Henry, Joseph (1797–1878)

US physicist, inventor of the electromagnetic motor in 1829 and of a telegraphic apparatus. He also discovered the principle of electromagnetic induction, roughly at the same time as Michael Faraday, and the phenomenon of self-induction. The unit of inductance, the henry, is named after him.

Born in Albany, New York, Henry studied at Albany Academy, where he became professor 1826, moving 1832 to New Jersey College (later Princeton). He was the Smithsonian Institution's first director, from 1846.

By 1830, he had made powerful electromagnets by using many turns of fine insulated wire wound on iron cores. In that year he anticipated Faraday's discovery of electromagnetic induction (although Faraday published first). In 1835 he developed the relay (later to be much used in electric telegraphy).

In astronomy, Henry studied sunspots and solar radiation, and his meteorological studies at the Smithsonian led to the founding of the US Weather Bureau.