282 HENEY AND THE TELEGEAPII. 



previoiisly effected by a small galvanic element." And in another later 

 paper, lie repeated to tlie same effect : "After reading an account of 

 the galvanometer of Schweigger, the idea occurred to me that a much 

 nearer approximation to the theory of Ampere could he attained by in- 

 sulating 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." 



The electro-magnet figured and described by Sturgeon (in his commu- 

 nication of ]!fovember, 1825,) consisted of a small bar or stout iron wire 

 bent into a f| or horse-shoe form, havhig a copper wire wound loosely 

 around it in eighteen turns, with the ends of the wire dipping into mer- 

 cury-cups connected with the respective poles of a battery having 130 

 square inches of activ^e surface. This was undoubtedly the most effi- 

 cient electro-magnet then in existence. 



In June of 1828, Henry exhibited to the Albany Institute a small-sized 

 electro-magnet closely wound with silk-covered copper wire about one- 

 thirtieth of an inch in diameter. By thus insulating the conducting 

 wire, instead of the magnetic bar or core, he was enabled to employ a 

 compact coil in close juxtaposition from one end of the horse-shoe to the 

 other, obtaining thereby a much larger number of circuits, and with 

 each circuit more nearlj^ at right angles with the magnetic axis. The 

 lifting power of this magnet is not stated, though it must obviously have 

 been much more powerful than the one described by Sturgeon. 



In March of 1820, Henry exhibited to the Institute a somewhat larger 

 magnet of the same character. "A round piece of iron about one-quar- 

 ter of an inch in diameter was bent into the usual form of a horse-shoe, 

 and instead of losel}' coiling around it a few feet of wire, as is usually 

 described, it was tightly wound with 35 feet of wire covered with silk, 

 so as to form about 400 turns ; a pair of small galvanic plates which 

 could be dipped into a tumbler of diluted acid, was soldered to the ends 

 of the wire, and the whole mounted on a stand. With these small plates 

 the horse-shoe became much more powerfully magnetic than another of 

 the same size and wound in the usual manner, by the application of a 

 battery composed of 28 plates of copper and zinc each 8 inches square. " 

 In this case the coil was wound upon itself in successive layers. 



To Henry, therefore, belongs the exclusive credit of having first con- 

 structed the magnetic "spool" or "bobbin," that form of coil since 

 universally employed for every application of electro-magnetism, of in- 

 duction, or of magneto-electrics. 



In the latter part of 1829, Henry still further increased the magnetic 

 power derived from a single galvanic pair of small size, by a new arrange- 

 ment of the coil. " It consisted in using several strands of wire each 

 covered with silk, instead of one." Employing a horse-shoe formed 

 from a cylindrical bar of iron half an inch in diameter and about ten 

 inches long, wound with 30 feet of tolerably fine copper wire, he found 



