Joseph Henry

Joseph Henry was an American physicist and inventor who became the first secretary of the Smithsonian Institution and later served as president of the National Academy of Sciences. While building more powerful electromagnets in the early 1830s, he discovered the property of self-inductance, and though Michael Faraday published his own related discovery of mutual induction first, Henry's independent electromagnetic research was substantial in its own right. He turned the electromagnet into a genuinely practical instrument and used it to build an early electric doorbell in 1831 and an electrical relay in 1835, and that relay supplied the working principle Samuel Morse and Charles Wheatstone built into the practical electric telegraph. The SI unit of electrical inductance, the henry, is named for him.