Henry, Joseph (1797–1878), American physicist. Henry was a schoolteacher in Albany, New York, and later a professor of mathematics and natural philosophy at Princeton. He conducted a series of investigations into electric phenomena, particularly those related to magnetism. His first major achievement was building a powerful electromagnet, for which he developed insulated wire in order to make a closely wound coil of several layers. He is also credited with constructing primitive versions of the telegraph and the electric motor. In 1846 he became the first secretary and director of the Smithsonian Institution in Washington, D.C. In recognition of his work, his name was given to the henry, the unit of electrical inductance, by international agreement in 1893.