Skip to Content

SI Units

MSL is the New Zealand home for the International System of Units, the 'SI'

The International System of Units, the SI, is the internationally agreed basis for expressing measurements at all levels of precision and in all areas of science, technology, and human endeavour.

Revision of the SI in 2018

The 'SI' will undergo a major revision in 2018 that will be agreed and implemented by the international metrology community.  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. 

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 Boltzman 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 contunuity with present measurements. The changes to the 'SI' will provide a springboard for future innovation.

For further information on the 'SI' and the changes can be found on the BIPM website.

 

Brief History

The creation of the decimal metric system at the time of the French Revolution and the subsequent deposition of two platinum standards representing the metre and the kilogram, on 22 June 1799, in the Archives de la République in Paris can be seen as the first step in the development of the present International System of Units.

After the signing of the Metre Convention on 20 May 1875, work began on the new international prototypes of the metre and kilogram. In 1889 these international prototypes, together with the astronomical second as the unit of time constituted a three-dimensional mechanical unit system.  With the base units metre, kilogram and second it was known as the MKS system.

In following years the ampere, the kelvin and the candela were added as base units, respectively, for electric current, thermodynamic temperature and luminous intensity. The name International System of Units, with the abbreviation SI, was given to the extended system in 1960.  In 1971 the current version of the SI was completed by adding the mole as the base unit for amount of substance.