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May 21, 2019

The new definitions of the International System of Units come into effect!

May 20, 2019, World Metrology Day, marked the culmination of many years of work that now makes it possible to have a measurement system based entirely on fundamental natural constants.

Kilogramme étalon

Last November, the delegations of all the signatory countries to the Metre Convention voted to use fundamental constants for the definitions of the kilogram, ampere, mole and kelvin. These units are now based on constants of nature that are immutable in space and time, freeing us from material artifacts that had shown their limits.

We can take as an example the case of the kilogram which, before this date, was defined according to the mass of an iridium platinum cylinder kept in Sèvres at the Bureau International des Poids et Mesures. Despite the precautions taken since 1889 to preserve this object in the best conditions, it was found that it had undergone variations. The Planck constant, a quantum physics constant on which mass units are now based, should not, by definition, be subject to any variations.

It was therefore chosen to use the Planck constant for the definition of the kilogram, the Boltzmann constant for the kelvin, the Avogadro constant for the mole and finally the elementary charge for the ampere.


The core values of the International System of Units

Units Defining constants Symbol Numérical value
kilogram Planck constant h 6,626 070 15 x 10–34 J s
metre Speed of light in vacuum c 299 792 458 m s-1
second Hyperfine Transition frequency of caesium atom ∆νCs 9 192 631 770 Hz
ampere Elementary charge e 1,602 176 634 x 10–19 C
kelvin Boltzmann constant k 1,380 649 x 10–23 J K-1
mole Avogadro constant NA 6,022 140 76 x 1023 mol-1
candela Luminous efficacy Kcd 683 lm W-1


These new definitions have been made possible through extensive research around the world over many years. The LNE, and the French metrology network it manages, has played a major role in these redefinitions. It has one of the three watt balances in the world that helped to set the Planck constant. Our work in fundamental electrical metrology has led to the development of a quantum current generator in accordance with the new and improved definition. And finally, the work of the LCM has made it possible to develop an acoustic thermometer for the new definition of the kelvin.

>> Consult our complete file on the International System of Units


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