Electrical power generators are devices that convert mechanical, chemical, or other forms of energy into electrical energy. The most common type of electrical power generator, a bicycle dynamo, uses electromagnetic induction to convert mechanical energy into electrical energy. These simple devices are essentially reversed electric motors with a rotor that carries one or more coils surrounded by a magnetic field, typically supplied by a permanent magnet or electromagnet. In other electrical power generators, mechanical energy from steam turbines moves the rotor, which induces an electric current in the rotor coil. Electrical power generators that provide direct current (DC) typically include a mechanical switch or commutator that switches the current every half-rotation so that the rotor remains unidirectional. Large, modern generators or alternators in power stations provide alternating current (AC) for general distribution. Specialized electrostatic generators such as Wimshurst machines and, on a large scale, van de Graaff generators are principally used for special applications such as particle accelerators that require high voltages but low current.