The definitions of four SI base units – the kilogram, ampere, kelvin and mole – have been changed.
The new definitions are based on fixed numerical values of the Planck constant (h), the elementary charge (e), the Boltzmann constant (k) and the Avogadro constant (NA) respectively. They came into force on 20th May.
The seven SI base units are:
- time: second (s)
- length: metre (m)
- mass: kilogram (kg)
- electric current: ampere (A)
- thermodynamic temperature: kelvin (K)
- amount of substance: mole (mol)
- luminous intensity: candela (cd)