Princeton

Henry's successes were, of course, recognized and in November 1832 he be­came the new Professor of Natural Philosophy at the College of New Jersey, now better known as Princeton University. One of those who recommended him simply said. "He has no equal".

Ten years later Henry turned to examining the discharges from a Leyden Jar capacitor. By studying the way in which the discharge magnetized steel needles within wire coils, he correctly deduced them to be oscillatory. He then teetered on the brink of one of the greatest scientific discoveries.

He observed that a single spark, about an inch long, in an upper room, caused a needle to be magnetized inside a coiled circuit in the cellar, a perpen­dicular distance of 30 feet. He wrote that he was "disposed to adopt the hypothesis of an electrical plenum, and from the foregoing experiment it would appear that the transfer of a single spark is sufficient to disturb perceptibly the electricity of space throughout at least a cube of 400,000 feet of capacity".

The effect, he said, was almost comparable "with that of a flint and steel in the case of light". As Oliver Lodge later commented, "Comparable it is indeed, for we now know it to be the self same process".

By 1851 Henry could assert that the effects are "being propagated wave fashion" and "to a surprising distance". One of his students recorded in 1844 that sparks produced from "the Electrical Machine in the College Hall" affected the surrounding electricity "through the whole village".

Other experiments confirm that Henry was generating, propagating and detecting electromagnetic waves and was evolving a qualitative theory of the ether. Maxwell's later theory of electromagnetism began with Faraday's ideas, but it could equally have begun with Henry's.

Recognition of Henry's achievements suffered because he was initially slow to publish his work. Later in his career when he published more quickly he used a journal respected in America but then less well known in Europe. Conse­quently Europeans were slow to learn of his discoveries. However, when the Henry was suggested as the unit of inductance at the International Congress of Electricians in 1893, it was proposed by a Frenchman and seconded by a Briton.

Electromagnetism, in its widest sense, was not Henry's only interest. At various time he studied astronomy, geophysics, meteorology, anthropology and ethnology. And at the Smithsonian he proved to be an able administrator.

Joseph Henry: he teetered on the brink of one of the greatest discoveries...

His marriage, to his cousin Harriet, was long and happy and their declining years were enhanced by the care of their three daughters, the only survivors of six children.

After almost 50 years of near-perfect health, Henry's final illness made itself known in December 1877. His doctor gave him six months and he died on May 13, 1878. Friends raised and invested $40 000, the income going first to Henry and then his surviving dependants with the capital eventually passing to the National Academy. With such terms, even Henry could not reject it.

When Henry had tried to settle his account with his doctor he was told, "There are no debts for the dean of American science". The old man was moved. "I have always found the world full of kindness to me", he said, "and now here it is again."