Ampere balance
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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, and the force between the two coils is measured. This is used to calculate the magnitude of the current.
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. An improved apparatus called the watt balance sidesteps this problem and is now used instead.