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SI units

the units of measurement generally accepted for all scientific and technical uses; together they make up the International System of Units. (See also metric system.) The abbreviation SI, from the French Système International d'Unités, is used in all languages. There are seven base SI units, defined by specified physical measurements, and two supplementary units. Units are derived for any other physical quantities by multiplication and division of the base and supplementary units. The derived units with special names are shown in the accompanying table.

SI is a coherent system. This means that units are always combined without conversion factors. The derived unit of velocity is the meter per second (m/s); the derived unit of volume is the cubic meter (m3). If you know that pressure is force per unit area, then you know that the SI unit of pressure (the pascal) is the unit of force divided by the unit of area and is therefore equal to 1 newton per square meter.

The metric prefixes can be attached to any unit in order to make a unit of a more convenient size. The symbol for the prefix is attached to the symbol for the unit, e.g., nanometer (nm) = 10−9 m. The units of mass are specified in terms of the gram, e.g., microgram (μg) = 10−9 kg.

Only one prefix is used with a unit; the use of units such as the millimicrometer is no longer acceptable. When a unit is raised to a power, the power applies to the prefix as well, e.g., a cubic millimeter (mm3) = 10−9 m3. When a prefix is used with a ratio unit, it should be in the numerator rather than in the denominator, e.g., kilometers/second (km/s) rather than meters/millisecond (m/ms). Only prefixes denoting powers of 103 are normally used. Hecto-, deka-, deci-, and centi- are usually attached only to the metric system units gram, meter, and liter.

Owing to the force of tradition, one noncoherent unit, the liter, equal to 10−3 m3, or 1 dm3, is generally accepted for use with SI. The internationally accepted abbreviation for liter is the letter l; however, this can be confused with the numeral 1, especially in typescript. For this reason, the capital letter L is also used as a symbol for liter. The lower case letter is generally used with prefixes, e.g., dl, ml, fl. The symbols for all other SI units begin with a capital letter if the unit is named after a person and with a lower case letter otherwise. The name of a unit is never capitalized.


William Thomson, Scottish physicist, 1824-1907. See: kelvin, Kelvin scale.

kel·vin (K),

A unit of thermodynamic temperature equal to 273.16-1 of the thermodynamic temperature of the triple point of water. See: Kelvin scale.
[Lord Kelvin]


/kel·vin/ (K) (kel´vin) the base SI unit of temperature, equal to 1/273.16 of the absolute temperature of the triple point of water.


(K) (kel'vin)
A unit of thermodynamic temperature equal to 1/273.16 of the thermodynamic temperature of the triple point of water.
See also: Kelvin scale
[Lord Kelvin]


device marked at regular intervals, used for measuring;
  • celsius scale; centigrade scale; °C temperature scale with 100 gradations between freezing (0°C) and boiling points (100°C) of pure water, when recorded at sea level

  • fahrenheit scale; °F temperature scale with 180 gradations between freezing (32°F) and boiling points (212°F) of pure water, when recorded at sea level

  • kelvin scale; K temperature scale measured in degrees Celsius from absolute zero; 0K is equivalent to -273.16°C, 0°C equates to 273K, and 100°C equates to 373K

References in periodicals archive ?
Outdoor light begins to get bluish at about 5,600 degrees Kelvin and goes up from there.
Some instruments will operate in cryogenic chambers chilled to 77 degrees Kelvin, or as cold as liquid nitrogen.
Luminys lighting systems offer up to 500,000 watts of intensity from a single source, with high quality light (5000 degrees Kelvin and 96+ CRI) that can dim from 100% to 3% of maximum output without a shift in color temperature.
Using radio telescopes, the research team led by Associate Professor Tomoharu Oka has found four "warm, dense (more than 50 degrees Kelvin, more than 10,000 hydrogen molecules per cubic centimeter)" masses of molecular gas at the center of the Milky Way Galaxy.
operational infrared sounders provide about 20 infrared channels of information and characterize atmospheric temperature profiles to an accuracy of two to three degrees Kelvin.
Our observations reveal, for the first time, the one-to-one connection between plasma that is heated to millions of degrees Kelvin and the spicules that insert this plasma into the corona," said McIntosh.
For the initial research detailed in the technical paper, MIT scientists used Star-P's efficient parallelization system to obtain the thermodynamic properties of 1,3-butadiene -- a prevalent air toxin and petroleum combustion by-product -- for temperatures ranging from 50 to 500 degrees Kelvin.
a leading developer of superconducting microelectronics technology and the Digital-RF[TM] product line, successfully demonstrated the operation of its high performance superconducting circuits in a modified Lockheed Martin compact four-stage, pulse tube cryocooler operating at 4 degrees Kelvin.
2 degrees Kelvin, or slightly above absolute zero - the temperature at which atoms almost freeze to a standstill, and only can jiggle a little bit.
Temperatures near the equator were found to reach a frigid 80 degrees Kelvin (minus 316 Fahrenheit), as expected.
We believe that in the future there could be a large potential market for MgB2 wire in 'dry' MRI and energy research magnets operating without liquid helium at temperatures greater than 10 degrees Kelvin.
The GOFs can retain 1 percent of their weight in hydrogen at a temperature of 77 degrees Kelvin and ordinary atmospheric pressure-roughly comparable to the 1.