During his experiments with electromagnetism, Henry discovered the property of inductance in electrical circuits, which was first recognized at about the same time in England by Michael Faraday, who was the first to publish on the subject. In honor of Henry, the SI unit of inductance bears his name. One henry equals the inductance of a circuit with an induced voltage of one volt and an inducing current that changes one ampere per second.
Henry was born to Scottish immigrants living in Albany, New York, on December 17, 1797. His father was a laborer and his family struggled financially. Henry was only 8 when his father died, and at that time moved to the town of Galway, where his grandmother helped rear him. After an elementary school education, Henry began an apprenticeship with a watchmaker. Though eventually he would choose a different vocation, the apprenticeship provided him with a hands-on education in mechanics and instrument construction that would later serve him well. When he was 16, Henry read a book on experimental philosophy that inspired an interest in science. Yet he considered a stage career before finally entering the Albany Academy in 1819, where his natural scientific abilities became apparent.
While attending the academy, Henry tutored other students to help make ends meet. Henry had a knack for explaining and demonstrating scientific concepts, and frequently during those years helped his own teachers teach. In 1826, Henry officially became a professor at the academy. He taught mathematics and natural philosophy, and in his hours outside of the classroom carried out much of his most significant scientific work. At first, he focused his energies on terrestrial magnetism, which soon led him to broaden his scope to other types of magnetism.
Henry attended a lecture at which the English scientist William Sturgeon demonstrated a new type of magnet. It was created by the flow of an electric current and ceased to be magnetic when the current stopped. Henry decided to experiment with these electromagnets, and greatly improved the design. He insulated the wire, and tightly coiled it around the iron core of the electromagnet. By using more wire, he created greater magnetic field strength. Henry became well known for the powerful electromagnets he constructed and often demonstrated them before large crowds. An electromagnetic he built at the behest of Yale College was able to lift more than 2,000 pounds, a very impressive feat at the time. Later Henry electromagnets were capable of suspending in excess of 3,500 pounds.
While working with his powerful electromagnets, Henry first observed inductance. This property of a circuit is observable when a varying magnetic field generates an electromotive force, or voltage, in that circuit or in a nearby circuit. When two circuits are involved, the phenomenon is generally referred to as mutual inductance. The discovery of this type of inductance is usually credited to Faraday, whereas the discovery of self inductance, which involves a single circuit, is most commonly ascribed to Henry.
Also in the early 1830s, Henry built a simple contraption that was a predecessor of the modern direct current motor. The contraption was a novelty and did not perform any useful work, but possessed all the basic components found in later motors, including a field generated by a magnet, a commutator and an electromagnetic armature. The motion produced by the device was not rotation, however, but rather a rocking movement that occurred at about 75 vibrations per minute.
In 1832, Henry left the Albany Academy to accept a professorship at the College of New Jersey, which later became Princeton University. There he devised an early telegraph that allowed communication between his laboratory and his home located on campus. He sometimes used the relay for such mundane purposes as ordering lunch. Henry did not attempt to commercialize his telegraphing system and, as a result, today Samuel Morse is more commonly associated with the invention of the telegraph than Henry. Other projects carried out by Henry at the college included additional work in electromagnetism as well as the study of sunspots, investigations into acoustics and experimental work in ballistics.
When the United States Congress founded the Smithsonian Institution in 1846, the board chose Henry, the best-known American scientist alive at the time, to be its first secretary. In this new capacity, Henry had less time to carry out independent research, a loss he deeply regretted. Yet, his administrative work at the Smithsonian, which gave him access to the highest tiers of government, made him extremely influential. Moreover, many of the procedures and policies Henry established at the Smithsonian remain in place today. During the Civil War, he served as a technical adviser to President Lincoln; the large group of volunteers Henry organized to share weather observations from across the country eventually evolved into the National Weather Service. Henry was also influential in the establishment of the National Academy of Sciences. The society elected Henry its second president.
Henry passed away on May 13, 1878, in Washington, D.C. The considerable attendance at his funeral, which included the U.S. President Rutherford B. Hayes, members of the Supreme Court, renowned scientists and many other illustrious figures, was a testament to the tremendous impact made by Henry and his work.