Earth's surface: approximately 980.665 dynes, 9.806 65 millinewtons, or about 0.002 2045 pounds of force in the English system. Gran a traditional German weight unit, varying in size but typically about 60 milligrams. Grand U.S. slang for 1000, especially the sum of 1000 dollars. Grano a traditional Italian weight unit, varying in size but typically about 50 milligrams. The plural is grani. Grav (g) another name for the unit of acceleration usually called the g (see above). Gray (Gy) the SI unit of radiation dose. Radiation carries energy, and when it is absorbed by matter the matter receives this energy. The dose is the amount of energy deposited per unit of mass. One gray is defined to be the dose of one joule of energy absorbed per kilogram of matter, or 100 rad. The unit is named for the British physician L. Harold Gray (1905-1965), an authority on the use of radiation in the treatment of cancer. Great gross a traditional unit of quantity equal to a dozen gross, or 1728. Great hundred a traditional unit of quantity equal to 120 instead of 100. The great hundred equals 6 score or 2 shocks. Merchants in medieval England used both the great hundred and the gross (144) in specifying quantities, but more recently the gross has been much more common. Great year the Platonic year. Gregorian year a unit of time equal to exactly 365.2425 days, the average length of a year in the Gregorian calendar currently used throughout the world. See year [2]. Grit a measure of fineness for abrasive materials such as sandpaper, sanding belts, or the finer materials used to polish optical surfaces. Originally the grit number was the number of holes in a standard screen; if the screen had, say, 240 holes, then the particles that would pass through the screen were described as 240 grit. However, very fine abrasive particles (such as 500 to 1000 grit) are too small to be screened in this way, so the measure is defined by tables giving the average particle size in micrometers for each grit size. A table of grit sizes is provided. Gros a traditional French weight unit equal to 3 deniers (about 3.824 grams). Gross (gro or gr) a unit of quantity equal to a dozen dozen, or 144. This commercial unit has been in use since at least the 1400's. Gross ton (GT) the name "gross ton" is used in at least two ways: (1) as another name for the British Imperial or long ton of 2240 pounds (see ton [1]), and (2) as another name for the register ton, a unit of volume equal to 100 cubic feet (see ton [3]), in describing the entire interior volume of a ship as opposed to the cargo-carrying capacity. To avoid confusion, it is better to use "long ton" for use (1) and "gross register ton" for use (2). Gry a proposed unit of distance in the English traditional system. The name was first used in June 1679 by the philosopher John Locke (1632-1704) as a unit equal to 0.001 foot, 0.01 inch, or 0.1 line in a decimalized distance system. (Thomas Jefferson, who was very familiar with Locke's writings, later proposed a similar system in the U.S., but he called 0.001 foot a point rather than a gry.) In 1813, the gry was revived in another decimal measurement scheme in Britain. All these ideas failed, but the gry had some limited use in the nineteenth century as a unit equal to 0.1 line or 1/120 inch (0.211 667 millimeter). Long forgotten, the gry has recently come back into the limelight in connection with a puzzle, circulating on the Internet, which asks for three English words ending in -gry. The word "gry" is from the ancient Greek, where it meant "a trifling amount." Gsm a common but non-standard symbol for grams per square meter (g/m2), the metric unit of density for paper and for fabric. Paper density measured in this unit is often called grammage. Gt, gtt traditional pharmicist's abbreviations for a drop. Originally, gt was the singular (1 gt) and gtt the plural. G/t symbol for grams per tonne (metric ton), a unit of proportion equal to 0.001 g/kg or 1 part per million by mass. Gunter's chain the traditional surveyor's chain equal to 4 rods; see chain. ##END_OF_RECORD H Planck's constant, equal to approximately 0.662 606 876 x 10-33 joule second, a fundamental constant of physics also used as a unit of "action" or of angular momentum in particle physics. The unit was defined by the German physicist Max Planck (1858-1947), who showed in 1900 that at atomic and subatomic scales energy occurs in discrete packets called quanta. Each quantum has energy h·f, where f is the frequency of the radiation in hertz (see below). Hacienda a large traditional unit of land area in Mexico and the southwestern U.S. The word also refers to a large estate, ranch, or plantation. As a unit, it equals 5 square leguas or 125 million square varas. Using the Texas definition of the vara, this would be about 8960 hectares or 22 140 acres (34.59 square miles). Using the shorter Mexican vara, it would be about 8778 hectares or 21 690 acres. Hair's breadth a common informal unit of distance. Human hairs vary considerably in width, depending on age, color, genetics, and other factors. An average hair is approximately 70 to 100 micrometers (µm) in diameter, so, as a rough standard, a hair's breadth is 100 µm or 0.1 millimeter (mm). Halakim (hl) plural of helek (see below). Half a unit of proportion equal to 1/2. The English word "half," like the German prefix "halb-," is often placed before the name of a unit to create a combination that functions as a new unit equal to 1/2 the old one. Half dozen, half hour, and half gallon are typical and common examples. Half life a unit of relative time measuring the rate at which a radioactive substance decays, or (more generally) the rate of decrease for any process that decreases exponentially. In the case of radioactivity, the half life is the time required for the activity to be reduced by half. After a second half life, the activity is again reduced by half, so it is then 1/4 the original activity. To reduce the activity to 0.1% of the original amount requires about 9.966 half lives. Half step a unit used in music to describe the ratio in frequency between notes. Equal to 1/12 octave, the half step measures the difference between adjacent notes in the standard 12-tone scale, as on a piano keyboard. Two notes differ in frequency by a half step if the higher one has frequency equal to 21/12 = 1.0595 times the frequency of the lower one. Hand a traditional unit of distance, now used mostly to measure the height of horses. One hand equals 4 inches, 1/3 foot, or 10.16 centimeters. Hank a traditional measure of length for yarn. The length of yarn in a hank varies with the market and the material; for example, a hank of cotton yarn traditionally included 840 yards (768 meters) of yarn, while a hank of wool yarn was 560 yards (512 meters). In the U.S., however, a hank of wollen yarn is generally 1600 yards (1463 meters). For both cotton and wool, the hank is equal to 7 leas or to 12 cuts. In retail trade, a hank is often equal to 6 or 7 skeins of varying size. Hardness a measure of the hardness of a metal or mineral. We all know what it means for a substance to be "hard," but it's not so easy to quantify and measure this property. The Mohs hardness scale is used in geology to give a rough estimate of hardness by testing which minerals are able to scratch the sample. In metallurgy, samples are tested for hardness by machines that indent the surface under a controlled pressure. The resulting measurement is often computed as the force applied divided by the surface area of the indentation. The Brinell, Vickers, Rockwell, and Knoop tests are among the techniques used. Plastics, rubber, and similar materials are tested with instruments called durometers and the resulting readings are often designated duro. The arbitrary pencil hardness scale is also used to measure the hradness of paints and other semi-soft coatings. Hartley (Hart) a unit of information content used in information and communications theory. The hartley is similar to the shannon. If the probability of receiving a particular message is p, then the information content of the message is -log10 p hartleys. For example, if a message is a string of 5 letters or numerals, with all combinations being equally likely, then a particular message has probability 1/365 and the information content of a message is 5(log10 36) = 7.7815 hartleys. One hartley equals log2 10 = 3.321 928 shannons or loge 10 = 2.302 585 nats. The unit honors the American engineer Ralph V.L. Hartley, who introduced the idea of measuring information content in a famous 1928 paper. Hartree (Eh) a unit of energy used in atomic and moleclar physics, equal to about 4.3598 x 10-18 joule or 27.212 electron volts. The unit is named for the British physicist and mathematician Douglas R. Hartree (1897-1958). Hat size in the metric world, the size of a hat is simply the circumference C of the head it's meant to cover, in centimeters. The traditional U.S. hat size was defined to equal C/pi, in inches; this would be the diameter if the head were circular in cross section. Thus the U.S. hat size equals the metric size divided by 2.54pi, which is almost exactly 8. Traditional British hat sizes are equal to U.S. sizes minus 1/8. Thus metric size 60 is equivalent to U.S. size 60/8 = 7 1/2 and British size 7 3/8. A table is provided. Hat trick an informal unit of quantity equal to 3. The unit is used in sports in counting goals, especially goals scored by the same player. It originated in cricket but is more familiar to North Americans in ice hockey. No one seems to know for sure why 3 goals came to be called a hat trick. According to one popular account, cricket clubs in England sometimes awarded a hat to a bowler who took 3 wickets with successive balls--but some people think this was an effect of the expression rather than a cause. Head (hd) an informal unit of length, equal to the approximate length of a horse's head, used in expressing the results of a horse race. Heaped bushel a traditional unit of volume in the United States, the heaped bushel is just what its name implies: the volume of a bushel container filled to overflowing. Officially, the heaped bushel was supposed to equal 1.278 regular, or "struck" bushels; this is a volume of 2748.237 cubic inches, 1.5904 cubic feet, or 45.036 liters. In practice, the heaped bushel was frequently interpreted as 1.25 bushels, which is equal to exactly 10 dry gallons, 2688.025 cubic inches, 1.5556 cubic feet, or 44.049 liters. Heat index (HI or HX) a measure of the combined effect of heat and humidity on the human body. U.S. meteorologists compute the index from the temperature T (in °F) and the relative humidity H (as a fraction; that is, H = 0.65 if the relative humidity is 65%). The formula used is HI = -42.379 + 2.04901523 T + 1014.333127 H - 22.475541 TH - .00683783 T2 - 548.1717 H2 + 0.122874 T2H + 8.5282 TH2 - 0.0199 T2H2. Heating degree day (HDD) see degree day. Heat unit an alternate American name for the British thermal unit (Btu). This is not the same as the Celsius heat unit (Chu). Hebdo- an obsolete metric prefix denoting ten million (107). It was coined from the Greek hebdomos, seventh. A group of seven is sometimes called a hebdomad, a word related to the French hebdomadaire, weekly. Hect- or hecto- (h-) a metric prefix denoting 100, coined from the Greek word hekaton for one hundred. Hectare (ha) the customary metric unit of land area, equal to 100 ares. One hectare is a square hectometer, that is, the area of a square 100 meters on each side: approximately 107 639.1 square feet, 11 959.9 square yards, or 2.471 054 acres. Hectobar (hbar) a rarely-used metric unit of pressure. One hectobar equals 100 bars, 10 megapascals, or about 1450.3 pounds per square inch. Hectogram (hg) a common metric unit of mass, equal to 100 grams or about 3.5274 ounces. Hectoliter (hL or hl) a common metric unit of volume. The