4 BELL SYSTEM TECHNICAL JOURNAL 



known, his researches arc of the most enduring character, and for all 

 time must enter intimately into the Hves of all civilized mankind. 

 He was without peer among the American physicists of his time, and 

 it is well attested by every record that he was a man of varied culture, 

 of large breadth and liberality of views, of generous impulses, of 

 great gentleness and courtesy of manner, combined with equal firm- 

 ness of purpose and energy of action. 



Let us now turn to Henry's investigations of electro-magnetism, 

 which were among his earliest scientific undertakings. He began 

 his career in 1826 in New York State at the Albany Academy, where 

 he had only the apparatus he could construct with his own hands 

 and, out of each year, but a single month uninterrupted by other 

 duties to devote to his researches. It was there — independently of 

 Faraday and on some fundamental points prior to him — that Henry 

 discovered the laws of current induction. At the same time he under- 

 took a study of the electromagnet which prepared the way for not 

 only the telephone and telegraph, but also for all types of dynamos 

 and motors. 



The electromagnet was discovered by Sturgeon in England, but 

 Henry's contributions to our knowledge of it were so great that after 

 his work, a powerful instrument suitable for many uses replaced 

 what had been a feeble toy. When he started his work on the electro- 

 magnet its design was not understood; when he had completed his 

 work he had developed a magnet, the design of which was understood 

 and which could be adapted, according to the rules which he laid 

 down, to a multitude of purposes. 



With reference to the making of electromagnets, Henry pointed 

 out the improvements which resulted from insulating the conducting 

 wire itself, instead of the rod to be magnetized, and by covering the 

 whole surface of the iron with a series of coils in close contact. This 

 was effected by insulating a long wire with silk thread, and winding 

 this around the rod of iron in close coils from one end to the other. 

 The same principle was extended by employing a still longer insulated 

 wire, and winding several strata of this over the first, care being taken 

 to insure the insulation between each stratum by a covering of silk 

 ribbon. By this arrangement the rod was surrounded by a compound 

 helix formed of a long wire of many turns instead of a single helix of a 

 few turns. 



Thus Henry laid down the rules, which, in general, are followed 

 today in the construction of commercial electromagnets; namely, 

 that the wire should be insulated, that it should be wound in layers, 

 and that there should be several lavers, one above the other. He 



