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Henry, Joseph

american wire current source

(1797–1878) US physicist: pioneer of electromagnetism.

Strangely, no American after did much for the study of electricity for 75 years, when Henry did a great deal. In many ways Henry is the traditional American of folklore; tall, handsome and healthy, he was still a strenuous researcher at 80. Growing up in Albany in New York State with a widowed mother, he was not fond of schoolwork and at 15 was apprenticed to a watchmaker, but the business soon failed. For a year he wrote plays and acted in them, and then by chance read a book on science which reshaped his life. He attended the Albany Academy, did well and spent a period as a road engineer before taking a job as teacher of mathematics at the Academy, researching on electricity in his spare time. His research gave him enough reputation to secure a post at the College of New Jersey (which became Princeton) in 1832, teaching a full range of sciences.

In 1825 devised an electromagnet with a varnished soft iron core wrapped by separate strands of uninsulated wire. Henry in 1829 much improved this by using many turns of thin, insulated, wire. (He supervised the making of one at Yale in 1831 that would lift a tonne.) Also in 1831 he made the first reciprocating electric motor, as ‘a philosophic toy’. Like his magnets it was powered by batteries, his only source of current. He had wire in plenty, from an unknown source, and used miles of it. In 1830 he discovered electromagnetic induction, ‘the conversion of magnetism into electricity’. also discovered it independently soon after, and published first. However, Henry, in 1832, was the first to discover and to publish on self-induction, and the unit is named after him. A coil has a self-inductance of 1 henry (H) if the back emf in it is 1 volt when the current through it is changing at 1 ampere per second. In 1835 he introduced the relay, which made long-distance electric telegraphy practical, an important step in North America.

When he was 49, Henry became first director of the Smithsonian Institution. This had a curious history. James Smithson was an unrecognized bastard son of the Duke of Northumberland. Resentful of his position, he was determined that ‘my name shall live in the memory of man when the Northumberlands … are extinct and forgotten’ and he therefore left a large fortune to go to the USA (with which he had no links of any kind) to found ‘an Establishment for the increase and diffusion of knowledge’. Henry shaped it well; it became ‘the incubator of American science’ and he was the model administrator. A strict Calvinist, he resisted patents or wealth for himself and refused for 32 years to increase his salary of $3500.

Henry, William [next] [back] Henry, Aaron(1922–1997) - Civil rights activist, state government official, Chronology, Representation and 1964 Democratic Convention

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