hectoliter equals 100 liters, 0.1 cubic meter, 26.417 U.S. liquid gallons, 21.999 British imperial gallons, or 3.5315 cubic feet. Hectometer (hm) a metric unit of distance equal to 100 meters, 328.084 feet, or 36.4538 yards. This unit isn't used much in everyday life in metric countries, but it appears in various scientific contexts. Hectopascal (hPa) a metric unit of pressure equal to 100 pascals or 0.1 kilopascal (kPa). The hectopascal, used almost entirely in measurements of air pressure, is identical to the millibar (mb). The millibar has been used to measure air pressure for many years, but it is not an SI unit. Although meteorologists in various countries use the hectopascal as a kind of alias for the millibar, the proper SI unit for air pressure is the kilopascal. Heer a traditional measure of length for linen and woolen yarn, equal to 2 cuts or 1/6 hank (see above). This is equivalent to 80 yards (73.152 meters). Hefner candle (HC) or Hefnerkerze (HK) a former unit for measuring the intensity of light. The unit is named for F. von Hefner-Altenack (1845-1904), who invented a laboratory light standard, the Hefner lamp, which burned isopentyl acetate to provide light of intensity 1 HK. One Hefner candle is equivalent to approximately 0.902 candela (cd). Helek (hl) a traditional Hebrew unit of time equal to 1/18 minute or 10/3 seconds. The plural is halakim. Halakim are used in formulas establishing the molad, the instant of new moon, which marks the start of the month in the Hebrew calendar. Hemidemisemiquaver the shortest unit of relative time in music, equal to 1/8 quaver, 1/64 whole note or 1/128 breve. Hemina a Roman unit of liquid volume, equal to 1/2 sextarius or about 265.6 milliliters (0.561 U.S. pint or 0.467 British Imperial pint). Hemisphere a traditional unit of solid angle equal to 1/2 sphere, 2pi steradians, or about 20 626.48 square degrees. Henry (H) the SI unit of electric inductance. A changing magnetic field induces an electric current in a loop of wire (or in a coil of many loops) located in the field. Although the induced voltage depends only on the rate at which the magnetic flux changes, measured in webers per second, the amount of the current depends also on the physical properties of the coil. A coil with an inductance of one henry requires a flux of one weber for each ampere of induced current. If, on the other hand, it is the current which changes, then the induced field will generate a potential difference within the coil: if the inductance is one henry a current change of one ampere per second generates a potential difference of one volt. The henry is a large unit; inductances in practical circuits are measured in millihenrys (mH) or microhenrys (µH). The unit is named for the American physicist Joseph Henry (1797-1878), one of several scientists who discovered independently how magnetic fields can be used to generate alternating currents. The plural is sometimes spelled henrys, but in English it is correct to spell it henries. Heptad a unit of quantity equal to 7. Hertz (Hz) the SI unit of frequency, equal to one cycle per second. The hertz is used to measure the rates of events that happen periodically in a fixed and definite cycle; the becquerel, also equal to one "event" per second, is used to measure the rates of things which happen randomly or unpredictably. Multiples of the hertz are common: the frequencies of radio and television waves are measured in kilohertz (kHz), megahertz (MHz), or even gigahertz (GHz), and the frequencies of light waves in terahertz (THz). The unit is named for the German physicist Heinrich Rudolf Hertz (1857-1894), who proved in 1887 that energy is transmitted through a vacuum by electromagnetic waves. Hexad a unit of quantity equal to 6. Hexit a unit of information equal to 4 bits or 1/2 byte. A string of 4 bits has 16 possible states (0-15) and is usually represented as a single base-16, or hexadecimal, digit (in the hexadecimal system the letters A through F are used to represent the numbers 10 through 15, respectively). A hexit of data is also known as a nibble, a tetrad, or a quadbit. Hide a very old English unit of land area, dating from perhaps the seventh century. The hide was the amount of land that could be cultivated by a single plowman and thus the amount of land necessary to support a family. Depending on local conditions, this could be as little as 60 acres or as much as 180 acres (24-72 hectares). The hide was more or less standardized as 120 acres (48.6 hectares) after the Norman conquest of 1066. The hide continued in use throughout medieval times, but it is now obsolete. The unit was also known as the carucate. Hogshead (hhd) a traditional unit of volume for liquids. Originally the hogshead varied with the contents, often being equal to 48 gallons of ale; 54 of beer; 60 of cider; 63 of oil, honey, or wine; or 100 of molasses. In the United States, a hogshead is defined to hold 2 barrels, or 63 gallons; this was the traditional British wine hogshead. It is equal to exactly 14 553 cubic inches, or about 8.422 cubic feet (238.48 liters). In the British imperial system, the hogshead equals 1/2 butt, or 52.5 imperial gallons (8.429 cubic feet, or 238.67 liters). Thus the British imperial and American hogsheads are almost exactly the same size. No one seems to know for sure how this unit got its unusual name. Homestead a historic unit of area in the United States, equal to 160 acres (64.75 hectares). Under the Homestead Act passed by Congress in 1862, settlers in the western states were allowed to take title to a homestead of 160 acres of land by registering a claim, settling on the land, and cultivating it. A homestead equals 1/4 square mile, or 1/4 section in U.S. government terminology. Hoppus foot, hoppus board foot traditional units of volume in British forestry. In a 1736 manual of practical calculation, the English surveyor Edward Hoppus advised foresters to estimate the volume of wood in a log of length L and girth (circumference) G as L·(G/4)2. Since the correct formula is L·G2/(4·pi), the resulting figure, called the hoppus volume, is smaller than the actual volume of the log. It seems like a mistake, but in fact not all the wood in a log can be used, and the hoppus volume is a fairly reasonable estimate of the usable volume of wood in the log. Volume measurements made using the hoppus formula are stated in hoppus feet. In effect, this makes the hoppus foot a unit of volume equal to 4/pi = 1.273 cubic feet or 0.036 054 cubic meter. Similarly, the hoppus board foot is equal to 1/12 hoppus foot or 1.273 board feet, which is almost exactly 3 liters (0.00300 cubic meter). The British forestry industry changed its unit of timber measurement from hoppus feet to cubic meters in 1971. Hoppus ton (HT) a traditional unit of volume in British forestry. One hoppus ton is equal to 50 hoppus feet or 1.8027 cubic meters. Shipments of tropical hardwoods from Southeast Asia, especially shipments of teak from Myanmar (Burma), are still stated in hoppus tons. Horsepower (hp) a unit of power representing the power exerted by a horse in pulling. The horsepower was defined by James Watt (1736-1819), the inventor of the steam engine, who determined after careful measurements that a horse is typically capable of a power rate of 550 foot-pounds per second. This means that a horse, harnessed to an appropriate machine, can lift 550 pounds at the rate of 1 foot per second. Today the SI unit of power is named for Watt, and one horsepower is equal to approximately 745.6999 watts. (In Britain this is rounded off to exactly 745.7 watts, and slightly different values are also used in certain industries.) Outside the U.S., the English word "horsepower" is often used to mean the metric horsepower, a slightly smaller unit. See also: boiler horsepower, electric horsepower, water horsepower. Horsepower hour (hp hr) a unit of work or energy equal to the work done at the rate of 1 horsepower for 1 hour. The horsepower hour equals 1 980 000 foot pounds or approximately 2.685 megajoules, 2545 Btu, 641.1 (large) Calories, or 745.7 watt hours. Hounsfield unit (HU) a unit used in medical imaging (CT or MRI scanning) to describe the amount of x-ray attenuation of each "voxel" (volume element) in the three-dimensional image. The voxels are normally represented as 12-bit binary numbers, and therefore have 212 = 4096 possible values. These values are arranged on a scale from -1024 HU to +3071 HU, calibrated so that -1024 HU is the attentuation produced by air and 0 HU is the attenuation produced by water. Tissue and bone then produce attenuations in the positive range. The reading in Hounsfield units is also called the CT number. The unit is named for the British engineer Godfrey Hounsfield, who demonstrated the first CT scanner in 1972. For this invention he received the Nobel Prize in medicine in 1979. Hour (h or hr) [1] a traditional unit of time, equal to 60 minutes, or 3600 seconds, or 1/24 day. The custom of dividing the daylight into 12 hours goes back at least as far as the Babylonians, who liked to divide units by 12 because groups of 12 are easily divided into halves, thirds, or fourths. Later, when people wanted to express times at night, it was natural to divide the night into 12 hours as well, making 24 hours in the day. The word comes from an ancient Greek word hora which originally meant a season, especially a religious season, and hence a "defined" period of time. Hour (h or hr) [2] a unit of angular measure used in astronomy, equal to 1/24 circle or 15°. Objects are located in the sky by latitude and "right ascension". Latitude is measured in degrees from the equator to the poles, just as it is on the surface of the earth. Longitude is replaced by right ascension, measured in hours from the longitude, traditionally known as the First Point of Aries, at which the Sun crosses the equator on its northward journey in the spring. House in astrology, each sign of the Zodiac is called a house; thus the house can be considered a unit of angle measure equal to 1/12 circle or 30°. Hu a traditional Chinese unit of liquid volume. The hu contains about 51.77 liters, 13.676 U.S. gallons, or 11.389 British imperial gallons. Hubble a unit of distance sometimes used in astronomy. The hubble is a gigantic unit, equal to 109 light years. This is 9.4605 x 1021 kilometers (9.4605 yottameters, if you please), 5.8785 sextillion miles (U.S.), 63.240 x 1012 astronomical units, or 306.595 megaparsecs. In practice, most astronomers use the megaparsec for measuring such stupendous distances. The unit honors the American astronomer Edwin Hubble (1889-1953), who discovered the uniform expansion of the Universe explained by the Big Bang theory. Hue a numerical measure of color; see color units. Hundred [1] a unit of quantity. In commercial use in old England the term "hundred", or cent (C), did not always mean an even 100; sometimes it meant 120 (the "great hundred") or some other number, depending on the item. For fish, the exact number in a hundred varied with the species. Hundred [2] an old English unit of area equal to 100 hides (see above). This is roughly 12 000 acres, 5000 hectares, or 18.75 square miles. The hundred is approximately the area of a village with its associated fields, so the name "hundred" came to mean a minor division of a shire or county. This use carried over to the American colonies, where, for example, many of the early settlements in Virginia were called hundreds. Hundredweight (Cwt) a traditional unit of weight equal to 1/20 ton. The hundredweight is the English version of a commercial unit used throughout Europe and known in other countries as the quintal or the zentner. In general, this unit is larger than 100 pounds avoirdupois, so to fit the European market the hundredweight was defined in England as 112 pounds avoirdupois (about 50.8023 kilograms) rather