Joseph Henry (1797-1878), American physicist. Henry made numerous discoveries in the field of electromagnetism. In 1829 he made the most powerful electromagnet then known. A year later he discovered electromagnetic induction while following up his earlier work. He made this discovery independently of Faraday, but Faraday published his results first. The unit of inductance was called the henry in his honour. In 1832 he published his paper on self-inductance. He also developed the electrical relay, which solved the problem of high resistance in long wires. This made long-range communication feasible. He was the first director of the Smithsonian Institute.