Henry, Joseph (1797-1878), a United States physicist and the first director of the Smithsonian Institution. His experiments and inventions in electricity in the 1820's and 1830's helped hasten its use for power and communication. In 1830, he observed electromagnetic induction, one of the physical processes upon which the operation of electric generators and motors depends. (It was independently discovered the next year by the English scientist Michael Faraday, who was first to publish his findings.) In 1832, Henry discovered self-induction, a phenomenon that affects the way in which an electric current changes in a circuit.

Henry greatly improved the design of electromagnets, increasing their strength by using insulated wire to make coils with a large number of turns. He developed the electric relay in 1835, which helped lead to the development of the telegraph. He never applied for a patent.

Henry was born near Albany, New York. From 1832 to 1846 he taught natural philosophy (physics) at the College of New Jersey (now Princeton University). At the Smithsonian Institution (1846-78), Henry was the United States' first national coordinator of scientific research. His initiation of weather reporting by telegraph led to the organization of the U.S. Weather Bureau (now the National Weather Service). The henry, the unit of electrical inductance, was named in his honor in 1893.