HENRY Joseph, American physicist: b. Albany, N. Y., 17 Dec. 1797; d. Washington, D. C., 13 May 1878. He was educated at the Albany Academy, after graduation undertook the study of chemistry, anatomy and physiology with a view to adopting the medical profession. During the years 1824-25, he contributed occa sional scientific papers to the Albany Institute, his especial subjects being chemistry and me chanics, and was appointed assistant engineer on the survey instituted for a road between Lake Erie and the Hudson. In the spring of 1826 he was elected teacher of mathematics and natural philosophy in the Albany Academy and in the latter part of 1827 read a very important paper before the Albany Institute, 'On Some Modifi cations of the Electro-Magnetic Apparatus.' He made his first public demonstration of his mag netic discoveries in exhibiting before the Insti tute small electromagnets wound with silk-cov ered wire. These magnets had a greatly multi plied lifting power over any that had yet been known. In this lay the essential point of his first discovery, for he was undoubtedly the ear liest physicist to adopt insulated or silk-covered wire for the magnetic coil, and to employ spool winding for the limb of the magnet. He demon strated also for the first time, by a very intelli gent experiment, the difference of action in a quantity magnet excited by a quantity battery of a single pair, and an intensity magnet with a long fine wire coil excited by an intensity battery of many elements, having their resistances suit ably proportioned. The first of these two forms was not capable of being employed for tele graphic purposes, while the intensity magnets with their attachments could be so applied. The quantity magnets which he exhibited caused a good deal of excitement in the scientific world. Their attractive power was at that time quite unprecedented. One of them had sufficient power to raise as much as 3,500 pounds.
Henry was the first to show that iron could be magnetized at a distance, and to invent a suitable combination of magnet and battery for the production of this result. In 1831 he made this experimental demonstration. He suspended a mile of insulated copper wire round a chamber in the Academy, and so placed a hell at one ex tremity of it that it was struck by the polarized armature of an intensity battery connected with the other extremity. This was the earliest ex ample of the magnetic telegraph, for the gal vanometer or needle had been the principle on which all preceding experiments had been con ducted. It was not long after that he invented
a machine, and finally constructed it, which is recognized as the first electromagnetic engine with automatic pole-changer. In 1832 after re peated experiments he discovered how to give greater intensity to a magnetic discharge by the induction of a current on itself in a long spiral or helical wire. These progressive steps in mag netic science gained for him an extended reputa tion, and soon after the publication of the last experiment in Silliman's 'American Journal of Science' Henry was elected professor of nat ural philosophy in Princeton College. The dis covery of the spiral or helical conductor sug gested to him further experiments, and his extended researches and their results were an nounced by him in a paper published 1834, under the tide 'On the Influence of a Spiral Conductor in Increasing the Intensity of Electricity from a Galvanic Arrangement of a Single Pair.' He supplemented these discoveries by many others, and by his experiments produced electrical com binations which were undoubtedly precursors of later relay and receiving magnets, while his dem onstration of the conditions and range of in duction from electrical currents, and the succes sive orders of induction in the passage of fric tional electricity, as well as his discovery of the oscillatory nature of electricity, paved the way for that great scientific and practical resolution which was to consummate by the genius of Morse and his confreres.
In 1846 Henry was called. to a new sphere of activity, in which he exhibited his usual zeal and enthusiasm. The Smithsonian Institution had just come into existence, and during the formative period of the great museum, he was appointed to be its secretary. The office did not so far engross his attention as to make him neglectful of practical work in science. He found time to investigate the acoustics of pub lic buildings, meteorological changes of the at mosphere and methods for telegraphic trans mission of meteorological observation from all points of the continent. From 1868 up to his death he was president of the National Acad emy of Sciences, and of the Philosophical So ciety of Washington from 1871, when it was first organized.