On November 16, 2018, at the General Conference on Weights and Measures, in Versailles, Paris, the world’s standard definition of the kilogram, the ampere, the Kelvin and the mole was changed forever, after representatives from 60 countries voted to redefine the International System of Units (SI) for weight, current, temperature and amount of chemical substance.
The definition of the kilogram, the International Prototype of the Kilogram (IPK), a cylinder of a platinum alloy stored at the International Bureau of Weights and Measures (BIPM) in France, will now be retired after more than 130 years.
During the conference organised by BIPM, the representatives decided that all SI units will now be in terms of constants that describe the natural world.
This will assure the future stability of the SI and open the opportunity for the use of new technologies, including quantum technologies, to implement the definitions.
The changes, which will come into force on May 20 next year, will bring an end to the use of physical objects to define measurement units.
The new definitions impact four of the seven base units of the SI: the kilogram, ampere, kelvin and mole; and all units derived from them, such as the volt, ohm and joule.
Although the size of these units will not change, 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.
