Joseph Henry (born December 17, 1797,Albany,New York, U.S.—died May 13, 1878, Washington, D.C.) was one of the first great American scientists afterBenjamin Franklin. He aided and discovered several important principles ofelectricity, includingself-induction, a phenomenon of primary importance inelectronic circuitry.

While working withelectromagnets at the Albany Academy (New York) in 1829, he made important design improvements. By insulating the wire instead of the iron core, he was able to wrap a large number of turns of wire around the core and thus greatly increase the power of themagnet. He made anelectromagnet forYale College that could support 2,063 pounds, a world record at the time.

Henry also searched forelectromagnetic induction—the process of convertingmagnetism into electricity—and in 1831 he started building a large electromagnet for that purpose. Because the room at the Albany Academy in which he wanted to build his experiment was not available, he had to postpone his work until June 1832, when he learned that British physicistMichael Faraday had already discoveredinduction the previous year. However, when he resumed his experiments, he was the first to notice the principle of self-induction.

In 1831 Henry built and successfully operated, over a distance of 2.4 km (1.5 miles), atelegraph of his own design. He became professor of natural philosophy at theCollege of New Jersey (laterPrinceton University) in 1832. Continuing his researches, he discovered the laws upon which thetransformer is based. He also found that currents could be induced at a distance and in one case magnetized a needle by using a lightning flash 13 km (8 miles) away. That experiment was apparently the first use ofradio waves across a distance. He aidedSamuel F.B. Morse in the development of the telegraph by giving him 8 km (5 miles) ofcopper wire and writing a letter toCongress in 1842 encouraging it to support an 80-km (50-mile) test line. By using a thermogalvanometer, a heat-detection device, he showed thatsunspots radiate less heat than the general solar surface.

In 1846 Henry became the first secretary of theSmithsonian Institution, Washington,D.C., where he organized and supported acorps of volunteerweather observers. The success of the Smithsonian meteorological work led to the creation of the U.S.Weather Bureau (later Service). One ofLincoln’s chief technical advisers during theU.S. Civil War, he was a primary organizer of theNational Academy of Sciences and its second president. In 1893 his name was given to the standard electrical unit of inductive resistance, thehenry.