Dictionary of Principal Units

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- KELVIN (K)
- The SI unit of thermodynamic temperature. The 13
^{th}General Conference on Weights and Measures (CGPM 1967) adopted the name kelvin (symbol K) and defined the unit as follows:

The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

The term*degree Kelvin*was officially dropped in 1967. Thus, the symbol is K and not^{o}K.

- KILOGRAM (kg)
- The SI unit of mass. The 1
^{st}General Conference on Weights and Measures (CGPM 1889) legalized the international prototype of the kilogram and declared:

This prototype shall henceforth be considered to be the unit of mass.

This international prototype made of platinum-iridium is kept by the International Bureau of Weights and Measures at Paris under conditions specified by the 1^{st}CGPM in 1889. A duplicate in the custody of the National Bureau of Standards at Washington D.C. is the mass standard for the United States.**The kilogram is the only base unit still defined by an artifact.**A Kilogram equals (1) 1 000 grams; (2) 2.205 pounds; (3) 9.842 x 10^-4 long tons; or (4) 1.102 x 10^-3 short tons.

- KILOMETER (km)
- 1 kilometer = 103 meters = 0.621 miles (approximately)

- KNOT (kn)
- A unit of speed. One knot equals 1 nautical
mile per hour. (A knot
also equals 6 080.2 feet/hour or 1.151 statute miles/hour.)

- LAMBERT (L)
- A unit of luminance. One lumen per square
centimeter leaves a
surface whose luminance is 1 lambert in all directions within a
hemisphere. (The candela per square
meter is the preferred unit of
luminance.)

- LIGHT YEAR (ly)
- A unit of distance. One light year equals the distance that a
light ray (photon) would travel in 1 year.
1 light year equals 9.46 070 x 10
^{15}meters or 5.87 848 x 10^{12}miles.

- LITER (l)
- An SI unit of volume. One liter equals 10
^{-3}cubic meters. (A liter also equals (1) 1 000 cubic centimeters; (2) 0.035 31 cubic feet; (3) 61.02 cubic inches; (4) 1.308 x 10^{-3}cubic yard; (5) 0.2642 U.S. liquid gallon; (6) 1.057 U.S. quarts; or (7) 0.2050 Imperial gallons.)

- LUMEN (lm)
- The SI unit of luminous flux. The flux through a unit solid
angle (1 steradian) from
a uniform point source of 1 candela.

- LUMEN PER SQUARE FOOT (lm/ft2)
- A unit of illuminance and also a unit of luminous
excitation.

- LUMEN PER SQUARE METER (lm/m2)
- The SI unit of luminous excitation.

- LUMEN PER WATT (lm/W)
- The SI unit of luminous efficacy.

- LUMEN SECOND (lm s)
- The SI unit of quantity of light.

- LUX (lx)
- THe SI unit of illuminance. One lux equals 1 lumen
per square meter.

- MAXWELL (Mx)
- The CGS unit of magnetic flux. One maxwell equals 1 gauss
per square centimeter, or 1 magnetic line of force. (One maxwell
equals 0.001 kiloline, or 10
^{-8}weber. Use of the SI unit, the weber, is preferred.)

- METER (m)
- The SI unit of length. The 17th General Conference on Weights
and Measures (CGPM, 1983) abolished the former definition of the
meter and adopted a new definition which reads:

The meter is the length of the path traveled by light in vacuum during a time interval of 1/299 792 458 of a second.

The old prototype of the meter, which was legalized by the 1st CGPM in 1889, is still kept at the International Bureau of Weights and Measures (BIPM).The meter was formerly defined as 1 650 763.73 wavelengths in vacuum of the orange-red line of the spectrum of Kr-86 (Krypton). A meter also equals (1) 100 centimeters; (2) 3.281 feet; (3) 39.37 inches; (4) 0.001 kilometer; (5) 5.396 x 10

- MHO (mho)
- A unit of conductance (and admittance). The conductance of a
conductor whose resistance is 1 ohm. (The name siemens
(S) is also used for this quantity.)

- MICROMETER (mu m)
- A unit of length. One micrometer equals one millionth
(10
^{-6}) of a meter. (The term micron formerly used for this unit no longer is preferred.)

- MICRON
- A unit of length. The micron is an antiquated form of
micrometer.

- MIL (mil)
- A unit of length. One mil equals one-thousandth of an
inch.

- MILE, STATUTE (mi)
- A unit of length. One mile equals 5 280 feet.
One statute mile also equals (1) 1.609 kilometers; (2) 1 760
yards; (3) 6.336 x 10
^{4}inches; or (4) 0.8684 nautical miles.

- MILE, NAUTICAL (nmi)
- A unit of length. One nautical mile equals 1.1516 statute
miles. One nautical mile also equals (1) 6 080.27 feet; (2)
1.853 kilometers; or (3) 2 027 yards.

- MILLIMETER (mm)
- A unit of length. One millimeter = 10
^{-3}meter.

- MILLIMETER PF MERCURY (mmHg)
- A unit of pressure.

- MINUTE (of TIME) (min)
- A unit of time. One minute equals 60 seconds.
(Time may also be designated by means of superscripts, as in 9
m30 ^{x}, where there otherwise will be no confusion with abbreviations.)

- MOLE (mol)
- The SI unit of amount of substance. The following definition
of the mole, adopted by the 14th General Conference on Weights and
Measures (CGPM 1971):

The mole is the amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.

One mole is an amount of a substance, in specified mass units, equal to the molecular weight of that substance. (Examples are the gram mole and the pound mole.)

- NEPER (Np)
- A dimensionless unit for expressing the ratio of two voltages,
two currents, or two power values in a logarithmic manner. The
number of nepers is the natural (Naperian) logarithm of the square
root of the ratio of the two values being compared. Thus, the
neper uses the base of 2.718 28 in contrast with the bel
(or decibel) which uses
the common-logarithm base of 10. One neper equals 8.686 decibels.

- NEWTON (N)
- The SI unit of force. One newton is the force that will impart
an acceleration of 1 meter per
secondkilogram. One newton equals
10
^{5}dynes.)

- NIT (nt)
- A unit of luminance which is synonymous with candela
per square meter.

- OERSTED (Oe)
- The CGS unit of magnetic field strength. The magnetic field
produced at the center of a plane circular coil of 1 turn and of
radius 1 centimeter,
which caries a current of 0.5PI abamperes.
Use of the SI unit, the ampere
per meter, is preferred.

- OHM (omega)
- The SI unit of resistance (and of impedance). The resistance
of a conductor such that a constant current of 1 ampere
in it produces a voltage difference of 1 volt
between its ends.

- OUNCE (oz)
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