The International System of Units.

The name Système International d'Unités (International System of Units) with the international abbreviation SI was adopted by the Conférence Générale des Poids et Mesures (CGPM) in 1960. It is a coherent system based on the seven base units (CGPM 1960 and 1971) listed in table 1 below (or GIF image).

The SI is a coherent system of units, which is to say that any derived unit is an exact multiple of the base units. Specific names and symbols have been given to several derived SI units.

Definitions of the SI base units

The primary definitions of the SI base units are in French. Their current definitions, along with an English translation, are given below:

metre; mètre

Le mètre est la longueur du trajet parcouru dans le vide par la lumière pendant une durée de 1/299 792 458 de seconde. (17th CGPM (1983), Resolution 1).

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

kilogram; kilogramme

Le kilogramme est l'unité de masse; il est égal à la masse du prototype international du kilogramme. (1st CGPM (1889) and 3rd CGPM (1901)).

The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.

Note: This internatational prototype is made of platinum-iridium and is kept at the International Bureau of Weights and Measures, Sèvres, France.

second; seconde

La seconde est la durée de 9 192 631 770 périodes de la radiation correspondant à la transition entre les deux niveaux hyperfins de l'état fondamental de l'atome de cesium 133. (13th CGPM (1967), Resolution 1).

The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.

ampere; ampère

L'ampère est l'intensité d'un courant constant qui, maintenu dans deux conducteurs parallèles, rectilignes, de longueur infinie, de section circulaire négligeable, et placés à une distance de 1 mètre l'un de l'autre dans le vide, produirait entre ces conducteurs une force égale à 2×10^-7 newton par mètre de longueur. (9th CGPM (1948), Resolutions 2 and 7).

The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2×10^-7 newton per metre of length.

kelvin; kelvin

Le kelvin, unité de température thermodynamique, est la fraction 1/273,16 de la température thermodynamique du point triple de l'eau. (13th CGPM (1967), Resolution 4).

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

The 13th CGPM (1967, Resolution 3) also decided that the unit kelvin and its symbol K should be used to express both thermodynamic temperature and an interval or a difference of temperature.

In addition to the thermodynamic temperature (symbol T) there is also the Celsius (symbol t) defined by the equation

t=T-T₀ where T₀=273.15 K. Celsius temperature is expressed in degree Celsius; degré Celsius (symbol °C). The unit 'degree Celsius' is equal to the unit 'kelvin', and a temperature interval or a difference of temperature may also be expressed in degrees Celsius.

mole; mole

1°. La mole est la quantité de matière d'un système contenant autant d'entités élémentaires qu'il y a d'atomes dans 0,012 kilogramme de carbone 12.

2°. Lorsqu'on emploie la mole, les entités élémentaires doivent être spécifiées et peuvent être des atomes, des molécules, des ions, des électrons, d'autres particules ou des groupements spécifiés de telles particules. (14th CGPM (1971), Resolution 3).

1. The mole 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.

2. 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 particle.

Note: In this definition, it is understood that the carbon 12 atoms are unbound, at rest and in their ground state.

candela; candela

La candela est l'intensité lumineuse, dans une direction donnée, d'une source qui émet une radiation monochromatique de fréquence 540×10¹² hertz et dont l'intensité énergétique dans cette direction est 1/683 watt par stéradian. (16th CGPM (1979), Resolution 3).

The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of a frequency 540×10¹² hertz and has a radiant intensity in that direction of (1/683) watt per steradian.

SI derived units

The SI derived units with special names are listed in table 2.

Table 2. SI derived units.

Quantity	Name	Symbol	Expression in SI base units
plane angle	radian [1]	rad	m m-1 = 1
solid angle	steradian [1]	sr	m2 m-2 = 1
frequency	hertz	Hz	s-1
force	newton	N	m kg s-2
pressure	pascal	Pa	m-1 kg s-2
energy, work, quantity of heat	joule	J	m2 kg s-2
power, radiant flux	watt	W	m2 kg s-3
electric charge, quantity of electricity	coulomb	C	s A
electric potential, potential difference, electromotive force	volt	V	m2 kg s-3 A-1
capacitance	farad	F	m-2 kg-1 s4 A2
electrical resistance	ohm		m2 kg s-3 A-2
electrical conductance	siemens	S	m-2 kg-1 s3 A2
magnetic flux	weber	Wb	m2 kg s-2 A-1
magnetic flux density	tesla	T	kg s-2 A-1
inductance	henry	H	m2 kg s-2 A-2
Celsius temperature	degree Celsius	°C	K
luminous flux	lumen	lm	cd m2 m-2 = cd
illuminance	lux	lx	cd m2 m-4 = cd m-2
activity (of a radionuclide)	becquerel	Bq	s-1
absorbed dose [2], specific energy imparted, kerma	gray	Gy	m2 s-2
dose equivalent [2]	sievert	Sv	m2 s-2

Notes:

1. The radian and steradian are now derived units, their separate designation as supplementary units was abrogated by the CIPM 1995.
2. The dose equivalent is equal to the absorbed dose multiplied by dimensionless factors defining the relative biological effectiveness of the radiation. Although the gray and sievert have the same expression in terms of base units, they measure conceptually distinct quantities.