than 100 pounds. This definition apparently dates from about the middle of the 1300's. The British hundredweight was divided into 4 quarters [1] of 28 pounds, 8 stone of 14 pounds, or 16 cloves of 7 pounds each. In the United States, where the currency was decimalized and there wasn't so much need to align the unit with the quintal and zentner, the hundredweight came to equal exactly 100 pounds (about 45.3592 kilograms). The U.S. hundredweight seems to have been invented by merchants around 1840. To distinguish the two hundredweight units, the British version is often called the long hundredweight and the American is called the short hundredweight or cental. The C in the symbol is of course the Roman numeral 100. Hvat a traditional unit of distance in Croatia equal to 1.8965 meters, identical to the Austrian klafter [1]. The word means "reach," and the unit is intended as a person's maximum arm span, like the English fathom and French toise. Hyl an obsolete MKS unit of mass. One hyl is the mass that is accelerated at one meter per second per second by one kilogram of force (kgf). Since 1 kgf = 9.806 65 newtons, the hyl is equivalent to 9.806 65 kilograms. The name of the unit comes from an ancient Greek word for matter. ##END_OF_RECORD i a mathematical unit equal to the square root of -1. Although often called the imaginary unit, i is quite real in many applications. For example, in vector geometry it is used to represent a counterclockwise rotation by 90°. The Swiss mathematician Leonhard Euler (1707-1783) introduced the symbol i for the imaginary unit in 1777. IBU an abbreviation for international bitterness (or bittering) unit, a unit used in brewing beer and ale. One IBU is equal to 1 part per million of isohumulone, an acid (derived from hops) that provides the bitterness in the brew. Measurements from 0 to about 70 IBU are possible, but most beers measure between 10 and 30 IBU. Icfm abbreviation for inlet cubic feet per minute, a unit traditionally used to measure the capacity of air compressors. Immi a traditional Swiss unit of liquid volume. The immi has been brought into the metric system and now equals exactly 1.5 liters (about 3.17 U.S. liquid quarts). Imperial a large wine bottle holding about 6 liters, 8 times the volume of a regular bottle. Also called a methuselah. Imperial units the units of the British imperial system, adopted by Parliament in 1828. The basic units of the system are the foot, the avoirdupois pound, and the imperial pint. Inch (in or ") [1] a traditional unit of distance equal to 1/12 foot or exactly 2.54 centimeters. The Old English word ynce is derived from the Latin uncia, meaning a 1/12 part; thus "inch" and "ounce" actually have the same root. The inch was originally defined in England in two ways: as the length of three barleycorns laid end to end, or as the width of a man's thumb at the base of the nail. The barleycorn definition is peculiarly English, but the thumb-width definition is generic. In fact, in many European languages the word for inch also means thumb: examples include the Dutch duim, Swedish tum, French pouce, and Spanish pulgada. In the history of English units the inch seems to come before the foot: after the Norman conquest of 1066 the foot was defined to equal 12 inches, rather than the inch being defined as 1/12 foot. Inch of mercury (in Hg) a traditional unit of atmospheric pressure. In the United States, atmospheric pressure is customarily expressed as the height of a column of mercury exerting the same pressure as the atmosphere. When a traditional mercury barometer is used, this height is read directly as the height of the mercury column. These readings must be corrected for temperature since mercury, like most liquids, tends to expand as it warms. The conventional equivalent of one inch of mercury is 0.491 153 pounds per square inch or 3.386 38 kilopascals (33.8638 millibars). In the symbol for the unit, Hg is the chemical symbol for mercury; it comes from the Latin word hydrargyrum ("water-silver") for the liquid metal. Inch of water column (in WC) a traditional unit of pressure, used in plumbing to describe both water and gas pressures. The conventional equivalent of one inch of water is 249.0889 pascals, which is 2.490 889 millibars, about 0.036 127 pounds per square inch (psi) or about 0.073 556 inches (1.868 32 millimeters) of mercury. Inch of water gauge (in wg or "wg) another common name for the inch of water column. The word "gauge" (or "gage") after a pressure reading indicates that the pressure stated is actually the difference between the absolute, or total, pressure and the air pressure at the time of the reading. Inch pound (in·lbf or in lb) a traditional unit of work or energy, equal to 1/12 foot pound, about 0.112 985 joule or 1.0709 x 10-4 Btu. In d. abbreviation for the Latin in die, daily, a unit of frequency used in medical prescriptions. Inhour (ih or inhr) a unit used in nuclear engineering to describe the "reactivity" of a nuclear reactor. In a reactor, fast-moving neutrons break apart atoms of uranium or plutonium; the fission of these atoms releases additional neutrons which keep the reaction going. The ratio R between the number of neutrons created and the number consumed in each cycle of fission must be very close to 1 in order for the reaction to be controlled. The reactivity is the difference k = R - 1 between this ratio and 1. One inhour is the reactivity which will cause the number of neutrons to increase by a factor of e = 2.71828 in one hour; a reactivity of t inhours will cause the number of neutrons to increase by a factor of e in 1/t hours. The exact size of the unit varies according to the design of the reactor. Enrico Fermi (1901-1954), the Italian-American physicist who built the first nuclear reactor, introduced this unit in 1947; its name is an acronym for "inverse hours." Other reactivity measures include the dollar and the milli-k. International foot the current foot unit of the English-speaking countries, equal to exactly 30.48 centimeters. See survey foot for additional information. International nautical mile the nautical mile as currently defined by international agreement, equal to exactly 1852 meters or 6076.11549 feet. This long name is sometimes used to distinguish the current nautical mile from older units. International rubber hardness degree (IRHD) a unit used to measure the hardness of rubber and similar materials (technically known as elastomers). Measurements are made using an IRHD durometer, and the results are usually similar, but not identical, to readings made with the older Shore "A" durometer. The International Organization for Standardization (ISO) and the American Society for Testing and Materials (ASTM), among other standards agencies, have published IRHD test procedures. International unit (IU) a unit used to measure the activity (that is, the effect) of many vitamins and drugs. For each substance to which this unit applies, there is an international agreement specifying the biological effect expected with a dose of 1 IU. Other quantities of the substance are then expressed as multiples of this standard. Examples: 1 IU represents 45.5 micrograms of a standard preparation of insulin or 0.6 microgram of a standard preparation of penicillin. Consumers most often see IU's on the labels of vitamin packages: in standard preparations the equivalent of 1 IU is 0.3 microgram (0.0003 mg) for vitamin A, 50 micrograms (0.05 mg) for vitamin C, 25 nanograms (0.000 025 mg) for vitamin D, and 2/3 milligram for vitamin E. Please note: for many substances there is no definite conversion between international units and mass units (such as milligrams). This is because preparations of those substances vary in activity, so that the effect per milligram of one preparation is different from that of another. Internet time a global decimal time system proposed by the Swatch Corporation. See beat for more information. Ips an abbreviation for inches per second (in/s), a traditional unit of velocity equal to 2.54 centimeters per second. Ipy an abbreviation for inches per year (in/yr), a traditional unit for corrosion rates. Irish acre a traditional unit of land area in Ireland, equal to 160 square Irish perches (see next entry). This is equivalent in English units to 7840 square yards, 70 560 square feet, or about 1.6198 English acre (0.6555 hectare). The colpa, a traditional Irish unit of pasturage, is approximately equal to the Irish acre. Irish mile the traditional mile in Ireland is 6720 feet, which is 1.272 727 English mile or 2.048 256 kilometers. The discrepancy arose because the Irish perch, or rod, was standardized at 21 feet instead of the English figure of 16.5 feet. Just as in England, the Irish chain was equal to 4 perches (84 feet instead of 66 feet), the Irish furlong was equal to 10 chains (840 feet instead of 660 feet) and the mile was equal to 8 furlongs. Iron a traditional unit measuring the thickness of leather used in making shoes, especially the soles of the shoes. One iron is equal to 1/48 inch (0.5292 millimeter), so a sole 1/4 inch thick is described as "12 iron." The origin of this unit is unclear; comments from knowledgeable readers would be welcome! ISO the "short name" of the International Organization for Standardization. Based in Switzerland, the ISO sets international industrial standards, including standards for the use of units of measurements. For ISO film ratings, see ASA. IT calorie common name for the international steam table calorie, a unit of energy equal to exactly 4.1868 joules. See calorie. ##END_OF_RECORD jag a traditional British name for a small load, especially a small load of hay. Never standardized, the jag represented roughly 20-25 bushels (0.7-1.0 cubic meters). Jansky (Jy) a unit used in radio astronomy to measure the strength, or more precisely the flux density, of radio signals from space. In measuring signal strength, it's necessary to take into account both the area of the receiving antenna and the width of the frequency band in which the signal occurs. Accordingly, one jansky equals a flux of 10-26 watts per square meter of receiving area per hertz of frequency band (W/m2Hz). Although it is not an SI unit, the jansky is approved by the International Astronomical Union and is widely used by astronomers. It honors Karl G. Jansky (1905-1950), the American electrical engineer who discovered radio waves from space in 1930. The jansky is sometimes called the flux unit. Jar a traditional unit of electric capacitance, approximately equal to the capacitance of one of the Leiden jars used in electrical experiments as long ago as the eighteenth century. Benjamin Franklin is said to have measured the storage power of his electrical equipment in jars. There are 9 x 108 jars in a farad, so 1 jar is approximately 1.1 nanofarad. Jerib or djerib a traditional unit of land area in the Middle East and southwestern Asia. The jerib originally varied considerably from one area to another. In modern times it has become identified with the hectare in many countries, including Turkey and Iran. In Afghanistan, however, it is usually equal to 1/5 hectare (2000 square meters or 0.494 acre). Jerk a unit of change in acceleration sometimes used by engineers. This is not as silly as it sounds, because when we're in a vehicle and feel a jerk, we are in fact experiencing a change in the acceleration of the vehicle. One jerk is equal to a change in acceleration of one foot per second per second in one second, that is, 1 ft/sec3. One jerk equals 0.3048 m/s3 or about 0.03108g /sec. Jeroboam a large wine bottle holding about 3 liters, 4 times the volume of a regular bottle. A jeroboam is sometimes called a double magnum. Jersey foot a traditional unit of distance in Jersey, one of the Channel Islands. Also called the pied-perche, the jersey foot is equal to 11 English inches or 0.9167 English foot. The