JOSEPH HENRY, (1797-1878), an eminent American physicist, was born in Albany, the capital of the State of New York, on the 17th of December 1797. He received his education at an ordinary school, and afterward at the Albany academy, which enjoyed considerable reputation for the thoroughness of its classical and mathematical courses. On finishing his academic studies he contem-plated adopting the medical profession, and prosecuted his studies in chemistry, anatomy, and physiology with that view. He occasionally contributed papers to the Albany institute, in the years 1824 and 1825, on chemical and mechanical subjects; and in the latter year, having been unexpectedly appointed assistant engineer on the survey of a route for a State road from the Hudson river to Lake Erie, a distance somewhat over 300 miles, he at once em-barked with zeal and success in the new enterprise. This diversion from his original bent gave him an inclination to the career of civil and mechanical engineering; and in the spring of 1826 he was elected by the trustees of the Albany academy to the chair of mathematics and natural philosophy in that institution. In the latter part of 1827 he read before the Albany institute his first important contribution, " On some Modifications of the Electro-Magnetic Apparatus." Struck with the great im-provements then recently introduced into such apparatus by Mr William Sturgeon of Woolwich, he had still further extended their efficiency, with considerable reduction of battery-power, by adopting in all the experimental circuits (where applicable) the principle of Schweigger's " multi-plier," that is, by substituting for single wire circuits, voluminous coils (Trans. Albany Institute, October 10, 1827, vol. i. pp. 22, 23). In June 1828 and in March 1829 he exhibited before the institute small electro-magnets closely and repeatedly wound with silk-covered wire, which had a far greater lifting power than any then known. Henry appears to have been the first to adopt insulated or silk-covered wire for the magnetic coil; and I also the first to employ what may be called the " spool" winding for the limbs of the magnet. He was also the first to demonstrate experimentally the difference of action between what he called a "quantity" magnet excited by a " quantity " battery of a single pair, and an " intensity " magnet with long fine wire coil excited by an " intensity " battery of many elements, having their resistances suitably proportioned. He pointed out that the latter form alone was applicable to telegraphic purposes. A detailed account of these experiments and exhibitions was not, however, published till January 1, 1831 (Silliman's Am. Jour. Sci., xix. 400-408). Henry's " quantity" magnets acquired considerable celebrity at the time, from their unprecedented attractive power,one (August 1830) lifting 750 pounds, another (March 1831) 2300, and a third (1834) 3500.
Early in 1831 he arranged a small office-befl to be tapped by the polarized armature of an " intensity " magnet, whose coil was in continuation of a mile of insulated copper wire, suspended about one of the rooms of his academy. This was the first instance of magnetizing iron at a distance, or of a suitable combination of magnet and battery being so arranged as to be capable of such ac-tion. It was, therefore, the earliest example of a true " magnetic " telegraph, all preceding experiments to this end having been on the galvanometer or needle principle. About the same time he devised and constructed the first electromagnetic engine with automatic polechauger (Silliman's Am. Jour. Sci., July 1831, xx. 340-343 ; and Sturgeon's Annals Electr., 1839, iii. 554). Early in 1832 he discovered the induction of a current on itself, in a long helical wire, giving greatly increased intensity of discharge (Silliman's Am. Jour. Sci., July 1832, xxii. 408). In 1832 he was elected to the chair of natural philosophy in the New Jersey college at Princeton. In 1834 he continued and extended his researches " On the Influ-ence of a Spiral Conductor in increasing the Intensity of Electricity from a Galvanic Arrangement of a Single Pair ;" a memoir of which was read before the American Philosophical Society, February 6, 1835 (Trans. Am. Phil. Soc, v. 223-231, n.s.) In 1835 he com-bined the short circuit of his monster magnet (of 1834) with the small "intensity" magnet of an experimental telegraph wire, thereby establishing the fact that very powerful mechanical effects could be produced at a great distance by the agency of a very feeble magnet used as a circuit maker and breaker, or as a " trigger" the precursor of later forms of relay and receiving magnets. In 1837 he paid his first visit to England and Europe. In 