Page:The Biographical Dictionary of America, vol. 05.djvu/234

 HENRY

HENRY

few coils of insulated wire. At a great expense of battery power it could lift a few pounds; it was useless iu the arts and inadequate for tele- graphic purposes. Henry con- verleil it into two distinct in- struments. The one, with a long fine continuous wire, which is the sensitive instru- ment, that in the long cir- cuit of the telegraph responds to distant influence, Henry '© ^ _ *• called an intensity' magnet, becauso to act tlius at a dis- tance it must be connected with an intensity battery. The other, wound with many separate coils of short thick wire, was incapable of action at a distance, but could be endowed with great strength. In 1831 he constructed a magnet capable of sustaining a weight of 3G00 pounds when excited by the current from a single cell battery occupying less than one cubic foot of space. Tiiis he called a quantity magnet because it required a (quantity battery, and it is this magnet, with its especial battery, which forms the short local circuit of the telegraph. The necessary connection of the magnets with their respective batteries forms the independent dis- coverv of Henry of the law of proportion between the projectile force of the battery and the resist- ance in the wire of the magnet and length of circuit. Henry published an account of liis mag- nets and pointed out the practical application of his principles to the telegraph in 1831 in the American Journal of Science. The same year he transmitted signals through a wire over a mile long, causing a bell to ring at the further end of the wire. This length of wire was sufficient to illastrate to his pupils his principles which in 1898 allowed a message to be sent around the world. At Princeton, Henry stretched his tele- graphic wires across the college gi'ounds, com- municating with Mrs. Henry in his residence. He made there, in 1833, his largest magnet, called "Big Ben " by the students, and showed how an intensity magnet, when excited by dis- tant influence, might be made to open or close the circuit of this powerful quantity magnet, bringing it thus into action at a distance. This device of opening one circuit by means of another is used in the telegraph in the relay, to call into action, to continue the line to another intensity circuit, and to bring into play the local quantity circuit. It was used by Henry from 1833 at Princeton, having been perhaps invented by him earlier. He showed by it how powerful effects might be produced at a distance, causing his magnet in the laboratory to lift and drop thou- sands of pounds by electrical communication from his residence on the opposite side of the

college grounds, the same principle as was sub- sequently used in working machinery from a distance. In his telegraphic experiments Henry used the earth for the return current at least as early as 1835, being the first to complete the circuit in this way. In 1837 Wheatstorie and Cook were struggling vainly with tiie telegraph, having discarded the electro-magnet, an essential element of their project, finding it incapable of action at a distance. Henry, then in London, came to their assistance w-ith liis two magnets and explained to Wheatstone the principles of his combinations, in this waj' rendering practi- cable the telegraph iu England. On his return to America he aided Morse through personal interviews, by the exhibition of his own tele- graphic ai>paratus in Princeton, and by advice and sympathy by letter, until the telegraph was introduced. The Morse instrument was a local tj'pe recorder; it could not act at a distance until Dr. Leonard T. Gale applied Henry's principles and made the invention practicable. Not essen- tial to the telegraph, it was superseded by the phonetic system of hitting a metal sounder by means of a moving bar. the simple device of Henry in his Albany telegraph of 1831, in which a bar of iron, vibrating between the poles of an electro-magnet, strvick a bell. All the essential elements of the electro-magnetic telegraph, viz., the phonetic device of liitting a metal sounder, the two magnets, the one in the long, the other in the shoi-t circuit, their connection with their respective batteries, the opening of one circuit by means of another, — could have been patented by Henry as early as 1833 had he been so inclined, but he refused to tie up for his own use discov- eries which he hoped might benefit the world. In 1831 he invented his electro-magnetic engine for maintaining continuous motion by means of an automatic pole-changer, which proved an im- portant step in the development of the art of converting the electric current into mechanical power. Henry entered the field of the induction of currents as early as 1827, obtainingsparks from a common magnet before he made his electro- magnets, thus making the discovery of magneto- electricity before Faradaj-, who announced it in 1832. In 1829 or 1830 he discovered the "extra current," discovered by Faraday in 1834. In 1831 he obtained induced currents and sparks with his electro-magnets, as he had with the common magnets, and winding a reel with a mile of wire to be revolved between the arms of a huge mag- net, anticipated the dynamo. In 1832 he made other valual)le experiments in this line and published his first paper upon the subject in the Proceedinqs of tiie American Piiilosophical society. In 1834 he pursued in Princeton the subject of the " extra current " with copper rib-