The SI system of units defines seven **SI base units**: fundamental physical units defined by an operational definition.

All other physical units can be derived from these base units: these are known as SI derived units. Derivation is by dimensional analysis. Use SI prefixes to abbreviate long numbers.

## No circular references

Note that there are two base units above which do not appear to be defined purely in terms of other base units.

- The ampere is defined in terms of newtons. However, one newton is 1 kg · m · s
^{-2}.
- The candela is defined in terms of hertz, watt and steradians. One hertz is 1 s
^{-1}, one watt is 1 J · s^{-1} = 1 kg · m^{2} · s^{-3} and the steradian is the dimensionless solid angle subtended at the centre of a sphere of radius *r* by a portion of the surface of the sphere having an area *r*^{2}.

Therefore there are no circular references in the definition of the base units.

## See also

## External links

- BIPM http://www1.bipm.org/en/si/base_units/
- NPL - Kilogram http://www.npl.co.uk/mass/faqs/kilogram.html
- NIST -SI http://physics.nist.gov/cuu/Units/index.html

Last updated: 05-02-2005 21:13:18

Last updated: 05-03-2005 17:50:55