524 The Smithsonian Institution 



iron were placed in a glass tube around which a wire was 

 coiled so that the adjacent rings did not touch each other, 

 they would become magnetic on the passage of a current 

 of electricity through the wire. Thus Oersted's discovery, 

 that an electrical current would influence a magnet, was sup- 

 plemented by Arago's, that it would 2\so produce a magnet. 

 Three or four years later another notable step in advance was 

 made by Sturgeon, in England, who produced for the first 

 time what has since been known as an "electro-magnet." 

 He bent a bar of soft iron into the shape of a horseshoe, thus 

 bringing the poles into the same plane for greater conve- 

 nience ; and he dispensed with the glass tube used by Arago 

 by varnishing his iron core, thus insulating the coils of naked 

 wire, which he wound in a spiral about it. But the most 

 powerful electro-magnets made by Sturgeon's method were 

 insignificant compared with what Henry was able to produce 

 a few years later. Instead of varnishing the iron core and 

 using naked wire, he insulated the copper-wire itself by cover- 

 ing it with silk, and this enabled him to coil the wire closely 

 and to make two or more layers about the core. This had 

 the effect of enormously increasing the strength of the mag- 

 nets produced, and Henry at once recognized the importance 

 of the discovery. But he carried the investigation much fur- 

 ther, examining into the relation of the battery to the mag- 

 net, developing two forms of the latter, which he called 

 "quantity" and "intensity" magnets, and by the aid of the 

 latter succeeded in making visible and audible signals at the 

 end of a long line, which had been declared to be impossible 

 by Barlow. He actually set up in the hall of the Albany 

 Academy a line more than a mile in length, through which 

 signals were transmitted without difficulty, and the principles 

 involved were so well understood by Henry that even then, 

 in 1832, he confidently declared that transmission through any 



