Board of Trade current balance. This current balance is also known as an ampere balance or Kelvin balance. It is an electromechanical apparatus used for the precise measurement of the ampere, a unit of electric current. The current to be measured is passed through two coils (seen around a marble cylinder at lower left) and the magnetic force between the two coils is measured to give the magnitude of the current. The current balance was invented by William Thomson, the First Baron Kelvin, in 1882. Photographed in 1950.