jersey foot is divided into 12 jersey inches, making 13.09 jersey inches to an English foot. Jiffy a unit of time used in computer engineering. A jiffy is the length of one cycle, or tick, of the computer's system clock. In the past, this was often equal to one period of the alternating current powering the computer: 1/60 second in the U.S. and Canada, usually 1/50 second elsewhere. More recently the jiffy has become standardized, more or less, as 0.01 second (10 milliseconds). The word jiffy, with its ordinary meaning of an instant or very brief time, appeared in English during the eighteenth century, but its origin is not known. Jigger a unit of volume for liquor, usually considered equal to 1.5 (U.S.) fluid ounces or 44.360 milliliters. Jin a traditional unit of weight in China, comparable to the English pound. During the European colonial era the jin was identified with the catty, a Malay unit widely used in various forms throughout East and Southeast Asia. Like the catty, the jin was then equal to 1 1/3 pounds or 604.79 grams. Traditionally, it was divided into 16 liang. In modern China, however, the jin is a metric unit equal to exactly 500 grams (1.1023 pounds) and divided into 10 liang. The kilogram itself is usually called the gongjin, or "metric jin." The spellings chin and gin also have been used for the jin. Jitro a traditional unit of land area in the Czech Republic, identical to the Austrian joch (see below). Jo an informal unit of area used in Japan to measure the size of rooms in houses and apartments. One jo is the area of a traditional tatami mat, 180 by 90 centimeters or 1.62 square meters (1.94 square yards). Joch a traditional unit of area in German speaking countries, especially in Austria. One joch is the area of a square 40 klafters (about 83 yards) on a side. This comes to 0.5755 hectare or about 1.422 acres. The plural is joche. Joch is also the word for a yoke in German, so this unit represents an area that could be plowed in a day by a yoke of oxen. In what is now the Czech Republic this unit was known as the jitro; in Croatia it is the jutro. Jones a unit of detectivity, that is, the ability of an electronic device to detect radiant energy such as light waves or infrared radiation. In a 1959 paper, R. Clark Jones defined the "specific detectivity" of a device to be D* = [square root (Aw)]/N, where A is the area of the detector, w is the frequency bandwidth, and N is the power of the noise generated by the device. The quantity D* is measured in the complex unit cm·Hz1/2/W, customarily called the "Jones." In modern equipment, detectivities are often quite large, in the range of 109 to 1012 Jones. Joule (J) the SI unit of work or energy, defined to be the work done by a force of one newton acting to move an object through a distance of one meter in the direction in which the force is applied. Equivalently, since kinetic energy is one half the mass times the square of the velocity, one joule is the kinetic energy of a mass of two kilograms moving at a velocity of 1 m/sec. This is the same as 107 ergs in the CGS system, or approximately 0.737 562 foot-pound in the traditional English system. In other energy units, one joule equals about 9.478 170 x 10-4 Btu, 0.238 846 (small) calories, or 2.777 778 x 10-4 watt hour. The joule is named for the British physicist James Prescott Joule (1818-1889), who demonstrated the equivalence of mechanical and thermal energy in a famous experiment in 1843. Although Joule pronounced his name "jowl", the unit is usually pronounced "jew'l". Journal a traditional unit of land area in France, equal to the area that could be plowed in a day (jour is the French word for day). The unit varied from one region to another, generally in the range 0.3-0.45 hectare (0.75-1.1 acres). The juchart (see below) was a very similar unit used in Switzerland and southern Germany. JTU abbreviation for Jackson turbidity unit, a unit formerly used in measuring water quality. The turbidity of water was measured by lighting a candle under a tall glass tube and filling the tube with the water sample until a observer looking down the tube could no longer see the candle flame through the water. The height of the water column determines the turbidity in Jackson turbidity units, using a table constructed for this purpose. Turbidity is usually measured now in a different unit, the NTU, using an instrument called a nephelometer. Juchart or juchert a traditional unit of land area in southern Germany and German-speaking Switzerland. Like the Austrian joch and the French journal (see above), the juchart represents an area that could be plowed in a day by a yoke of oxen. The juchart varied considerably from place to place, but often it was about 4000 square meters, very close to the size of the English acre. In Bavaria, the juchart was standardized early in the nineteenth century at 3407.27 square meters (0.8420 acre). In Switzerland, after the introduction of the metric system in the mid nineteenth century, the juchart was generally understood to equal 3600 square meters (0.8896 acre). The joch, the journal, and the juchart are ultimately derived from a Roman unit, the jugerum, which was smaller, about 2500 square meters. The unit is also called the tagwerk ("day's work"). Jug an informal name for the Scots pint, a unit of volume equal to about 1.791 U.S. liquid quarts or 1.695 liters. Julian day (JD) a continuous count of days beginning with January 1, 4713 BC (-4712 CE). Julian days are named for the French scholar Julius Scaliger (1540-1609), who introduced the idea in 1582 (the same year the Gregorian calendar was proclaimed). They are often used by astronomers and sometimes used by historians to provide a precise date for an event, independent of all calendar systems. Scaliger picked 4713 BC for the start of the count because this was earlier than all known historical records and happened to be a convenient starting point for several chronological and astronomical cycles. The Julian day is taken to begin at noon Universal Time. The first moment of the year 2001 CE, at the International Date Line, is also the start of JD 2 451 910. Julian epoch (J) a measure of time used in astronomy. The word epoch comes from Greek, and means a fixed or standard instant of time. Other times are stated with reference to this fixed time using years and fractions of years. In 1984, astronomers agreed to fix the standard epoch at 12 hours Universal Time of 2000 January 1 (JD 2 451 545.0). This instant is designated J2000.0. Other times are specified with reference to this time using a year of length 365.25 days, the average length of the year in the Julian calendar. This means that J2001.0, for example, is 18 hours Universal Time of 2001 January 1 (exactly 365.25 days after J2000.0). Julian year the average length of the year in the Julian calendar, equal to exactly 365.25 days. See year [2]. Jupiter a unit of mass, now being used in astronomy to express the masses of new planets being discovered in orbit around various stars. It's equal to the mass of the planet Jupiter, estimated to be about 1.899 x 1024 metric tons, or, if you please, 1.899 yottatonnes (Yt). By coincidence, this is approximately 0.001 Sun (0.000 955 Sun, to be more exact). Jutro a traditional unit of land area in Croatia, the jutro equals 5754.64 square meters (1.422 acres), identical to the Austrian joch (see above) and Czech jitro. This is the area of a square 40 hvati on a side. The word means "morning," that is, it represents the area that could be plowed in one morning. The plural is jutra. ##END_OF_RECORD K an informal abbreviation for one thousand used in expressions where the unit is understood, such as "10K run" (10 kilometers) or "700K disk" (700 kilobytes or kibibytes). Note that "K" is also the symbol for the kelvin (see below). Also note that the symbol for the metric prefix kilo- (1000) is actually k-, not K-. Kaf a symbol for 1000 acre feet. This symbol is commonly used in reservoir management in the U.S. 1 kaf = about 1.2335 million cubic meters. Kappland a traditional unit of land area in Sweden. The kappland is equal to 1/32 tunnland or 1750 square Stockholm feet (kvadratfot); this is equivalent to 154.26 square meters or about 184.50 square yards. Karat (kt) a traditional measure of proportion equal to 1/24, used by U.S. jewelers to express the purity of gold alloys. Thus "14-karat gold" is legally required to be at least 14/24, or 58.3%, gold. In Britain the unit is spelled carat, just like the weight unit for diamonds and other precious stones. American jewelers spell the unit of gold purity with the "k" and the weight unit with the "c" in order to distinguish more clearly between them. (In German, both units are spelled with the "k".) Katal (kat) a unit of catalytic activity used especially in the chemistry of enzymes. A catalyst is a substance that starts or speeds a chemical reaction. Enzymes are proteins that act as catalysts within the bodies of living plants and animals. A catalyst has an activity of one katal if it enables a reaction to proceed at the rate of one mole per second. The unit, pronounced "cattle," was added to the International System at the 21st General Conference of Weights and Measures in October 1999. Kati a traditional Malaysian unit of weight, usually spelled catty in English. Kattha or katta a traditional unit of land area in South Asia, equal to 20 dhurs or 1/20 bigha. Like the bigha, the kattha varied in size from region to another. In Nepal, where the unit is still in use, the kattha equals about 338 square meters or 442 square yards. Kayser (K) a CGS unit used to measure light and other electromagnetic waves. The "wave number" in kaysers equals the number of wavelengths per centimeter; thus 1 kayser equals 100 per meter (m-1). The unit honors J. H. G. Kayser (1853-1940), who compiled a giant atlas of chemical spectra. The unit is often abbreviated K, although this conflicts with the symbol for the kelvin. Keddah a traditional Egyptian unit of liquid volume also used in other parts of the Middle East. The keddah is equal to about 2.0625 liter (about 2.18 U.S. liquid quarts or 1.815 British imperial quarts). Keel a traditional British unit of weight for coal. After considerable variation, the keel of coal was standardized in 1695 as 21.2 long tons, or 47 488 pounds (21.5402 metric tons). This is the approximate weight of coal carried at that time by barges on the river Tyne in northern England; the barges were also called keels, from the Dutch word kiel for such a ship. Keg [1] a traditional unit of volume or quantity, varying with the item contained in the keg. A keg of herring, for example, contains 60 fish. A keg of wine is frequently 12 U.S. gallons (about 45.42 liters), and a keg of beer is 15.5 U.S. gallons (about 58.67 liters). "Keg" comes from an old Norse word for a small barrel. Keg [2] a traditional unit of weight for nails. A keg of nails weighs 100 pounds and thus has a mass of about 45.359 kilograms. Kelvin (K) the SI base unit of temperature, previously called the degree Kelvin (°K). One kelvin represents the same temperature difference as one degree Celsius. In 1967 the General Conference on Weights and Measures defined the temperature of the triple point of water (the temperature at which water exists simultaneously in the gaseous, liquid, and solid states) to be exactly 273.16 kelvins. Since this temperature is also equal to 0.01 °C, the temperature in kelvins is always equal to 273.15 plus the temperature in degrees Celsius. The kelvin equals exactly 1.8 degrees Fahrenheit. The unit is named for the British mathematician and physicist William Thomson (1824-1907), later known as Lord Kelvin after he was named Baron Kelvin of Largs. He is best remembered for his pioneering work on the physics of heat, but he was also a strong advocate of the metric system; his support helped