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Joseph Henry. Portrait of the American physicist Joseph Henry (1797-1874) in his early thirties. In 1829 Henry made a much improved electromagnet, with a varnished soft iron core wrapped with many turns of thin, insulated wire. In 1831 he designed the first reciprocating electric motor. In 1830 he discovered electromagnetic induction but was beaten to publication by Faraday. Henry is, however, credited with the discovery of self- induction, and the unit is named after him. In 1835 he introduced the relay which made long- distance electric telegraphy practical, an important step in North America. He was the first director of the Smithsonian Institute.
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