Joseph Henry, an American physicist, born in Albany, N. Y., Dec. 17, 1797. He received a common school education, and after a course of study in the Albany academy, in 1826 he was appointed professor of mathematics in that institution. In 1827 he began a series of experiments in electricity, and in 1828 published an account of various modifications of electro-magnetic apparatus. He was the first to prove by actual experiment that in the transmission of electricity for great distances the power of the battery must be proportioned to the length of the conductor. He was also the first actually to magnetize a piece of iron at a distance, and invented the first machine moved by the agency of electro-magnetism. (See Electro-Magnetism.) In March, 1829, he exhibited to the Albany institute electromagnets which possessed magnetic power superior to that of any before known, and subsequently he constructed others on the same plan, one of which, now in the cabinet of the college at Princeton, N. J., will sustain 3,G00 pounds, with a battery occupying about a cubic foot of space. In 1831, in some experiments at the Albany academy, he transmitted signals by means of the electro-magnet through a wire more than a mile long, causing a bell to sound at the further end of the wire.

An account of these experiments and of his electromagnetic machine was published in Silliman's "American Journal of Science" in 1831, vol. xix., in which Prof. Henry pointed out the applicability of the facts demonstrated by his experiments to the instantaneous conveyance of intelligence between distant points by means of a magnetic telegraph, several years before such a telegraph was brought into practical operation by Prof. Morse. In 1832 he was appointed professor of natural philosophy in the college of New Jersey at Princeton, where he continued his experiments and researches.

In his first course of lectures in that institution in 1833 he mentioned the project of the electro-magnetic telegraph, and demonstrated that the electro-magnet might be used to produce mechanical effects at a distance. In February, 1837, he went to Europe, and in April of that year visited Prof. Wheatstone of King's college, London, to whom he explained his discoveries and his method of producing great mechanical effects at a distance, such as the ringing of church bells 100 miles off, by means of the electro-magnet. In 1840, on the organization of the Smithsonian institution at Washington, Prof. Henry was appointed its secretary, a post which he still holds (1874), and which gives him the principal direction of the institution. His discoveries in physics are numerous, some of the most important of which are described in various scientific articles in this work. He is the author of "Contributions to Electricity and Magnetism" (4to, Philadelphia, 1839), and many papers in the "American Philosophical Transactions," in Silliman's "Journal," in the "Journal of the Franklin Institute," and in other scientific periodicals.