establish the now-familiar electrical units such as the ohm, volt, and farad. Ken a traditional Japanese unit of length comparable to the English fathom. The ken equals 6 shaku, which is about 1.818 meters (5.965 feet). The ken is the length of a traditional tatami mat. Kerat a traditional Middle Eastern unit of length, equal to about 9/8 inch or 2.86 centimeters. The unit has the same Arabic root as the carat. Key slang for kilo, meaning kilogram. Kgr an obsolete (but rather common) symbol for the kilogram. The proper symbol is kg. Kibi- (Ki-) a binary prefix meaning 210 = 1024. This prefix, adopted by the International Electrotechnical Commission in 1998, was supposed to replace kilo- for binary applications in computer science. Thus 1024 bytes of storage is officially a kibibyte, not a kilobyte. However, computer professionals generally dislike this unit (they say it sounds like a cat food) so the ambiguity in the size of a kilobyte persists. The prefix is a contraction of "kilobinary." The symbol Ki-, rather than ki-, was chosed for uniformity with the other binary prefixes (Mi-, Gi-, etc.). Kilderkin an old British unit of volume equal to 1/2 barrel or 2 firkins. Based on the current British barrel, this would be 18 (imperial) gallons, which is about 2.9 cubic feet or 78 liters. Older kilderkins were generally in the range of 16-18 gallons. The word comes from a Dutch word for a small cask. Kilo a common informal name for a kilogram. Kilo- (k-) [1] a metric prefix meaning 1000. The prefix is a modification of chilioi, the Greek word for a thousand. Kilo- (k-) [2] in measuring the memory of a computer, the prefix kilo- often means 210 = 1024 instead of 1000. By a 1998 resolution of the International Electrotechnical Commission, the new prefix kibi- (Ki-) should replace kilo- for 210. However, this doesn't seem to be happening. Kiloampere (kA) a unit of electric current equal to 1000 amperes. Kilobar (kbar or kb) a metric unit of pressure, used particularly in industrial applications and in geology for measuring high pressures. The kilobar equals 1000 bars, 100 megapascals, or about 14 503 pounds per square inch. (Note: in the investment world a kilobar is not a unit; it is a bar of gold, silver, or platinum weighing 1 kilogram.) Kilobase (kb) a unit of genetic information equal to the information carried by 1000 pairs of the base units in the double-helix of DNA; also used as a unit of relative distance equal to the length of a strand of DNA containing 1000 base pairs. Kilobecquerel (kBq) a unit of radioactivity equal to 1000 atomic disintegrations per second or 27.027 nanocuries (nCi). Kilobit (kbit or kb) a unit of information generally equal to 1024 bits or 128 bytes; this unit is now supposed to be called a kibibit. Kilobyte (kB) a unit of information equal to 1000 bytes. As a unit of computer storage, however, the kilobyte is usually equal to 1024 bytes, although this should now be called a kibibyte. Kilocalorie (kcal) an ambiguous metric unit of energy. The ambiguity arises because there are two "calories" in common use, identified in this dictionary as the calorie (the small or gram calorie equal to 4.1868 kilojoules) and the Calorie (the large or kilogram calorie equal to 4.1868 megajoules). The term kilocalorie properly means 1000 calories, which is the same as 1 Calorie. In other words, "kilocalorie" is the correct name for the unit known in nutrition simply as the "calorie." Kilocurie (kCi) a unit of radioactivity equal to 1000 curies or 37 terabecquerels (TBq), that is, 37 trillion atomic disintegrations per second. The strength of the powerful radiation sources used in cancer therapy are customarily stated in kilocuries. Kilocycle (kc) 1000 cycles; a term sometimes used as an informal name for the kilohertz. Kilodalton (kDa) a unit of mass equal to 1000 atomic mass units. See dalton. Kiloelectronvolt (keV) a unit of work or energy used in physics, equal to 1000 electronvolts. Kilofoot (kft) a traditional unit of distance equal to 1000 feet or exactly 304.8 meters. This odd combination of a metric prefix and an English unit is used in telecommunications to describe cable lengths and transmission distances. Kilogauss (kGs) a metric unit of magnetic flux density equal to 1000 gauss or 0.1 tesla. The strength of industrial magnets and solenoids is often expressed in kilogauss, although this unit is being gradually replaced by the tesla. Kilogram (kg) the base unit of mass in the SI and MKS versions of the metric system. The kilogram is defined as the mass of the standard kilogram, a platinum-iridium bar in the custody of the International Bureau of Weights and Measures (BIPM) near Paris, France. Copies of this bar are kept by the standards agencies of all the major industrial nations, including the U.S. National Institute of Standards and Technology (NIST). One kilogram equals exactly 1000 grams, or about 2.204 622 6 pounds. Kilogram calorie (kcal or kgcal) the "large calorie" or "food calorie" used in nutrition, equal to 1000 ordinary ("gram") calories. The correct name for this unit is kilocalorie. Kilogram meter (kgf·m or kg·m) a metric unit of work or energy equal to 9.806 65 joules (J). This is the work done by one kilogram of force (see below) acting through a distance of one meter. The kilogram meter is also used as a unit of torque equal to 9.806 65 newton meters (N·m). Kilogram mole, kilogram molecule, kilogram molecular weight (kgmol or kgmole) various older names for a unit of the amount of a chemical compound. One kilogram mole of a compound is the number of kilograms of the compound equal to the molecular weight of a molecule of that compound measured in atomic mass units. The correct name for this unit is the kilomole (kmol). Kilogram of force (kgf) a unit of force equal to the gravitational force on a mass of one kilogram. One kilogram of force equals 9.806 65 newtons, or 2.204 622 6 pounds of force in the traditional English system. Using this unit revives the old confusion between mass and weight, one of the worst features of traditional measurement systems, so it is really a very bad idea. However, kilograms of force have been used rather frequently in engineering and physics. This unit is also called the kilopond. Kilogram per square centimeter (kgf/m2 or kg/m2) a common metric unit of pressure equal to 98.0665 kilopascals (see below). Similarly, one kilogram per square meter is equal to 9.806 65 pascals. Kilohertz (kHz) a common unit of frequency equal to 1000 per second or 1 per millisecond. AM radio stations have signal frequencies measured in kilohertz. Kilojoule (kJ) a common metric unit of work or energy, comparable to the British thermal unit (Btu). In fact, one kilojoule equals approximately 0.947 817 Btu. In other energy units, the kilojoule is also equivalent to 0.238 846 kilocalories, 0.277 778 watt hour, or 737.562 foot-pounds in the traditional English system. Kiloline a metric unit of magnetic flux, equal to 1000 lines [2] or 10 microwebers. Kiloliter (kl or kL) a metric unit of volume. The kiloliter is identical to the cubic meter: it equals about 35.3147 ft3, 1.307 95 yd3, 264.17 U.S. gallons, 219.99 British Imperial gallons, 7.497 U.S. bushels, or 6.049 British imperial bushels. Kilomega- (kM-) an obsolete metric prefix denoting 109 (1 U.S. billion). This prefix has been replaced by giga- (G-). Kilometer (km) a common metric unit of length or distance. One kilometer equals exactly 1000 meters, about 0.621 371 19 mile, 1093.6133 yards, or 3280.8399 feet. Oddly, higher multiples of the meter are rarely used; even the distances to the farthest galaxies are usually measured in kilometers. The unit is sometimes pronounced with the accent on the first syllable and sometimes on the second. Both are acceptable; there are no official pronunciations for SI units. Kilometer per hour (km/h) a common metric unit of speed or velocity. 1 km/h is equal to 5/18 meter per second, 0.621 371 19 miles per hour or 0.911 344 42 feet per second. The only correct symbol for this unit is km/h; variations such as kmph or kph should not be used. Kilomole (kmol) a unit of amount of substance equal to 1000 moles. One kilomole of a compound is the number of kilograms of the compound equal to the molecular weight of a molecule of that compound in atomic mass units. The kilomole was formerly called the kilogram mole. Kilonewton (kN) a common metric unit of force, the kilonewton equals 1000 newtons; it is a force that will accelerate a mass of 1 metric ton at the rate of 1 meter per second per second. One kilonewton equals 101.972 kilograms of force, 224.809 pounds of force or 7233.01 poundals. Kilohm a unit of electric resistance equal to 1000 ohms. Kilo-oersted (kOe) a CGS unit of magnetic field strength equivalent in MKS units to 79 577.472 ampere-turns per meter. The unit, used for stating the field strengths of industrial magnets, is almost always spelled with the hyphen. Kiloparsec (kpc) a unit of distance used in astronomy, the kiloparsec equals 1000 parsecs, 3261.631 light years, 3.085 678 x 1016 kilometers, or about 19.18 quadrillion miles. The Solar System is located about 8 kiloparsecs from the center of the Milky Way galaxy. Kilopascal (kPa) a common metric unit of pressure. One kilopascal equals 1000 pascals (Pa), 10 millibars(mb), or about 0.145 038 pounds per square inch (lbf/in2 or psi), 20.8855 pounds per square foot, 7.502 millimeters of mercury (mm Hg), 0.2953 inches of mercury (in Hg), 4.015 inches of water column (in WC), or 0.3346 foot of head (ft hd). Kilopond (kp) another name for the kilogram of force (kgf) (see above). Kilorad (krad) a common unit of radiation dose equal to 1000 rads, 10 grays, or 10 joules of energy per kilogram of mass. Kilosecond (ks or ksec) a unit of time equal to 1000 seconds (16 minutes 40 seconds). Although it is never used in ordinary life, the kilosecond has many uses in science. One day is equal to 86.4 kiloseconds. Kiloton (kt or kton) [1] an ambiguous unit of mass, ambiguous because it may refer to 1000 U.S. tons (907 185 kilograms), 1000 British tons (1 016 047 kilograms), or 1000 metric tons (1 million kilograms, or 1 gigagram). To avoid this confusion, the metric unit should be written kilotonne. Kiloton (kton) [2] a unit of explosive energy equal to the energy released by exploding 1000 U.S. tons (2 million pounds) of TNT. This is about 4.18 terajoules (4.18 x 1012 joules) or 4 billion Btu. Kilovolt (kV) a common unit of electric potential equal to 1000 volts. Electric distribution lines operate at potentials of anywhere from several to several hundred kilovolts. Kilovolt ampere (kV·A) a common unit of load in power engineering, equal to 1000 volt amperes. Kilowatt (kW) a common metric unit of power, equivalent to 1000 watts, about 1.341 022 horsepower, or 737.562 foot-pounds per second. Kilowatt hour (kW·h or kw hr) the commercial unit of electric energy. One kilowatt hour represents the amount of energy delivered a rate of 1000 watts over a period of one hour. Since the watt is 1 joule/sec and there are 3600 seconds in an hour, the kilowatt hour is equivalent to exactly 3.6 megajoules of energy, or about 3412.141 Btu, 859.846 (kilogram) Calories, or about 2.655 million foot pounds. Kiloyard (kyd) a traditional unit of distance equal to 1000 yards (exactly 914.4 meters). This unit is used in the U.S., British, and Commonwealth navies in describing ship distances and target ranges. Kiloyear (kyr) a unit of time equal to 1000 years, commonly used in archaeology, paleontology, climatology, and related sciences. Of course, a kiloyear is the same as a millennium. Kin the Japanese version of the catty, a common weight unit of the Far East. The Japanese identified this unit with a traditional unit equal to about 1.323 pounds or almost exactly 600 grams; this is about 0.75% smaller than the Chinese catty. Kip [1] an informal unit of mass, sometimes used