Ampere balance

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(left) Rosa-Dorsey type Ampere balance at the US National Bureau of Standards (now NIST) in 1927. The current coils are visible under the balance, attached to the right balance arm.
(right) Ampere balance constructed at the former National Institute of Advanced Industrial Science and Technology, Japan, now in the National Museum of Nature and Science, Tokyo, Japan.

The ampere balance (also current balance or Kelvin balance) is an electromechanical apparatus used for the precise measurement of the SI unit of electric current, the ampere. It was invented by William Thomson, 1st Baron Kelvin.

The current to be measured is passed in series through two coils of wire, one of which is attached to one arm of a sensitive balance. The magnetic force between the two coils is measured by the amount of weight needed on the other arm of the balance to keep it in equilibrium. This is used to calculate the magnitude of the current.

The main weakness of the ampere balance is that the calculation of the current involves the dimensions of the coils. So the accuracy of the current measurement is limited by the accuracy with which the coils can be measured, and their mechanical rigidity.

A more complicated version of an ampere balance, called the Kibble balance, that removes this source of inaccuracy by a calibration step, was invented by Bryan Kibble in 1975. This experimental device was developed at government metrology laboratories worldwide with the goal of providing a more accurate definition of the kilogram, the world's standard of mass. In this application, the Kibble balance functions in the reverse sense to the Ampere balance: it is used to weigh the International Prototype of the Kilogram, defining the kilogram in terms of an electric current and a voltage. Since voltage and current themselves are already defined in terms of fundamental physical constants like the Planck constant and the speed of light, this has created a new definition of the kilogram in terms of these fundamental constants.^{[citation needed]} This is a more stable definition than the previous one, which was based on the physical prototype kilogram which is vulnerable to damage and deterioration over the years.