Homopolar electric motor. Demonstration model of Faraday's 'electric motor', the first device to turn electrical energy into mechanical motion. The two containers would be filled with mercury and a permanent magnet mounted vertically in each. When a current was applied to the containers, the 'bridge' between them completed a circuit and the hanging arms would rotate around the magnet. This established the physics that would eventually lead to the invention of the electric motor. This model, made by Faraday in 1821, is now on display at the Royal Institution, London.