by engineers to express the amount of load on a structure. One kip equals 1000 pounds (453.59 kilograms). The name of the unit is an abbreviation of "kilopound." Kip [2] an informal unit of force equal to 1000 pounds of force or about 4.4482 kilonewtons. Kip [3] a symbol for 1000 inch pounds, used as a unit of energy or torque. In this usage one kip is equal to 83.333 foot pounds (lbf·ft), 112.985 joules (J), or, for torque, 112.985 newton meters (N·m). Kip [4] an old English word for a bundle of hides. It was sometimes used as a unit of quantity, usually equal to 50. Klafter [1] a traditional unit of distance in German-speaking countries, comparable to the English fathom. The Austrian klafter is equal to 1.8965 meters (6.22 feet). In Switzerland the klafter was brought into the metric system at exactly 1.8 meters (5.9055 feet). This unit is also called the faden. Klafter [2] a traditional German unit of volume for stacked firewood, comparable to the English cord. A klafter of wood was generally 1 klafter tall and 1 klafter long, but there was less agreement on its width, that is, the length of the logs. A common width was 3 fuss or 0.5 klafter, making the volume about 3.41 steres (cubic meters) or 0.941 cord by the Viennese definition, 3.34 steres or 0.921 cord in northern Germany. Klick, klik see click. Kmph a common but incorrect symbol for kilometers per hour. The correct symbol is km/h. Knoop hardness (HK or KHN) a measure of the hardness of a metal introduced by Knoop in 1939. The Knoop test is similar to the Vickers test in that a diamond penetrator is used to indent the sample being tested, but it uses a rhombohedral diamond rather than a pyramidal diamond point. It is similar to the Rockwell test in that the hardness measure is the depth of the penetration rather than its area. The result is measured in kilograms of force per square millimeter but should be stated as an empirical measurement, without units. Knot (kn or kt) a unit of velocity equal to one nautical mile per hour. Knots are customarily used to express speeds at sea, including the speed of the ship as well as the speeds of the wind and of the current. The word comes from the former method of measuring a ship's speed, which involved use of a knotted cord called the log line. One knot equals about 1.1508 miles per hour, exactly 1.852 kilometers per hour, or 0.5143 meters per second. The knot has also been used incorrectly as a name for the nautical mile. Since kt is the established symbol for the kilotonne, kn is the best choice as a symbol for the knot. Köddi a traditional Arabic unit of volume, equal to about 4/3 British imperial gallon or 7.58 liters. Koku a traditional Japanese unit of volume, equal to about 180.391 liters (39.68 British imperial gallons or 6.37 cubic feet). The unit originated as an estimate of the amount of rice needed to feed a person for a year. Kommerzlast German for "commercial load," now interpreted as a metric unit equal to exactly 3 tonnes (about 6613.9 pounds). Kon Korean name for the catty. Koyan a Far Eastern unit of weight equal to 4000 catty or 5333.33 pounds (8/3 short ton). Kph a common but incorrect symbol for kilometers per hour. The correct symbol is km/h. Krina a traditional unit of volume in Bulgaria, now expressed in the metric system as being equal to exactly 20 liters (4.40 British imperial gallons or 5.28 U.S. liquid gallons). Ksf, ksi symbol for kips (kilopounds) per square foot or per square inch, traditional engineering units of pressure or stress. 1 ksf = 47.880 257 kilopascals (kPa) and 1 ksi = 6.894 757 megapascals. Kulmet a traditional Latvian unit of volume equal to about 10.93 liters (2.40 British imperial gallons or 2.89 U.S. liquid gallons). A similar Estonian unit, the külimet, equals about 11.48 liters (2.53 British imperial gallons or 3.03 U.S. liquid gallons). Kunitz or Kunitz unit a unit used in biochemistry to describe the concentration or activity of the enzyme ribonuclease, which attacks ribonucleic acid (RNA). Ordinarily RNA absorbs ultraviolet light, so the activity of ribonuclease can be measured by observing the decrease of this absorption. One kunitz is the concentration of the enzyme causing a decrease of 100% in the absorption in a time of one minute, the measurement being made under certain standard laboratory conditions. The unit's name recognizes the Russian-American biochemist M. Kunitz, who proposed the standard test in 1946. Kvadrat- Scandinavian prefix meaning "square." In particular, a kvadratmeter is a square meter. The prefix is common to Danish, Norwegian, and Swedish. Kwan a traditional Japanese unit of weight equal to 6.25 kin (see above), which is about 8.27 pounds or 3.75 kilograms. Kya a common abbreviation (in English speaking countries) for "thousand years ago." The "k" is the metric symbol for kilo- (1000). The symbol kyr (kiloyear) is also used. Kyu a metric unit of distance used in typography and graphic design. The kyu, originally written Q, is equal to exactly 0.25 millimeter, about 0.71 point [2], or about 14.173 twips. The spelling "kyu" seems to have been introduced by the software company ##END_OF_RECORD labor a traditional unit of area in Latin American countries. The labor is equal to the area of a square 1000 varas on a side, or 0.04 legua. Using the Texas standard for the vara (33 1/3 inches), this is equivalent to 177.136 acres or 71.685 hectares. The word labor means work in Spanish, as it does in English. As a unit it represents the area that could be cultivated by a single farmer, somewhat like the old English hide. Lakh or lac a traditional unit of quantity in India, equal to 105 or 100 000. In India the lakh is used commonly instead of the million and commas are used to isolate the number of lakh; for example, the number 5 300 000 is called 53 lakh and written "53,00,000". See also crore. Lambda [1] a metric unit of volume equal to the cubic millimeter (mm3) or microliter (µL). The lambda has been used in chemistry in measuring very small samples. The symbol is the lower case Greek letter lambda. Lambda [2] a unit of relative distance used in the design of integrated circuits in microelectronics. These circuits are usually designed to be "scalable," so that the same design can be repeated on ever-smaller chips as technology improves. The "feature size" of a design is the width of its smallest element, and one lambda equals one half the feature size. Lambdas of a few tenths of a micrometer (micron) are common. Lambert (La or Lb or L) a CGS unit of luminance. Luminance is the luminous intensity of a surface, measured in units of light intensity per unit of area. One lambert is the luminance of a surface that emits or reflects one lumen per square centimeter. The lambert is a large unit, and practical measurements tend to be in millilamberts (mLa). One lambert equals 104/pi = 3183.099 candelas per square meter and 1 millilambert equals 3.183 099 candelas per square meter. (The difference between "luminance" and "illuminance" is the source of the light. If the light comes from the surface we speak of "luminance" and measure it in units of light intensity per unit of surface area. If the light comes from outside we speak of "illuminance" and measure it in units of light intensity time solid angle per unit of surface area.) Formerly luminance was measured in lamberts and illuminance in phots. In the SI, luminance is measured in candelas per square meter and illuminance in lux. The lambert honors the German physicist Johann Lambert (1728-1777), who showed that the illuminance of a surface is inversely proportional to the square of the distance from the light source. There has never been agreement on the symbol for the lambert; La is a good choice because it is unlikely to be confused with any other unit symbol. Lanac a traditional unit of land area in countries of the former Yugoslavia. Usage varies. In Serbia, the lanac is equal to about 0.56 hectare (1.38 acres), making it the Serbian equivalent of the Austrian joch and Croatian jutro. In Croatia, however, the lanac is a larger unit equal to 0.7193 hectare (1.777 acres). The word "lanac" means "chain", presumably referring to surveyors' use of chains in measuring land. Land mile the ordinary statute mile, equal to 5280 feet or 1609.344 meters, is sometimes called a "land mile" to distinguish it from the nautical mile. Lane meter a unit of deck area for "roll on/roll off" ships: cargo vessels designed so that containers or other cargo can be rolled on and off the decks of the ship. A lane is a strip of deck 2 meters wide. A lane meter is an area of deck one lane wide and one meter long, that is, 2 square meters (21.528 square feet). Langdo a traditional unit of land area in Bhutan. Like many European land units, the langdo is the area that a team of oxen can plow in a day. If the land is dry, a langdo is about 1/7 hectare or 1/3 acre; for a wet paddy field the langdo is about 1/10 hectare or 1/4 acre. Langley (Ly) a CGS unit of heat transmission equal to one thermochemical calorie per square centimeter, or exactly 41.84 kilojoules per square meter (kJ/m2). Named for the American astronomer Samuel P. Langley (1834-1906), the langley is used to express the rate of solar radiation received by the earth. Lap an informal unit of distance used in athletic competitions. In athletics ("track"), a lap is the length of one trip around a running track. This may vary from track to track, but at the level of serious competition most tracks have a standard length. In English speaking countries this was formerly 1/4 mile (1320 feet or 402.336 meters). Tracks used in most competitions today have a length of exactly 400 meters (1312.34 feet). In swimming, a lap is one tour (out and back) of the pool, a distance of exactly 100 meters (328.08 feet) in Olympic-size pools but only 50 meters (164.04 feet) in many recreational and "short-course" pools. Last an ancient northern European unit used in measuring large quantities, either by mass or volume or both. "Last" is the German word for "load," a meaning which also survives in the English word "ballast." Generally the last is approximately 4000 pounds (about 1800 kilograms) as a mass unit or 80 bushels (about 3 cubic meters) as a volume unit. In the U.S., a last of wool was formerly 12 sacks at 364 pounds per sack, or 4368 pounds (1981.3 kilograms). In the Netherlands, the last is a metric unit of volume equal to exactly 3 cubic meters. In Germany, where the last was previously a volume unit equal to 2.819 cubic meters, it's also a metric unit of mass equal to 2 metric tons (about 4409 pounds). In Britain, a Riga last of timber is 80 cubic feet (2.265 cubic meters) of square-sawn timber. Lb, lbf, lbm lb is the traditional symbol in English, Spanish, and Italian for the pound, derived from the Latin word libra for the Roman version of the same unit. The symbols lbf and lbm are used to distinguish between pounds of force and pounds of mass, respectively. Lea a traditional unit of length for yarn, varying with the weight of the fibers in the yarn. Typically a lea of wool is 80 yards (73 meters); a lea of cotton or silk 120 yards (110 meters); and a lea of linen 300 yards (274 meters). In all these cases, a lea is equal to 1/7 hank. A lea is sometimes called a skein. League a traditional unit of distance. Derived from an ancient Celtic unit and adopted by the Romans as the leuga, the league became a common unit of measurement throughout western Europe. It was intended to represent, roughly, the distance a person could walk in an hour. The Celtic unit seems to have been rather short (about 1.5 Roman miles, which is roughly 1.4 statute miles or 2275 meters), but the unit grew longer over time. In many cases it was equal to 3 miles, using whatever version of the mile was current. At sea, the league was most often equal to 3 nautical miles, which is 1/20 degree [2], 3.45 