# Measurement Dictionary of Principal Units

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## -F-

FAHRENHEIT (oF)
A unit of temperature. See DEGREE FAHRENHEIT

The SI unit of capacitance. A farad is the capacitance of a capacitor in which a charge of 1 coulomb produces a potential difference of 1 volt between the terminals.

FATHOM
A unit of length in the "English" system, once widely used to measure depths of bodies of water. A fathom was originally meant to equal the fingertip-to-fingertip distance when a "relatively large" man extends his arms horizontally. This makes a fathom equal to 2 yards, or 6 feet.

FERMI
A unit of length. One fermi equals 1 femtometer (= 10-15 meter).

FOOT (ft)
A unit of length in the "English" system. The origin of this unit pretty-much has to be what you suspect - the (average) length of the human foot. Foot-like units proliferated in many cultures, ranging (in modern terms) from less than 10 inches to more than 15 inches in length. The "modern" foot equals 12 inches, but is defined in terms of the meter. This unit should not be used for scientific or engineering purposes.

FOOTCANDLE (fc)
A unit of luminance. One footcandle equals 1 lumen per square foot. The name lumen per square foot is recommended for this unit. The SI unit, lux ( lumen per square meter) is preferred.

FOOTLAMBERT (fL)
A unit of luminance. One lumen per square foot leaves a surface whose luminance is 1 footlambert in all directions within a hemisphere. If luminance is measured in English units, the candela per square inch is preferred. However, use of the SI unit, the candela per square meter, is generally accepted.

FOOT.POUND (ft.lb)
A unit of work or (depending on context) a unit of torque.

Work: The work done by a force of one pound acting through a distance of 1 foot. The SI unit, the joule is preferred.

Torque: The torque exerted by a force of 1 pound acting through lever-arm of 1 foot.

## -G-

GAL (Gal)
A unit of acceleration. One Gal equals 1 centimeter per second per second.

GALLON (gal)
Because the gallon, quart, and pint differ in the United States and the United Kingdom, the use of this unit and term is generally discouraged for scientific purposes. An imperial gallon equals 1.200 95 U.S. gallons. One U.S. gallon equals 3.785 x 10-3 cubic meter.

GAUSS (G)
A unit of magnetic flux density, or magnetic induction. The ratio of the flux in any cross section to the area of that cross section, the cross section being taken normal to the direction of flow. One gauss equals 1 maxwell per square centimeter. The gauss is a unit of the CGS system. Use of the SI unit, the tesla, is preferred.

GILBERT (Gb)
A unit of magnetomotive force. One gilbert equals 0.4Pi(ni) where (ni) is an ampere-turn. The gilbert is a unit of the CGS system. Use of the SI unit, the ampere (or ampere turn), is preferred.

GRAIN (gr)
A unit of mass. One grain equals 0.064 80 gram. (One ounce, avoirdupois, equals 437.5 grains; 1 ounce, troy, equals 480 grains; 1 ounce, apothecaries', equals 480 grains. One pound, avoirdupois, equals 7 000 grains.)

GRAM (g)
The CGS unit of mass. One gram equals 1/1 000 th (10-3) kilogram.

## -H-

HAND
A unit of length in the "English" system of measurement. This ancient unit is still commonly used to measure the height of horses from the ground to the top of the shoulder. Originally, the hand was defined as the width of the four extended fingers, but during the reign of Henry VIII, the hand was defined as equal to 4 inches. (This is significantly larger than the width of most human hands.) This unit should not be used for scientific or engineering purposes.

HECTARE .
An SI unit of area. One hectare equals 100 are or 104 meter2, or 2.47 105 acres. 100 hectares = 1 kilometer2

HENRY (H)
The SI unit of inductance. One henry is the inductance of a circuit in which a current of 1 ampere induces a flux linkage of 1 weber.

HERTZ (Hz)
The SI unit of frequency. One hertz equals a frequency of one cycle per second.

HORSEPOWER (hp)
The "English" unit of power. The horsepower is considered an anachronism in science and technology. Use of the SI unit of power, the watt , is preferred. When used, 1 horsepower equals (1)42.44 Btu/min; (2) 33 000 foot-pounds/minute; or (3) 550 foot-pounds/second.

HOUR (h)
A unit of time. One hour equals 60 minutes, or 3 600 seconds.

## -I-

INCH (in)
A unit of length. The inch may have originally been defined as the width of a person's thumb (pressed down) at the base of the nail. H. Arthur Klein, in The World of Measurement (p. 54) quotes a proclamation attributed to King David I of Scotland (about 1150 C.E.):
The thowmys [thumbs] of iii [3] men, that is to say a mekill [big] man, and a man of messurable [moderate] statur, and of a lytell man. The thoums [another spelling for thumbs] are to be mesouret at [meaning across] the rut [root] of the nayll.

Alternatively, in 14th century England, during the reign of King Edward II, the inch was defined as:

three grains of barley, dry and round, placed end to end, lengthwise.

(Klein, p. 63). The inch was also defined as the combined length of 12 poppyseeds.

Today, one inch equals (by definition ) 2.54 centimeters. 12 inches = 1 foot.

INCH OF MERCURY (inHg)
A unit of pressure. One inch of mercury equals 3 386.4 newtons per square meter. (An inch of mercury also equals (1) 0.033 42 atmosphere; (2) 1.132 inches of water; (3) 345.3 kilograms/square meter (4) 70.73 pounds/square foot; or (5) 0.4912 pounds/square inch.)

INCH OF WATER (inH2O)
A unit of pressure. One inch of water equals 249.09 newtons per square meter. (An inch of water also equals (1)2.458 x 10^-3 atmosphere; (2) 0.073 55 inches of mercury; (3) 2.540 x 10^-3 kilogram/square centimeter; (4) 0.5781 ounce/square inch. The latter figures hold for a temperature of 4o Celsius.)

## -J-

JOULE (J)
The SI unit of energy. One joule is the work done by 1 newton acting through a distance of 1 meter. One joule equals (1) 1 watt-second; (2) 107 ergs; (3) 107 dyne. centimeters.)

JOULE PER KELVIN (J/K)
A unit of heat capacity and entropy.

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last update June 8, 2004 by JL Stanbrough