Watt balances: Providing a direct route from quantum electrical standards to the unit of mass

Dr. Stephan Schlamminger, National Institute of Standards and Technology (NIST), Gaithersburg, USA

In 1976, Bryan Kibble published an idea that improved the metrological link between mechanical and electrical power by orders of magnitude. The device used to implement this idea was named, in recognition of the unit of power, watt balance. The core tenet of Kibble’s idea is that the same physical principle that describes an electric motor can also be applied to explain a generator. Advances in the Josephson voltage standards and the discovery of the quantum Hall effect by von Klitzing in 1980 made it possible to express electrical power as a multiple of a product of two frequencies and the Planck constant. The combination of these quantum electrical standards and the watt balance provided a means to measure the Planck constant using a test mass with a precisely known mass value.

Forty years after Kibble’s publication, we are now on the verge of redefining the International System of Units (SI). Instead of being built upon base units, the new SI will rest on seven fundamental constants of nature whose values will be fixed. In the new system, the watt balance is one possibility to realize the unit of mass from a fixed numerical value of the Planck constant.