Joseph Henry (17.12.1797-13.05.1878) was an American physicist and a member of the National Academy of Sciences and its president (1866 — 78). He was born in Albany, where he studied at the Academy (1819 — 22) and later worked. In 1832 — 46 he was professor at Princeton College, and from 1846 director of the Smithsonian Institution.
His work concerned electromagnetism. He was the first to construct powerful horseshoe electromagnets (1828), using multilayer windings of insulated wire (their lifting capacity reached one tonne), and in 1831 he discovered the principle of electromagnetic induction (M. Faraday was the first to publish the discovery of induction). He built an electric motor (1831), detected (1832) the phenomenon of self-induction and extra currents, and established the causes that affect the inductance of a circuit. He invented the electromagnetic relay. He built a telegraph that worked over the grounds of Princeton College, and established in 1842 the oscillatory character of the discharge of a capacitor.
He was one of the organisers of the American Association for the Advancement of Science (its president in 1849) and of the Philosophical Society in Washington (president from 1871).