Self-excited dynamo

In 1866 and 1867, working independently, Werner Siemens in Berlin and Charles Wheatstone and Samuel Varley in London showed that a generator did not need permanent magnets. A small residual magnetism in iron electromagnets could be amplified by feeding part of the machine's own current back into its field coils, so that rotating a coil produced ever stronger current. Siemens named it the dynamo-electric principle, and presented it to the Berlin Academy in January 1867. In 1871 the Belgian Zenobe Gramme, working in Paris, built a ring-armature dynamo that gave a smooth, steady current and could be sold to industry.¹³

Michael Faraday's discovery of induction in 1831 showed that a moving magnet makes current, and the first generators of the 1830s and 1840s used permanent magnets, which were weak and bulky. The telegraph and electroplating industries wanted more current, and engineers such as Siemens, who ran an electrical firm that built telegraph lines, needed it. Ányos Jedlik in Hungary had built a self-exciting machine around 1861 without publishing, and this overlap reflects how ready the field was.¹

The dynamo changed electricity from a laboratory curiosity into an industrial power source. Gramme's machine could be reversed to work as an electric motor, a fact that became widely known at the 1873 Vienna exposition. Steam engines turned dynamos to run arc lamps in lighthouses, factories and city streets, and in 1882 Edison's Pearl Street Station used large dynamos to light lower Manhattan. Later alternators and turbines replaced these direct-current machines, but the principle, using electromagnetism and mechanical power to generate electricity, remains the basis of nearly all generation today.²