Electrical power is usually generated by electro-mechanical generators driven by steam produced from fossil fuel combustion,or the heat released from nuclear reactions; or from other sources such as kinetic energy extracted from wind or flowing water.The modern steam turbine invented by Sir Charles Parsons in 1884 today generates about 80 percent of the electric power in the world using a variety of heat sources.Such generators bear no resemblance to Faraday's homopolar disc generator of 1831,but they still rely on his electromagnetic principle（电磁原理） that a conductor（导体） linking a changing magnetic field（电磁场） induces a potential difference（电势差i.e.电压） across its ends
