Henry, Joseph, 1797–1878, American physicist, b. Albany, N.Y., educated at Albany Academy. He taught (1826–32) mathematics and natural philosophy at Albany Academy and was professor of natural philosophy (1832–46) at Princeton (then the College of New Jersey). From 1846 he served as the first secretary and director of the newly founded Smithsonian Institution; he introduced and developed many of its activities and established its general policies. Before assuming his responsibilities at the Smithsonian, he had made notable contributions to the physical sciences, especially in electromagnetism. Henry improved the electromagnet, increasing its strength and fitting it for practical use. He invented and operated the first electromagnetic telegraph, which formed the basis for the commercial telegraphic system. He discovered self-inductance, and the unit of inductance is often called the henry in his honor. Independently of Michael Faraday, he discovered the principle of the induced current, basic to the dynamo, transformer, and many other devices. Henry invented a small electromagnetic motor, and extended the work on induced currents to show that an induced current can be used to induce another current in a nearby circuit and that resulting currents in turn can induce others. His numerous other contributions include the institution of the weather report system.
See his Papers, ed. by N. Reingold et al. (15 vol., 1972–); biographies by S. R. Riedman (1961) and A. E. Moyer (1997).
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