Ampere balance
![(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.](/Images/godic/202412/20/Ampere_balance_19270440.jpg")
![](/Images/godic/202412/20/Current_balance0440.jpg")
![The ampere balance](/Images/godic/202412/20/Lord_Kelvins_Ampere_Balance0441.png")
![Slider of Kelvin's ampere balance](/Images/godic/202412/20/Slider_of_Kelvin_Ampere_Balance0441.png")
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.