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1831 __ Intensity magnet — the first magnetic telegraph
Joseph Henry (1797-1878)
Comment : In 1831, Joseph Henry suspended around the walls of a large class-room in the Academy a mile of copper wire, interposed in a circuit between a battery and an "intensity" magnet. At each excitation of the magnet, a rod which had been in contact with a limb of the soft iron core was repelled from it, and its other end struck a bell. Henry explained to his classes that signaling might be done in this way, but that was as far as he went with it. He did not formulate a code of language, nor attempt to develop an actual telegraph instrument; yet his simple circuit and bell had elements which no previous telegraph had had, and which were destined to be essential to the telegraph of the future. It is therefore claimed by those who think that Henry's contribution to the science has been underrated that it was he and not Morse who "invented" the telegraph. His biographer, Taylor, points out that this experimental apparatus "involved three significant and important novelties" : 1) It was the first electro-magnetic telegraph employing an intensity magnet capable of being excited at very great distances from a suitable "intensity" battery; 2) It was the first electro-magnetic telegraph employing the armature as the signaling device; or employing the 'attractive' power of the intermittent magnet, as distinguished from the 'directive' action of the galvanic circuit. That is to say, it was, strictly speaking, the first 'magnetic telegraph'. 3) It was the first 'acoustic' electro^magnetic telegraph. One practical convenience of the needle system was found to be the perfect silence of its indications; and hence, in almost every case a call-alarm was required to draw attention to its messages. How different would have been the estimate of Henry's labors (and especially the 'practical' estimation of subsequent patentees) if the modest discoverer and inventor had been worldly-wise enough to secure and early patent on these three indisputably original and most pregnant features of telegraphy.to contest whch no rival has ever appeared. In the decades immediately following, hundreds of patents were granted for improvements upon or modifications of the electro-magnetic telegraph, all of them dependent upon Henry's original invention. In 1832, Henry was called to the chair of natural philosophy in the College of New Jersey at Princeton, later to be developed into a famous university. Here he conceived a new form of galvanic battery and made his most powerful magnet. He also strung a wire between two buildings in the campus and did some more signaling. He describes it in a letter to a friend on the college faculty, Professor Samuel B. Dod, written after Professor Henry had gone to the Smithsonian Institution : « I think that the first actual line of telegraph using the earth as a conductor was made in the beginning of 1836. A wire was extended across the front campus of the college grounds, from the upper story of the Library Building to Philosophical Hall on the opposite side, the ends terminating in two wells. Through this wire signals were sent from time to time from my house to my laboratory [In his Princeton telegraph, the signals were conveyed by taps of a bell, as before]. The electro-magnetic telegraph was first invented by me in Albany in 1830. Professor Morse, according to his statements, conceived the idea of an electro-magnetic telegraph in his voyage across the ocean in 1832, but did not until some years afterward.1837.attempt to carry his ideas into practice ... At the time of making my original experiments on electro-magnetism in Albany, I was urged by a friend to take out a patent, both for its application to machinery and to the telegraph, but this I declined, on the ground that I considered it incompatible with the dignity of science to confine the benefits which might be derived from it to the exclusive use of any individual. In this, perhaps, I was too fastidious. In briefly stating my claims to the invention of the electro-magnetic telegraph, I may say that I was the first to bring the electro-magnet into the condition necessary to its use in telegraphy, and also to point out its application to the telegraph and to illustrate this by constructing a working telegraph, and had I taken out a patent for my labors at that time, Mr. Morse could have had no ground on which to found his claim for a patent to his invention. To Mr. Morse, however, great credit is due for his alphabet, and in bringing telegraphy to practical use. ». (Alvin F. Harlow)
French comment : Joseph Henry (1797-1878) est un physicien américain qui découvrit l'auto-induction et le principe de l'induction électromagnétique des courants induits. En 1832, il créa l'unité de mesure d'inductance électrique qui fut nommée le henry en son honneur. Henry expérimenta et améliora l'électroaimant, inventé en 1823 par l'Anglais William Sturgeon. Dès 1829, il avait développé des électroaimants d'une grande puissance de levée. En 1831, il fabriqua le premier télégraphe électromagnétique opérationnel. (Compiled from various sources).En 1830, Henry découvre qu'un courant peut être induit dans un conducteur par déplacement d'un champ magnétique ; principe de l'électromagnétisme qu'il ne publiera pas. Dès 1831, il démontre la possibilité de transmettre des messages à distance en utilisant simplement une source de courant, un interrupteur et un électro-aimant. Une pièce métallique générait des chocs audibles au même rythme que l'action de l'opérateur sur l'interrupteur. Henry présenta au public un appareil expérimental à Albany, dans l'état de New York et établit une liaison de plus de 150 mètres, démontrant ainsi la faisabilité du procédé. Mais il ne breveta pas son invention, pas plus qu'il ne lui trouva d'application pratique... Henry sera "doublé" par M. Faraday qui découvrira seul le phénomène d'induction magnétique (août 1831) et par S. Morse qui appliquera cette découverte à la transmission d'information (1832). On lui attribue malgré tout, la découverte de l'auto-induction (juillet 1832), phénomène fondamental en électromagnétisme. En 1893, son nom sera donné l'unité de résistance inductive. Le Henry (H) est l'unité du Système International correspondant à l'inductance électrique. (T. Thomasset, UTC, Université de Technologie Compiègne)
Source : Harlow, Alvin F. (1936), "Old Wires and New Waves- The History of the Telegraph, Telephone and Wireless", READ BOOKS, 2008, pp. 47-49.
Urls : http://www.utc.fr/~tthomass/Themes/Unites/Hommes/hen/Joseph%20Henry.pdf (last visited )

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