The watt balance is an electromechanical apparatus used for the precise measurement of the SI unit of electric current, the ampere.
The watt balance is a more accurate version of the ampere balance, in which the force between two current-carrying coils is measured and then used to calculate the magnitude of the current. The principle of the watt balance was proposed by B. P. Kibble of the UK's National Physical Laboratory (NPL) in 1975.
The main weakness of the ampere balance method is that the result depends on the accuracy with which the dimensions of the coils are measured. The watt balance method has an extra calibration step in which the effect of the geometry of the coils is eliminated, removing the main source of uncertainty. This extra step involves moving the force coil through a known magnetic flux at a known speed.
Versions of the watt balance built since about 1998, in the USA, UK, France and Switzerland, have achieved accuracies in the range from one part in 1 × 10−7 to one part in 1 × 10−8. The long-term goal of these experiments is to produce a new definition of the kilogram based on fundamental SI units, to replace the present definition based on the international prototype kilogram, a block of platinum/iridium owned by the International Bureau for Weights and Measures.