The New International Encyclopædia/Henry, Joseph

​HEN′RY, Joseph (1799–1878). An American physicist, born at Albany, N. Y. He was educated at the Albany Academy, where in 1826 he became professor of mathematics. Henry enjoyed the reputation of being one of the greatest of experimenters, and did more toward the development of the science of electricity than any other American since the time of Franklin. At the Albany Academy he developed the electromagnet, which had been invented a few years previously by Sturgeon of England. By insulating the wire with silk and constructing the apparatus according to certain original ideas, he obtained electromagnets of far greater power and efficiency than those of other experimenters, and also transmitted the current from the battery through a considerable length of wire to the magnet. In 1831 Henry sent a current through a mile of fine copper wire, and caused the armature of the electromagnet to be attracted and strike a bell, thus producing an audible signal. This is the first electromagnet telegraph, and Henry is to be regarded as the inventor of the principle now universally applied in modern practice. In further experiments at Princeton where Henry was appointed professor in 1832 he devised an arrangement of electromagnets and batteries, where the current transmitted to a considerable distance energized a magnet and attracted an armature which opened a ‘local’ circuit with its battery and caused a powerful electromagnet to perform work by allowing a weight to fall. This experiment contains the principle of the telegraph relay which made possible telegraphy over considerable distances. The apparatus was set up between Henry’s residence and laboratory at Princeton, and the earth was used as a return conductor for the first time. Henry was also the first to employ magnetic attraction and repulsion to produce motion, and constructed a simple magnetic engine which had the first automatic polechanger or commutator ever applied to the galvanic battery.

In the discovery of magnetic induction Henry was anticipated by Faraday (q.v.), but he was the first to notice the similar phenomenon of selfinduction. He also investigated the oscillations of electric discharges and other electrical phenomena. In 1846 he was chosen secretary of the Smithsonian Institution at Washington, a position that he held until his death. In 1849 he was elected president of the American Association for the Advancement of Science, and in 1858 he was chosen president of the National Academy of Sciences, of which body he was an original member. Upon the establishment of the Lighthouse Board in 1852 Professor Henry was appointed a member, and in 1871 became its head. He carried on in this capacity a number of important tests for the Government which resulted in the improvement of fog-signals and the various lights and lighthouses. He was also interested in meteorology, and in his reports, as secretary, he urged the Government to collect and distribute meteorological information. He suggested the use of the telegraph for this purpose, and for a number of years this important work was under the direction of the Smithsonian Institution. Terrestrial magnetism was also a subject of interest to Henry, and he not only participated in investigations on his own account, but urged upon the Government the importance of having such observations made. In acoustics, Professor Henry also carried on important researches, his attention being directed to this subject largely through his experiments with fog-signals. Henry enjoyed no small amount of European reputation, and in his trips abroad was enthusiastically received by English and Continental scientists.

Henry was involved in a controversy with S. F. B. Morse (q.v.) as regards the invention of the telegraph, but it is safe to state that the former is to be regarded as the originator of the principle, while Morse invented the instrument first used for this purpose. Henry’s collected writings are to be found in the Smithsonian Miscellaneous Collection, vol. xxx. (Washington, 1887). In volumes xx. and xxi. of the same series are to be found excellent biographical and memorial notices. Consult also Dickerson, Joseph Henry and the Magnetic Telegraph (New York, 1885).