Measurement
Dictionary of Principal Units

Click a letter below to go directly to that letter in the dictionary.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

-U-

-V-

VAR (var)

The SI unit of reactive power. The reactive power at the port of entry of a single-phase two-wire circuit when the product of (a) the rms (root mean square) value in amperes of the sinusoidal current, (b) the rms value in volts of the voltage, and (c) the sine of the angular phase difference by which the voltage leads the current is equal to 1.

VOLT (V)

The SI unit of voltage. The voltage between 2 points of a conducting wire carrying a constant current of 1 ampere, when the power dissipation between these 2 points is 1 watt. (V = W/A)

VOLTAMPERE (VA)

The SI unit of apparent power. The apparent power at the port of entry of a single-phase two-wire circuit when the product (a) the rms (root mean square) value of the current (in amperes) and (b) the rms value of the voltage (in volts) is equal to 1. (P = VA)

-W-

WATT (W)

The SI unit of power. One watt equals 1 joule per second. One watt equals: (1) 3.4192 Btu/hour; (2) 0.056 88 Btu/minute; (3) 10⁷ erg/second; (4) 44.27 foot-pounds/minute; (5) 0.7378 foot-pounds/second; (6) 1.341 x 10^-3 horsepower; (7) 1.360 x 10^-3 metric horsepower; (8) 0.014 kilogram-calories.minute; or (9) 0.001 kilowatt.

WATT PER METER KELVIN (W/(m K)

The SI unit of thermal conductivity.

WATT PER STERADIAN (W/sr)

The SI unit of radiant intensity.

WATT PER STERADIAN SQUARE METER (W/(sr^. m²)

The SI unit of radiance.

WATTHOUR (Wh)

A unit of power. One watthour equals 3 600 joules. (One watthour equals: (1) 3.413 Btu; (2) 3.60 x 10¹⁰ ergs; (3) 2 656 foot-pounds; (4) 859.85 gram-calories; (5) 1.341 x 10^-3 horsepower-hour; (6) 367.2 kilogram-meters; or (7) 0.001 kilowatt-hour.)

WEBER (Wb)

The SI unit of magnetic flux. The magnetic flux passing through an area of 1 square meter placed normal to a uniform magnetic field of magnetic flux density equal to 1 tesla. (Wb = T^.m²). If the flux linked by a circuit changes at a uniform rate of 1 weber per second, a voltage of 1 volt is induced i the circuit. (Wb = V^.s).

-X-

-Y-

YARD (yd)

A unit of length in the "English" system. Although the word yard comes from an Anglo-Saxon word for measuring rod or stick, the yard was originally defined as the distance from a person's nose to their outstretched fingertips, when their arm was extended horizontally. A yard equals 3 feet or 36 inches. The yard should not be used for scientific or engineering purposes.

YEAR (yr)

A unit of time. One year equals the time required (period) for the Earth to complete one orbit around the sun. One year equals 365.242 2 days.

-Z-

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References:

A good deal of the material in the Units Dictionary comes from Van Nostrand's Scientific Encyclopedia - Fifth Edition, p2258-2260.

The quoted definitions of most of the fundamental SI units is from Contemporary College Physics - 2nd Edition, Jones and Childers, Addison-Wesley Publishing Company, Appendix B: The International System of Units, p. 921

Historical information and information on "English" units comes from H. Arthur Klein, The World of Measurements, Simon & Schuster, 1974.

Several units conversions come from research done by Ben Brunner.

Information on astronomical units comes mainly from H.J. Gray and Alan Isaacs, A New Dictionary of Physics, Longman, 1975.

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last update October 24, 1996 by JL Stanbrough