MAGNETIC TELEGRAPH. 51 



see two sets of Henry's spools, and two batteries. 

 One is the "intensity battery and spool" first de- 

 scribed; and the coil of fine wire maybe, and often 

 is, several thousand feet long while the battery 

 is composed of more than a hundred cells. The 

 distance being so great they do not attempt to 

 send force enough through the intensity circuit to 

 operate a sounder, but only to open and close the 

 local circuit of Henry's quantity battery and 

 spool. That circuit consists of a battery of but 

 one or two cells of large surface, and a spool 

 with about a hundred feet of coarse wire wound 

 around its core. The intensity combination opens 

 and closes this quantity circuit, whose armature 

 strikes the sounder, just as the intensity armature 

 itself does on shorter lines. This obvious plan 

 Henry described and exhibited in Princeton to his 

 classes, long before any magnetic telegraph was 

 ever commercially constructed, or the convenience 

 of such an arrangement had resulted from the 

 great length to which the lines are stretched. 



Upon that apparatus there are but four names 

 to be written. Oersted, who discovered the effect 

 of the voltaic current upon the magnetic needle ; 

 Arago, who discovered that the voltaic current 

 could generate magnetism ; Sturgeon, who produced 

 the first electro-magnet; and Henry, who discovered 

 the conditions under which an electro-magnet 

 might be operated at a distance who invented the 