statute miles, or exactly 5556 meters. The British definition of the league is 3 statute miles (about 4828 meters) on land or 3 nautical miles at sea. In the U.S., leagues have never been used on land and the only definition is 3 nautical miles. However, many occurrances of the "league" in English-language works are actually references to the Spanish league (the legua), the Portuguese league (legoa) or the French league (lieue). For these units, see below on this page. Leap a traditional Welsh unit of distance equal to 6 feet 9 inches or 2.0574 meters. Legoa the Portuguese league, equal to 3 milhas (Portuguese miles). This is equal to about 3.836 statute miles or 6174.1 meters. Legua [1] the Spanish league. The traditional legua is equal to 5000 varas, which is close to 2.6 miles or 4.2 kilometers. Using the Texas definition of the vara, the legua is 2.6305 miles, 13889 feet, or 4233.4 meters. Using the traditional Spanish definition, it would be 2.597 miles, 13712 feet, or 4179.4 meters. Technically, this unit was abolished by Philip II in 1568, but it remained in rather wide use, especially in the Americas. During the late 18th and early 19th centuries, a league of 8000 varas (4.15 miles or 6680 meters) was legal in Spain. At sea, Spanish sailors used the usual marine league (3 nautical miles or 5556 meters) or Phillip's "geographical" league of 1/17.5 degree (3.429 nautical miles or 6350.5 meters). Legua [2] a traditional Spanish unit of area equal to one square legua [1]. In Spanish-speaking Latin America and the southwestern states of the U.S. land was customarily measured in leguas, with 1 legua equal to 25 labors (see above) or 25 million square varas. Using the Texas definition of the vara as the starting point, the legua is 4428.4 acres, 6.919 square miles, 1792 hectares, or 17.92 square kilometers. A slightly larger figure, 4439 acres (1796 hectares), is used in California. Larger sizes, between 1800 and 1900 hectares, were formerly used in some parts of South America. In Mexico and Texas, this unit is often called a sitio. Length (lg) an informal unit of distance. The distance between competitors in horse races, boat races, and similar situations is naturally expressed in lengths, with one length equal to the average length of a horse, boat, etc. In horse racing, the number of lengths by which one horse beats another is computed as 5 times the difference in their running times measured in seconds. This means the length is actually interpreted as a unit of time equal to 1/5 second. Lethal dose (LD) a measure used in pharmacology to express the percentage of a population killed by a dose of the substance being studied. The measurement is often given as a subscript. For example, the potency of a drug or pesticide is commonly expressed by stating the size of the LD50 dose: the amount of the substance that kills 50% of the test population. Li a traditional unit of distance in China. A Confucian proverb widely misquoted in the West as "a journey of a thousand miles begins with a single step" actually says "a journey of a thousand li begins with a single step." Although the traditional li was approximately 1/3 mile or 500 meters, the late imperial governments of China used a li of 1800 ch'ih, which is 2115 feet, about 0.401 mile, or 644.65 meters. In modern China, the li equals exactly 0.5 kilometer or 500 meters. In Chinese, the kilometer itself is usually called a gongli, or "metric li." Liang a traditional Chinese weight unit. During the European colonial period the liang was equal to 1/16 catty, 1/12 pound, or about 37.8 grams; this made it the same as a tael. In modern China, the liang equals 1/10 jin or 10 qian; this is exactly 50 grams (1.7637 ounces). Libra or libbra (lb) a traditional unit of weight in Italian, Spanish, and Portuguese speaking countries. The libra was the Roman unit from which the English pound is descended; the symbol "lb" for the pound comes from this unit. The Roman libra contained only 12 unciae (ounces) and was about 0.722 English pound. The traditional Italian libbra was of similar size. The Spanish and Portuguese units are larger, generally in the range from 1.011 to 1.016 English pound (very close to 460 grams). The Spanish libra equals 16 onzas, and the Portuguese libra equals 16 onças. The word "libra" is sometimes used now for the kilogram, a much larger unit. Lieue the French league. A variety of lieue units were used for land measurement in France, but generally these units were around 2.4-2.5 statute miles in length. In the 18th century, the legal unit was the lieue de poste, defined to equal 2000 toises or 2 milles (2.4221 miles or 3898 meters). In metric France the lieue is now considered to equal exactly 4 kilometers (2.4855 miles). See league (above). At sea, the lieue was often taken to equal 1/25 degree [2] or 2.4 nautical miles (4445 meters or 2.7619 miles); this unit was gradually replaced by the internationally recognized 3 nautical miles (5556 meters or 3.452 miles). Light watt a unit measuring the relative power output of a light source. Calculating the power delivered in the form of visible light is rather complicated. For a monochromatic (single frequency) light source such as a laser, the power in light watts equals 683V(l), where l is the wavelength of the light and V(l) is the relative power in watts per lumen (W/lm) required to produce a constant brightness sensation in the eye at wavelength l. Values of V(l) are defined by the International Commission on Illumination (CIE). The maximum value of V(l) is 1/683 W/lm = 1.464 mW/lm at the wavelength l = 555 nanometers (nm), the wavelength to which the eye is most sensitive. When the source delivers light over a range of frequencies (as a light bulb does), it is necessary to compute 683 times the integral of V(l) multiplied by the fraction of energy delivered at wavelength l. See also candela and talbot. Light year (ly) a unit of distance used in astronomy. One light year is the distance that light travels in one year through a vacuum (and, of course, most of the Universe is close to being a vacuum). Since light travels at 299 792 458 meters per second, and there are 31 556 925.9747 seconds in a year, one light year equals 9.460 528 405 x 1015 meters, or 9.460 528 405 trillion kilometers. One light year is approximately 5.880 trillion miles. Ligne a traditional unit of distance in French speaking countries, equal to 1/12 pouce (French inch) and corresponding closely to the English line [1]. The Swiss ligne is used throughout the world by watchmakers; it equals about 2.256 millimeters (0.0888 inch) and is divided into 12 douzièmes. In English the ligne is usually pronounced "lean" by those who don't hold to the French pronunciation. Ligula a Roman unit of liquid volume equal to 1/48 sextarius or about 11.07 milliliters. The word literally means "a lick." Line (li) [1] a traditional unit of distance equal to 1/12 inch (about 2.1167 millimeters). For measuring the thickness of buttons, there is also a smaller line equal to 1/40 inch (0.635 millimeter). The line is called the ligne (see above) in French, the linea in Spanish, the linie in German, and the liniya in Russian. Line (li) [2] a former name for the maxwell, the CGS unit of magnetic flux. The unit was called the line because magnetic fields were traditionally represented by lines depicting the direction of the field; the idea was to quantify the strength of these lines. This is a small unit, so fields were often measured in megalines; one megaline is equal to 0.01 weber. Line [3] a unit of distance equal to 1/14 inch, used in printing and advertising. This usage is short for agate line. Linear foot (or "lineal" foot) (ft or lf) terms used loosely to describe a one-foot length of any long, narrow object. The correct term is linear foot; the word "lineal" refers to a line of ancestry, not to length. Boards, pipes, and fencing are typical objects measured and sold by the linear foot. In the moving industry, a linear foot is a one-foot length of a moving van, usually a volume of about 72 cubic feet (roughly 2 cubic meters). Occasionally the term "linear foot" is used as an alternate name for the board foot, but this is appropriate only if the board is 12 inches wide. Terms such as linear meter and linear yard are used in a similar way to indicate one-meter or one-yard lengths. Link a traditional unit of distance used by surveyors, equal to 0.01 chain. In Britain, one link is exactly 0.66 feet, or 7.92 inches, or approximately 20.12 centimeters. In the U.S., both 66-foot and 100-foot chains have been used; for a 100-foot chain the link is the same as the foot. Liter or litre (L or l) the common metric unit of volume. The liter was originally defined to be the volume occupied by a kilogram of water, and the gram as the mass of a cubic centimeter of water. This would make the liter equal to exactly one cubic decimeter, that is, to the volume of a cube 0.1 meter (or 10 centimeters) on a side. Unfortunately, the physical objects constructed to represent the meter and kilogram disagreed slightly. As measured by the standard meter and standard kilogram, the standard liter turned out to be about 1.000 028 cubic decimeters. This discrepancy plagued the metric system for a long time. In 1901 an international congress accepted the discrepancy and formally defined the liter to be exactly 1.000 028 dm3. No one was particularly happy with such an awkward definition, so in 1964 the CGPM repealed the definition. In the SI, volumes are to be measured in cubic meters or power-of-ten multiples thereof, not in liters. However, the SI states that the liter "may be employed as a special name for the cubic decimeter." Throughout this dictionary, the liter is used as a name for exactly 1 cubic decimeter, 1000 cubic centimeters, or 0.001 cubic meter. In its new guise as the cubic decimeter, the liter is approximately 61.023 744 cubic inches. Compared to the customary volume units, the liter is a little more than a U. S. liquid quart (1.056 688 qt or 33.814 fluid ounces) but a little less than a U. S. dry quart (0.908 08 qt) or a British imperial quart (0.879 89 qt or 35.195 fluid ounces). Its name comes from a French volume unit, the litron, which was in turn derived from the Latin litra. Both the lower case letter l and the upper case L are accepted as symbols for the liter, but the U.S. Department of Commerce specifies that L be used, at least by businesses, to avoid confusion with the numeral 1. The unit is spelled liter in the U.S. and litre in Britain; there are many other spellings in various languages (see Spelling of Metric Units). Liter atmosphere (L atm) a unit of work or energy used in the study of confined gases. The behavior of gases is described, to a first approximation, by the ideal gas law PV = nRT. The ideal gas law is really an energy equation in which the left hand side, pressure P (in atmospheres) times volume V (in liters), measures the potential energy in the confined gas. One liter atmosphere is equal to 101.325 joules, 0.09605 Btu or 74.73 foot pounds. Livre a traditional unit of weight in French speaking countries and in Greece. The livre corresponds to the English pound and to the Spanish libra (see above). The livre is divided into 2 marcs or into 16 onces. The French livre varied from market to market, but the official standard from about 1350 to the introduction of the metric system was the livre poids de marc or livre de Paris of 489.5 grams (1.079 English pounds). In modern France, the livre is used as a metric unit equal to exactly 500 grams or 0.5 kilogram (1.1023 pounds). The traditional Greek livre is also about 500 grams. Load a traditional, generally informal, unit of volume. In landscaping and some construction trades a load often means a cubic yard (0.764555 m3). In ordinary language in the U.S., a load often means the volume of a pickup truck, a