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Units main page Fundamental SI units supplementary SI


There are seven fundamental quantities which are assumed to be mutually independent, on which the International System of Units (Le Systeme International d'Unités) or the SI is founded. These units are listed below:

Base quantity

Name

Symbol

length

meter

m

mass

kilogram

kg

time

second

s

electric current

ampere

A

temperature (thermodynamic)

kelvin

K

amount of substance

mole

mol

luminous intensity

candela

cd

  • Meter (1983) - It is the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a second. (This statement also implicitly defines 299792458 ms-1 as the exact speed of light in vacuum). The standard Meter was used to be the meter bar made of iridium-platinum alloy kept at the International Bureau of Weights and Measures (BIPM), Paris.

  • Kilogram (1901) - It is the unit of mass, equal to the mass of the international prototype of the kilogram. It is a standard artifact, made of platinum-iridium alloy kept at the BIPM.

  • Second (1967) - It is the duration of 9,192,631,770 periods of radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom.

  • Ampere (1948) - It is the constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and place one meter apart in vacuum, would produce between these two conductors a force equal to 2 x 10-7 newton per meter of length.

  • Kelvin (1967) - It is the fraction 1/273.16 of the thermodynamic temperature of the triple-point of pure water. In other words, the triple-point of pure water is defined to be 273.16 K.

  • Mole (1971) - It is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12 at rest and at the ground state. The elementary entities may be atoms, molecules, ions, electrons etc.

  • Candela (1979) - It is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity of (1/683) watt per steradian.

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