A power generator is a complex device that converts mechanical energy into electrical power for use in an external circuit. The motive energy includes steam turbines, gas turbines, water turbines, internal combustion engines and even hand cranks. The first electromagnetic generator, the Faraday disk, was built in 1831 by British scientist Michael Faraday. Generators provide nearly all of the power for electric power grids. The operating principle of electromagnetic generators was discovered in the years of 1831–1832 by Michael Faraday. The principle later called Faraday's law, is that an electromotive force is generated in an electrical conductor which encircles a varying magnetic flux.