Michael Faraday discovered the law of electromagnetic induction in 1831: an electric current is induced in a conductor when it is moved through magnetic lines of force. Faraday constructed a homopolar generator, now also known as a Faraday disk (disc, in England), which produces a relatively weak direct current (DC). This has been superseded by alternating current (AC) electrical generators, which power the machinery of the modern world.
This Demonstration shows a simulated Faraday disk dynamo. A copper disk in contact with two stationary brushes is manually rotated between the poles of a permanent magnet. This generates a current proportional to the rate of rotation, which can power a small light bulb. The disk can be rotated in either direction.