SI base unit: Meaning (information, definition, explanation, facts)

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.

Length (l)

Unit: metre (m)

One metre is defined as the distance light travels in a vacuum in 1/299,792,458 second. This standard was adopted in 1983 when the speed of light in vacuum was defined to be precisely 299,792,458 m/s.

Unit: kilogram (kg)

One kilogram is defined to be the mass of a specific cylinder of platinum-iridium alloy, kept at the International Bureau of Weights and Measures near Paris. There is an ongoing effort to introduce a definition by way of fundamental or atomic constants.

Unit: second (s)

One second is defined as the time required for 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of caesium 133. This definition was adopted in 1967.

Unit: ampere (A)

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.

The ampere is one of two base units (the other being the candela) that is not defined purely in terms of base units. One newton is 1 kg m s^-2, so there are no circular references.

Unit: kelvin (K)

The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. It is named after Lord Kelvin.

Unit: mole (mol)

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; its symbol is "mol".
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 particles.

Unit: candela (cd)

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

The candela is one of two base units (the other being the ampere) that is not defined purely in terms of base units. One hertz is 1 s^-1, one watt is 1 J s^-1 = 1 kg m² s^-2 and the steradian is the solid angle subtended at the centre of a sphere of radius r by a portion of the surface of the sphere having an area r². Therefore there is no circular reference here.