Dictionary of Principal Units

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- FAHRENHEIT (
^{o}F) - A unit of temperature. See DEGREE
FAHRENHEIT

- FARAD (F)
- 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.

- 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 10
^{4}meter^{2}, or 2.47 105 acres. 100 hectares = 1 kilometer^{2}

- 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.

- 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 4
^{o}Celsius.)

- 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) 10
^{7}ergs; (3) 10^{7}dyne^{.}centimeters.)

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

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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 Zlast update June 8, 2004 by JL Stanbrough