A magneto is a simple electrical AC generator: a device that converts mechanical energy into high-voltage alternating current. Magnetos are used when batteries are impractical, such as the ignition system of internal combustion engines, or for electrically detonating explosives.
A magneto has a primary winding in which a rotating magnet produces a low voltage, which in turn induces (see electromagnetic induction) a high voltage in a surrounding secondary winding. A make-and-break device in the primary winding creates the repeated changes of magnetic flux that induces the large voltage in the secondary winding. The magneto is connected to the rotating shaft of the engine. Initially, the inner magnet is rotated by hand (often by pulling on a spring-return cord); once running, the magnet is rotated by the engine itself.