In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electrons in the windings to flow through the external electrical circuit. It is somewhat analogous to a water pump, which creates a flow of water but does not create the water inside. The source of mechanical energy may be a reciprocating or turbine steam engine, water falling through a turbine or waterwheel, an internal combustion engine, a wind turbine, a hand crank, compressed air or any other source of mechanical energy.
Early 20th century alternator made in Budapest, Hungary, in the power generating hall of a hydroelectric station Early Ganz Generator in Zwevegem, West Flanders, Belgium
The reverse conversion of electrical energy into mechanical energy is done by an electric motor, and motors and generators have many similarities. In fact many motors can be mechanically driven to generate electricity, and very frequently make acceptable generators.