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8th decade

Celebrating 120 years of measurement science

1975:  Watt balance offers potential to redefine the kilogram

The principle of the watt balance was first proposed by Dr Bryan Kibble at NPL in 1975. Since then, two watt balances have been built and operated. The original balance operated in air and was used to determine the ampere in terms of the SI base units: the kilogram, the metre and the second. The second balance was designed to measure the Planck constant with the aim of redefining the kilogram. It operates in a vacuum to eliminate the effects of the atmosphere on mass and velocity measurements. It has enabled the Planck constant to be determined with the lowest uncertainty in the world, and underpins the redefinition of the kilogram in 2019.

The Kibble balance, as the watt balance is now known, is an instrument which compares electrical and mechanical power. It allows the SI unit of mass to be realised in terms of quantum electrical units, with relation to a fixed value of the Planck constant.

More accurate measurement facilitates advances in science and technology, which in turn allows more accurate measurement. These improvements must be underpinned by stable measurement standards and agreed definitions of measurement units.

NPL is working to improve the dissemination of the SI system of units to enable technology innovation.

Discover more about NPL's research into SI units

Discover more about NPL's work on the Kibble balance