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May 20, 2019: Kilo cylinder a casualty of changes affecting world of measurements

WORLD - Changes that end the world’s definition of the kilogram, the ampere, the kelvin and the mole come into force. The change follows a vote in Nov 2018, in Versailles, France, that redefined the International System of Units (SI).

The definition of the kilogram for more than 130 years, 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. It will be replaced by the Planck constant – the fundamental constant of quantum physics. While the stability of the IPK could only be confirmed by comparisons with identical copies, a difficult and potentially inaccurate process, the Planck constant is ready for use everywhere and always.

The decision, made at the General Conference on Weights and Measures, which is organised by the BIPM, means that all SI units will now be defined in terms of constants that describe the natural world. According to the organization, the change 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 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.
• The kilogram – will be defined by the Planck constant (h)
• The ampere – will be defined by the elementary electrical charge (e)
• The kelvin – will be defined by the Boltzmann constant (k)
• The mole – will be defined by the Avogadro constant (NA)

Although the size of these units will not change (a kilogram will still be a kilogram), 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.

The signing of the Metre Convention in 1875 created the BIPM and for the first time formalized international cooperation in metrology. The move laid the foundations for worldwide uniformity of measurement, historically focusing on and assisting industry and trade and later spreading into fields that include climate and energy.

#22981 Published: 01/30/2019

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