Wednesday, November 18, 2009

SI Base Units

Formal definitions of all SI base units are adopted by the CGPM. The first two definitions were adopted in 1889, and the most recent in 1983. These definitions are modified from time to time as science advances.

2.1.1 Definitions

Current definitions of the base units, as taken from the Comptes Rendus (CR) of the corresponding CGPM, are shown below indented and in a heavy sans-serif font. Related decisions which clarify these definitions but are not formally part of them, as taken from the Comptes Rendus of the corresponding CGPM or the Proc├Ęs-Verbaux (PV) of the CIPM, are also shown indented but in a sans-serif font of normal weight. The linking text provides historical notes and explanations, but is not part of the definitions themselves.

It is important to distinguish between the definition of a unit and its realization. The definition of each base unit of the SI is carefully drawn up so that it is unique and provides a sound theoretical basis upon which the most accurate and reproducible measurements can be made. The realization of the definition of a unit is the procedure by which the definition may be used to establish the value and associated uncertainty of a quantity of the same kind as the unit. A description of how the definitions of some important units are realized in practice is given on the BIPM website,

A coherent SI derived unit is defined uniquely only in terms of SI base units. For example, the coherent SI derived unit of resistance, the ohm, symbol ohm, is uniquely defined by the relation ohm = m2 kg s–3 A–2, which follows from the definition of the quantity electrical resistance. However any method consistent with the laws of physics could be used to realize any SI unit. For example, the unit ohm can be realized with high accuracy using thequantum Hall effect and the value of the von Klitzing constant recommended by the CIPM (see Appendix 1).

Finally, it should be recognized that although the seven base quantities – length, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity – are by convention regarded as independent, their respective base units – the metre, kilogram, second, ampere, kelvin, mole, and candela – are in a number of instances interdependent. Thus the definition of the metre incorporates the second; the definition of the ampere incorporates the metre, kilogram, and second; the definition of the mole incorporates the kilogram; and the definition of the candela incorporates the metre, kilogram, and second.

2.1.2 Symbols for the seven base units

The base units of the International System are listed in Table 1, which relates the base quantity to the unit name and unit symbol for each of the seven base units (10th CGPM (1954, Resolution 6); 11th CGPM (1960, Resolution 12); 13th CGPM (1967/68, Resolution 3); 14th CGPM (1971, Resolution 3)).

Table 1. SI base units

Base quantity
SI base unit
Name Symbol Name Symbol
length l, x, r, etc.
metre m
mass m kilogram kg
time, duration t second s
electric current I, i ampere A
thermodynamic temperature T kelvin K
amount of substance n mole mol
luminous intensity Iv candela cd

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