DISCOURSE OF W. B. TAYLOR. 217 



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 1829, Henry exhibited before the Institute a some- 

 what larger magnet of the game character. "A round piece of 

 iron about one-quarter of an inch in diameter was bent into the 

 usual form of a horse-shoe, and instead of loosely 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 bat- 

 tery 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 

 constructed the magnetic " spool " or " bobbin," that form of coil 

 since universally employed for every application of electro-magnet- 

 ism, of induction, or of magneto-electrics. This was his first great 

 contribution to the science and to the art of galvanic magnetization. 



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 arrangement of the coil. " It consisted in using several 

 strands of wire each covered with silk, instead of one." Employ- 

 ing a horse-shoe formed from a cylindrical bar of iron half an inch 

 in diameter and about 10 inches long, wound with 30 feet of toler- 

 ably fine copper wire, he found that with a current from only two 

 and a half square inches of zinc, the magnet held 14 pounds. 

 Winding upon its arms a second wire of the same length (30 feet) 

 whose ends were similarly joined to the same galvanic pair, he 

 found that the magnet lifted 28 pounds. " With a pair of plates 

 4 inches by 6, it lifted 39 pounds, or more than fifty times its own 

 weight."* On these results he remarks : 



* It must not be forgotten that at the time when this experimental magnet 

 was made, the strongest if not the only electro-magnet in Europe was that of 

 STURGEON, capable of supporting 9 pounds, with 130 square inches of zinc surface 

 in the battery. 



