# 2.02 Definitions for the seven base SI units

## Metre

One 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

One kilogram is equal to the mass of the International Prototype Kilogram. (This is a platinum iridium cylinder kept by the International Bureau of Weights and Measures at Sevres in France.)

## Second

One second is the duration taken for 9,192, 631,770 periods of a type of vibrational motion which occurs in caesium133 atoms at rest and at a temperature of 0K

Ampere (Amp)
One ampere is the current that flows when a magnetic force of 2 x 10-7 Newton per metre is produced between two parallel conductors of infinite length and negligible cross sectional area when separated by a distance of one metre of empty space.

## Kelvin

One Kelvin, is equal to 1/273.16 of the temperature of the triple point of water. The triple point of water occurs when it exists in all three states (gas liquid and solid) in equilibrium. (This only occurs at one specific temperature and pressure.)

## Mole

One mole is the amount of a substance that contains as many elementary particles as there are atoms in 0.012 kilogram of carbon 12.

## Candela

One candela is defined as the luminous intensity of a monochromatic light source of 540 x 1012HZ that radiates 1/683 watt of light radiation per steradian.