A current balance measures the force of repulsion between two wires, each carrying an electric current. An upper wire is fixed in place, and the wire directly below it is free to move. The currents are adjusted so that the electrical attractive force on the lower wire perfectly balances the gravitational force on the wire so that the wire levitates, at rest. Weights can be added to the lower current to increase the gravitation force. This device allows one to study the attractive electrical force between the wires as a function of the currents through the wires. Lord Kelvin’s original current balance design, used in the pictured device, dates from 1882. The current being measured passes through parallel circular loops of wire that each carry the same amount of current.
Property of the W&L Physics & Engineering Department
“Current Balance,” Digital Exhibits at Washington and Lee University Library, accessed September 24, 2017, http://omeka.wlu.edu/items/show/175.