International System of Units

The recommended practical system of units of measurement is the International System of Units (Système International d'Unités), with the international abbreviation SI.

SI

The SI was previously defined in terms of seven base units and derived units defined as products of powers of the base units. The seven base units were chosen for historical reasons, and were, by convention, regarded as dimensionally independent. They are

• Unit of time - second
• Unit of length - metre
• Unit of mass - kilogram
• Unit of electric current - ampere
• Unit of thermodynamic temperature - kelvin
• Unit of amount of substance - mole
• Unit of luminous intensity - candela.

Revision in SI

After decades of ground-breaking laboratory works, the world’s scientific and technical community, in a landmark and historic decision taken in the General Conference on Weights and Measures (CGPM), Versailles, France organised by International Bureau of Weights and Measures (BIPM) on 16 November 2018, has unanimously adopted the resolution to redefine four of the seven base units, the kilogram (SI unit of weight), Kelvin (SI unit of temperature), mole (SI unit of amount of substance), and ampere (SI unit of current).

• The kilogram – will be defined by the Planck constant (h)
• The ampere – will be defined by the elementary electrical charge (e)
• The kelvin – will be defined by the Boltzmann constant (k)
• The mole – will be defined by the Avogadro constant (NA)

The seven defining constants of the SI are:

• the caesium hyperfine frequency Cs
• the speed of light in vacuum c;
• the Planck constant h;
• the elementary charge e;
• the Boltzmann constant k;
• the Avogadro constant N_A; and
• the luminous efficacy of a defined visible radiation K_cd.

Although the size of these units will not change (a kilogram will still be a kilogram), the four redefined units will join the second, the metre and the candela to ensure that the set of SI base units will continue to be both stable and useful. The revised SI will maintain its relevance by facilitating technical innovations.

The new SI is being implemented worldwide from 20^th May 2019.

Source : BIPM