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Revision of the SI in 2018

21 November 2016

The International System of Units (SI) will undergo a major revision in 2018 that will be agreed and implemented by the international metrology community.

The changes to the SI involve no longer relying on artefacts to derive some units (such as the kilogram prototype in Paris) and instead use fundamental physical constants.

The use of constants in nature enables you to link from the smallest to the largest measurement quantities. It will tie measurements at the atomic (and quantum) scales to those at the macroscopic level. This introduces the appeal of a fundamental (“quantum”) basis for the changes.

There will still be the same seven base units (second, metre, kilogram, ampere, kelvin, mole, and candela). Of these, the kilogram, ampere, kelvin and mole will be redefined by choosing exact numerical values for the Planck constant, the elementary electric charge, the Boltzmann constant, and the Avogadro constant, respectively. The second, metre and candela are already defined by physical constants and it is only necessary to edit their present definitions.

The new definitions will improve the SI without changing the size of any units, thus ensuring continuity with present measurements.

The changes to the SI will provide a springboard for future innovation.

Further information on the changes can be found on the BIPM website.