varying unit. In Britain prior to modern times, a load was sometimes a standardized unit, but it varied with the commodity being carried. A typical size was 40 bushels (roughly 1.4 cubic meters). Long hundredweight the British hundredweight, equal to 112 pounds. Long ton the traditional British ton, equal to 2240 pounds. Longword a unit of information equal to 2 shortwords, 4 bytes or 32 bits. See also word [2]. Lot a traditional unit of weight in German speaking countries, equal to about 1/2 ounce or 15 grams. "Lot" is the German word for a lead plumbbob, so the unit represents a small lead weight. Lovibond color units see degree Lovibond. Lpf symbol for liters per flush, a specification found on toilets. U.S. government regulations now require the use of low-flush toilets of 6.0 Lpf or less. 1 Lpf = 0.264 U.S. gallon per flush (gpf). Lpi abbreviation for lines per inch, a unit used to state the resolution of display devices (such as television or computer monitor screens), or to state the line spacing of printed pages. Lug [1] an old English name for a rod (5.5 yards or 5.0292 meters). In some parts of England this unit represented a longer rod of 7 yards (6.4008 meters), a unit also called the great lug. Lug [2] a shallow box or crate for produce such as cherries, grapes, or peaches. The size of a lug varies with the item it contains. Typical lugs hold about 16-28 pounds (7-13 kilograms) of produce in a volume of roughly 1/3 bushel (about 12 liters). This unit seems to be particularly common in produce markets in the midwestern U.S. Lumberg an older name for the talbot, the unit of luminous (light) energy equal to 1 lumen second. Lumen (lm) the SI unit for measuring the flux of light being produced by a light source or received by a surface. The intensity of a light source is measured in candelas. One lumen represents the total flux of light emitted, equal to the intensity in candelas multiplied by the solid angle in steradians (1/(4·pi) of a sphere) into which the light is emitted. Thus the total flux of a one-candela light, if the light is emitted uniformly in all directions, is 4·pi lumens. "Lumen" is a Latin word for light. Lumen hour (lm h) a unit of quantity of light, equal to one lumen of light flux continued for one hour. The lumen second (lm s) is defined similarly. Lunar day another name for the tidal day, a unit of time equal to 24 hours 50 minutes used in tidal predictions. Lunar month, lunation names for the interval between two successive new moons, a unit of time equal to 29.530 59 days. See month [1]. Lusec a unit of power used to express the performance or leakage of vacuum pumps. One lusec represents a flow of one liter per second at a pressure of one micrometer (or micron) of mercury, or 1 L·µmHg/s. Since "u" is sometimes used as a symbol for the micron, the name of the unit is an acronym for liter-micron/second. One lusec is equivalent to 1.3156 atm·cm3/s or 133.3 Pa·cm3/s, which is the same as 133.3 microwatts. Luster, lustre, lustrum a traditional unit of time equal to 5 years. In ancient Rome the Lustrum was ceremony of expiation and purification for the whole population of the city, carried out every 5 years after the completion of the census. The use of luster or lustrum as a unit of time in English was fairly common in well-educated circles as long as "well-educated" meant classically educated; the unit has pretty much disappeared today. Lux (lx) the SI unit for measuring the illumination (illuminance) of a surface. One lux is defined as an illumination of one lumen per square meter or 0.0001 phot. Light measurement may seem a bit subtle, so it's useful to think about light originating at a point and shining upon a surface. The intensity of the light source is measured in candelas; the total light flux in transit is measured in lumens (1 lumen = 1 candela·steradian); and the amount of light received per unit of surface area is measured in lux (1 lux = 1 lumen/square meter). One lux is equal to approximately 0.09290 foot candle. ##END_OF_RECORD M[1] informal abbreviation for million in expressions where the base unit is understood, as in "500M hard drive" (500 megabytes or mebibytes). In chemistry, M is the symbol for "molar" (see below). M [2] the Roman numeral 1000, sometimes used in symbols to indicate athousand, as in Mcf, a traditional symbol for 1000 cubic feet. Given the widespread use of M to mean one million, this older use of M to mean 1000 is very confusing and should be scrapped. Ma a symbol for one million years, often used in astronomy and geology. The "a" stands for the Latin annum. Mace a traditional Chinese unit for weighing precious metals, especially silver. In the European colonial period, the mace was considered equal to 0.1 tael or liang; this would be 2/15 ounce or about 3.78 grams. Mach (M or Ma) a measure of relative velocity, used to express the speed of an aircraft relative to the speed of sound. The name of the unit is placed before the measurement. Thus "mach 1.0" is the speed of sound, "mach 2.0" is twice the speed of sound, and so on. (The actual speed of sound varies, depending on the density and temperature of the atmosphere. At 0 C and a pressure of 1 atmosphere the speed of sound is about 1088 ft/sec, 331.6 m/sec, or 741.8 mi/hr). The mach speed is important to the control of an aircraft, especially at speeds close to or exceeding mach 1.0. The unit, usually pronounced "mock" in English, is named for the Austrian physicist Ernst Mach (1838-1916). Maf or Maf a symbol for one million acre feet. This symbol, commonly used in reservoir management in the U.S., should be written Maf. 1 Maf =about 1.2335 billion (109) cubic meters. Magnitude (mag) [1] a unit traditionally used in astronomy to express the apparent brightness of stars, planets, and other objects in the sky. For centuries, the brightest stars were said to be of the "first magnitude," with fainter ones of the "second magnitude" and so on down to "sixth magnitude" for the faintest stars visible to the unaided eye. When it became possible to measure stellar brightnesses precisely, it was discovered that stars of a given traditional magnitude were roughly 2.5 times brighter than stars of the next magnitude. Astronomers agreed to define the magnitude scale so that a difference of exactly 5.0 mag corresponds to a brightness difference of exactly 100 times. A difference of 1.0 mag then corresponds to a brightness difference of the fifth root of 100 or about 2.512 times. The scale is upside down: brighter stars have lower, not higher magnitudes, in keeping with the historical origin of the scale. The zero point (0.0 mag) is set arbitrarily so that the stars historically listed as "first magnitude" have magnitude measurements of 1.5 mag or brighter. Magnitude (mag) [2] a unit used in earth science to measure the intensity of earthquakes. Geologists actually use several scales to measure earthquake intensity, but the one best known to the public is the Richter magnitude scale, developed in 1935 by Charles F. Richter (1900-1985) of the California Insitute of Technology. The Richter magnitude is computed from the measured amplitude and frequency of the earthquake's shock waves received by aseismograph, adjusted to account for the distance between the observing station and the epicenter of the earthquake. An increase of 1.0 in the Richter magnitude corresponds to an increase of 10 times in the amplitude of the waves. The most powerful earthquakes recorded so far had magnitudes of about 8.5. The Richter magnitude measures the intensity of the earthquake itself, not the intensity of the earthquake's effects: the effects also depend on the depth of the earthquake, the geology of the area around the epicenter, and many other factors. Earthquake effects are rated using the Mercalli scale (see below). Magnum a traditional unit of volume for wine, generally equal to 2 bottles. This is now exactly 1.5 liters (about 2.114 U.S. quarts). Mahnd a traditional Arab weight unit equal to about 2.04 pounds or 907 grams. Ml a Norwegian word for "measure," ml has been used as a name for various traditional Norwegian units. As a land measure, the ml is currently defined to be the same as the dekare, that is, exactly 1000 square meters (0.1 hectare or 0.2471 acre). The ml has also been used as a unit of volume equal to the dekaliter (10 liters). Mandel a traditional German unit of quantity equal to 15. Man hour a common unit of labor equal to the work of one person for one hour. The less restrictive term person hour is gradually coming into use. Manpower an informal unit of power equal to 0.1 horsepower or about 74.57 watts. The unit seems to have been invented by American engineers. Manzana a traditional unit of land area in Central America. The manzana is the area of a square 100 varas on a side; it thus varies according to the length of the vara. The Costa Rican manzana equals 0.698 896 hectares or about 1.727 acres. Very similar units are used in Guatemala, Honduras, and Nicaragua. The word manzana means an apple, but the unit is probably related to manzanar, orchard. Marathon a traditional unit of distance used in athletics. The length of a marathon is exactly 26 miles 385 yards, or about a millimeter less than 42.195 kilometers. Invented for the first modern Olympic Games at Athens in 1896, the marathon recalls the run made by Pheidippides in 490 BC to bring to Athens the news of the Greek victory over the Persians at the Battle of Marathon. However, the actual distance from Marathon to Athens is only about 36.75 kilometers. The 1896 run was exactly 40 kilometers from the Marathon Bridge to the Olympic Stadium. At the 1908 Olympics in London, a course of 26 miles 385 yards brought runners from Windsor Castle to White City Stadium. The marathons at the Olympic Games continued to vary until the 1924 Olympics in Paris, when the International Olympic Committee adopted the 1908 London distance as official. Marc, marco, or mark traditional units of weight in various countries of Western Europe. In each country the unit equals 1/2 the unit corresponding to the English pound. Thus the French marc equals 1/2 livre, 8 onces or about 244.75 grams; the Spanish marco equals 1/2 libra or about 230 grams; the German mark equals 1/2 pfund or about 280.5 grams; and the English mark equals 8 ounces or 226.8 grams. The English unit was used almost entirely for measuring precious metals. Mark twain see twain. Mas symbol for milliarcsecond, a unit of angular measure commonly used in astronomy. Masl a common symbol for "meters above sea level" used in geology and geography. Maund a traditional unit of weight in India and throughout South Asia. The maund varied considerably, but during the period of British rule in India it was standardized at about 82.286 pounds or 37.3242 kilograms. The maund is divided into 40 seers. Since 1980, Pakistan has used a metric version of the maund equal to exactly 40 kilograms (88.185 pounds); this would make the seer equal to the kilogram. Maxwell (Mx) a CGS unit of magnetic flux, equal to 10-8 weber. In amagnetic field of strength one gauss, one maxwell is the total flux across a surface of one square centimeter perpendicular to the field. This unit was formerly called the line [2]. The newer name honors the British physicist James Clerk Maxwell (1831-1879), who presented the unified theory of electromagnetism is 1864. MBF or MBM traditional symbols for 1000 (not one million) board feet, aunit of volume for timber equal to 250/3 = 83.333 cubic feet or 2.360 cubic meters. "BM" stands for "board measure." Mbsl a common symbol for "meters below sea level" used in geology and oceanography. MBtuh a symbol for 1000 (not one million) Btu (British thermal units) per hour, a unit traditionally used in the U.S. heating and air conditioning industry to state rates of heating or cooling. One MBtuh equals about 0.293 071 kilowatt. Mc