Ampere balance

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.

The main weakness of the ampere balance is that the calculation of the current involves the dimensions of the coils.

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 2019, the kilogram, ampere, kelvin, and mole were redefined in terms of fundamental constants, removing the dependence on physical objects.