1838 he made important investigations in regard to the conditions and range of induction from electrical currents,showing that induced currents, although merely momentary, produce still other or tertiary currents, and thus on through successive orders of induction, with alternating signs, and with reversed initial and terminal signs. He also dis-covered similar successive orders of induction in the case of the passage of frictionalelectricity (Trans. Am. Phil. Soc., vi. 303-337, n.s.). Among many minor observations, he discovered in 1842 the oscillatory nature of the electrical discharge, magnetizing about a thousand needles in the course of his experiments (Proc. Am. Phil. Soc, i. 301). He traced the influence of induction to sur-prising distances, magnetizing needles in the lower story of a house through several intervening floors by means of electrical discharges in the upper story, and also by the secondary current in a wire 220 feet distant from the wire of the primary circuit. The five num-bers of his Contributions to Electricity and Magnetism (1835-1842) were separately republished from the Transactions. In 1843 he made some interesting original observations on "Phosphorescence " (Proc. Am. Phil. Soc., iii. 38-44). In 1844, by experiments on the tenacity of soap-bubbles, he showed that the molecular cohesion of water is equal (if not superior) to that of ice, and hence, generally, that solids and their liquids have practically the same amount of ! cohesion (Proc. Am. Phil. Soc, iv. 56, 57, and 84, 85). In 1845 he showed, by means of a thermo-galvanometer, that the solar spots radiate less heat than the general solar surface (Proc Am. Phil. Soc, iv. 173-176).
In December 1846 Henry was elected secretary and director of the Smithsonian Institution, then just established. While closely occupied with the exacting duties of that office, he stitl found time to prosecute many original inquiries,as into the application of acoustics to public buildings, and the best construction and arrange-ment of lecture-rooms, into the strength of various building materials, &c. Having early devoted much attention to meteor-ology, both in observing and in reducing and discussing observa-tions, he (among his first administrative acts) organized a large and widespread corps of observers, and made arrangements for simul-taneous reports by means of the electric telegraph, which was yet in its infancy (Smithson. Report for 1847, pp. 146, 147). He was the first to apply the telegraph to meteorological research, to have the atmospheric conditions daily indicated on a large map, and to utilize the generalizations made in weather forecasts, and the first to embrace a continent under a single system,British America and Mexico being included in the field of observation. In 1852, on the reorganization of the American lighthouse system, he was ap-pointed a member of the new board ; and on the resignation of Admiral Shubrick as its chairman, Henry, in 1871, became the pre-siding officer of the establishmenta position he continued to hold during the rest of his life. His diligent investigations into the efficiency of various illuminants under differing circumstances, and into the best conditions for developing their several maximum powers of brilliancy, while greatly improving the usefulness of the line of beacons along the extensive coast of the United States, effected at the same time a great economy of administration. His equally careful experiments on various acoustic instruments also re-sulted in giving to his country the most serviceable system of fog-signals known to maritime powers. In the course of these varied and prolonged researches from 1865 to 1877, he also made important contributions to the science of acoustics ; and he established by several series of laborious observations, extending over many years and along a wide coast range, the correctness of Professor Stokes's hypothesis (Report Brit. Assoc., 1857, part ii. 27) that the wind exerts a very marked influence in refracting sound-beams. The complex conditions of such acoustic refraction he found to be exceedingly variable and curious (Report American Lighthouse Board, 1874, 1875, and 1877). From 1868 Henry continued to be annually chosen as president of the National Academy of Sciences ; and he was also president of the Philosophical Society of Washing-ton from the date of its organization in 1871.
Henry was by general concession the foremost of American physicists. He was a man of varied culture, of large breadth and liberality of views, of generous impulses, of great gentleness and courtesy of manner, combined with equal firmness of purpose and energy of action. He died at Washington, May 13, 1878. (S. F. B.)