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Besides machines in which electricity is excited by friction, electric machines are now common in which an electric current is generated by the revolution near the poles of a magnet or magnets of one or more soft-iron cores surrounded by coils of wire, these machines being known distinctively as magneto-electric machines. A dynamo-electric machine is a machine of this kind, in which the induced currents are made to circulate round the soft-iron magnet which produced them, thus increasing its magnetization. This again produces a proportionate increase in the induced currents, and thus by a successive alternation of mutual actions very intense magnetization and very powerful currents are speedily obtained. There are many forms of these machines, such as Gramme's, Siemens', Wilde's, Brush's, etc., used extensively in electric lighting, and as a motor for machinery, electric railways, etc.