Faraday disc generator postcard

Drawing of Faraday disc, the first electromagnetic generator, invented by British scientist Michael Faraday in 1831. The copper disc (D) rotated between the poles of a horseshoe shaped magnet (A), creating a potential difference between the axis and rim due to Faraday's law of induction. If an electrical circuit such as a galvanometer was connected between the binding posts (B) and (B') the motion induced a radial flow of current in the disc, from the axle toward the edge. The current flows into the spring contact (m) sliding along the edge of the disc, out of binding post (B') through the external circuit to binding post (B) , and back into the disc through the axle. Turning it in the opposite direction reverses the direction of current. The caption also says that passing a current from a Bunsen cell (battery) through it would cause the disc to turn, making it function as an electric motor. The labelled parts are given in the caption as: (A) inducing magnet (D) induced disc (B) binding-screw for current entering or exiting axis of disc (B') binding-screw for current entering or exiting circumference of disc (m) rubber (sliding spring contact) for edge of disc. The caption describes it as 'Foucault's and Le Roux's apparatus' so this picture was not drawn from Faraday's original machine, but one owned by Faraday's contemporary French physicist Léon Foucault. The Faraday disc was an inefficient generator because counter-currents flowed back through regions of the disc outside the magnetic field. It was the first homopolar generator