William Thomson's Current Balance
William Thomson (Lord Kelvin, 1824-1907)
patented the current balance in 1894. The balance can measure current or voltage using electromagnetic attraction and repulsion coupled with a standard beam balance mechanism.(1) The balance has with it a set of weights and a thermometer that is inserted through the hole in the top of the case and into the right coil. This instrument is an example of a centi-ampere balance. The high current terminals are at the back of the balance.
The beam balance mechanism consists of two rings connected to each end of the beam, with their planes horizontal to the beam. Each of the rings is free to move between a set of coils. These coils are joined in series, the current being measured passes through them, and the action of the two coils on the ring at one end is to depress the ring, while the action on the ring at the other end is to raise it. The beam, pivoted in the middle, is then turned around the horizontal axis and balanced by a sliding pointer and weight. (2)
"Sir Wm. Thomson's Patent Electric Balance J. White, Glasgow No. 233"

2)"Absolute Measurements In Electricity And Magnetism", Volume II,  pg 407-412, Andrew Gray, M.A., MacMillan and Co.,  1893


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