Joseph Henry

Joseph Henry was born in Albany, New York, in 1797 to a Scottish immigrant day-labourer who died when Henry was nine. He was sent to live with relatives in the Catskills, apprenticed to a watchmaker, and flirted briefly with a career in the theatre before a borrowed copy of George Gregory's *Lectures on Experimental Philosophy* — picked up almost by accident at age sixteen — set him on the path of science. He paid his way through the Albany Academy (a local secondary school) by tutoring, surveying, and road-building, and in 1826 was hired to teach mathematics and natural philosophy there. Between 1827 and 1832, while teaching a full load of classes, he conducted the experiments that made him the most important American physicist of the nineteenth century.

Henry's central insight was that a long coil of insulated wire, wrapped tightly around a soft-iron core, would produce an electromagnet far more powerful than the bare-iron, loosely-wound versions then in use. In 1831 he built an electromagnet for Yale College that lifted 2,063 pounds; a larger one for Princeton lifted 3,500. No previous device had come within a factor of ten of that performance. Working with these powerful magnets, he discovered electromagnetic self-induction in 1832 — the fact that a changing current in a coil induces a back-EMF in the same coil, resisting the change — independently of Faraday, who had discovered mutual induction in London a few months earlier. Henry also built the first practical electromechanical relay in 1835, the device that Samuel Morse would later adapt (without attribution) into the repeater that made long-distance telegraphy possible. In 1835 Princeton hired him as professor of natural philosophy. In 1846, when Congress at last decided what to do with the strange bequest left to the United States by the English chemist James Smithson, Henry was named the first Secretary of the Smithsonian Institution — effectively the chief administrative and scientific officer of the Republic's new national museum and research organisation.

Henry's greatest regret was publication delay. He discovered self-induction in 1832 but, in the characteristic reticence of American scientists of the period, did not publish until Faraday's 1834 papers on mutual induction made clear that an independent discovery claim would be lost unless asserted. By then European priority belonged to Faraday. When the SI unit system was revised in 1960, the unit of inductance (volt-seconds per ampere) was named the *henry* in his honour — a belated European acknowledgement of American priority. He died in 1878 in Washington, DC, and was buried with state honours. His Albany-era notebooks, now at the Smithsonian, contain the earliest recorded descriptions of back-EMF, the step-down transformer principle, and the sensitive-relay telegraph — three ideas on which most of twentieth-century electrical engineering rests.