Joseph Henry was one of the first great American scientists after Benjamin Franklin. Many parallels exist between his life and work and that of the English physicist Michael Faraday. Like Faraday, Henry was born into a poor family.Both received little formal education and both were apprenticed at an earlyage, Faraday to a book binder, and Henry to a watchmaker at the age of 13. Both, finally made lasting contributions to the field of electrical research.
Henry's interest in science was sparked by an odd coincidence. He had chasedhis pet rabbit underneath a church. Noticing that some floorboards were missing, 16-year-old Joseph climbed into the church and found a shelf of books. Hebegan looking through Lectures on Experimental Philosophy and was soon hooked on science. He entered the Albany Academy in New York and later began to teach at country schools to earn an income. He graduated from the Academy and was leaning toward studying medicine when a surveying job turned up, and that steered him toward engineering.
In 1826 Henry was back at the Albany Academy, but this time as a teacher of mathematics and science. In 1820, Danish physicist Hans Christian Oersted (1777-1851) had discovered that the flow of an electric current produced a magnetic field around the wire. This amazed scientists and many, including Henry and Faraday, began to experiment with magnetism. In 1829 Henry learned that William Sturgeon (1783-1850) had built an electromagnet that could lift nine pounds (4 kg). This was quite remarkable, but Henry believed he could create a magnet that was much stronger. The secret was to wrap more wire around the iron core, overlapping the levels. However, wire in that era was not insulated,and so wrapping one level over another caused a short circuit. Henry got around the problem by the laborious process of insulating the copper wire by hand, using strips of his wife's silk petticoats.
Now that he had insulated wire, Henry proceeded to experiment. By 1831 he hadcreated an electromagnet that could lift 750 pounds (340 kg). Later that same year he gave a demonstration at Yale University and lifted more than 2,000pounds (900 kg). For this feat he received an appointment as professor.
In addition to his large electromagnets, Henry also built small ones. In 1831he ran a wire more than one mile (1.6 km) and attached a device consisting of an electromagnet, a movable iron arm, and a spring. At the other end of thewire was a battery and key switch. When Henry pressed the key, activating the current, the distant electromagnet engaged, attracting a metal arm with a click. Releasing the key cut the electric flow, and the spring forced the armback to its rest position--the first telegraph.
There was a practical limit to the length of the wire that could be used--thelonger the wire, the greater the resistance, which resulted in less current.Georg Ohm (1787-1854) had devised a law by which resistance could be calculated. Henry found an easy way around the resistance problem in 1835 by inventing the electric relay.
Near the close of the 1830s, Henry had a chance encounter with a man who hadvery little electrical knowledge, but was extremely interested in electromagnets and relays. Henry believed that the discoveries of science should be forthe good of all mankind, so he never patented any of his devices. The man, Samuel F. B. Morse, received a great deal of advice and information from Henry.In 1840 Morse took out a patent on the electric telegraph and became a veryrich man.
This was not the first time Henry had been cheated. In 1830, the teaching load at the Albany Academy monopolized Henry's time to such an extent that he had only the month of August in which to conduct research and experiment. In August of that year Henry discovered the principal of electric induction, the process in which an electric current in one coil of wire can set up a currentin another coil. If the flow of electricity can produce a magnetic field, hereasoned, a magnetic field should be able to induce an electric current. Butwhen the end of August arrived, Henry had not completed his investigation. Hedecided to set the work aside and return to it the following August. It waswith considerable shock that Henry read Michael Faraday's announcement in 1831 of the discovery of electric induction. Rushing back to his experiments, Henry published a report of his own discovery, but he was too late; Faraday received credit for the discovery. To his own credit, Henry never argued about Faraday's priority, but privately he did resent that credit had gone elsewhere.
Henry, however, did get credit for including in his paper a discovery that Faraday had neglected to write about: the discovery of self-induction. A coil carrying electric current not only induces a flow in another coil, it can induce a current in itself. In 1834 Faraday discovered self-induction independently, but this time Henry got the credit. Estonian physicist Heinrich Lenz (1804-1865) also made the discovery independently and took it farther than both Henry and Faraday. Self-inductance became an important part in the design of electric circuits.
In 1831 Henry published a paper in which he described the working of an electric motor. Ten years earlier Faraday had built a motor, but it was little more than a toy. Henry's motor was more practical, but until Faraday developed the electric generator, there was no way to adequately power the motor. It isHenry's design that is used for motors in electrical appliances today.
In 1842 Henry anticipated a discovery that has been credited to Heinrich Rudolph Hertz. Henry discovered that he could magnetize needles in a basement with an electric spark from two floors above, correctly ascribing it to electromagnetic wave propagation. In another experiment, he magnetized a needle by utilizing a lightning flash eight miles away.
In addition to describing the mechanism of an electric motor, Henry was involved in many other endeavors. In 1846 he became the first secretary of the newSmithsonian Institution and encouraged the communication of scientific knowledge around the globe.Two years later he projected the image of the sun on ascreen, made careful measurements of temperature, and discovered that the mysterious sunspots were relatively cooler than the rest of the sun. He used thetelegraph to obtain weather reports from across the country, initiating a system that led to the founding of the United States Weather Bureau. During theCivil War he recommended the construction of ironclad warships, advice whichwas eventually followed. On May 13, 1878, Henry died in Washington, D.C., atthe age of 80. He was finally honored in 1893 when the International Electrical Congress agreed to name the unit of inductance the Henry.