International System of Units revised in historic vote

10 Dec, 2018

In a landmark decision representatives from 54 of the International Bureau of Weights and Member States voted to revise the International System of Units (SI), changing the world's definition of the kilogram, the ampere, the kelvin and the mole.

The decision, made at the 26th meeting of the General Conference on Weights and Measures (CGPM) in Versailles, France, means that all SI units will now be defined 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 20 May 2019, will bring an end to the use of physical artefacts to define measurement units.

The International Prototype of the Kilogram (IPK), a cylinder of platinum/iridium alloy conserved at the BIPM, which has been used as the definition of the kilogram for almost 130 years, will now be retired. It will be replaced by a definition based on the Planck constant - the fundamental constant of quantum physics.

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.

Several projects were initiated for replacing the platinum-iridium cylinder. In the framework of one of these projects, a new etalon was created in the form of an ideal polycrystalline silicon ball enriched by stable 28Si isotope, with a deviation from sphericity of less than 30 nanometers. Rosatom is participating in this project. Until the end of 2018, the Russian concern will supply the silicon-28 (28Si) isotope to Germany for the Kilogram-3 international research project on the creation of the a generation etalon of a mass.

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