The Three Secretaries 123 



was composed and the long circuit in which it was placed, it 

 was necessary to use an "intensity battery," — that is, a bat- 

 tery of many plates — for the reason that this battery pos- 

 sesses more electromotive force ; while with the other, formed 

 of many coils of short, thick wire, offering less resistance, a 

 "battery of quantity" should be employed. "I was the first," 

 he wrote, " to point out this connection of the two kinds of 

 battery with the two forms of the magnet in my paper in 

 Silliman's Journal, January, 1831, and clearly to state that 

 when magnetism was to be developed by means of a com- 

 pound battery, one long coil was to be employed, and when 

 the maximum effect was to be produced by a single battery 

 a number of single strands were to be used." 



The importance of this discovery of the necessary law of 

 proportion between the electromotive force in the battery and 

 the resistance in the magnet cannot be too highly estimated ; 

 not only does the telegraph depend upon this law, but every 

 action of galvano-magnetism. 



As has well been said by his daughter, " he married the 

 intensity magnet to the intensity battery, the quantity mag- 

 net to the quantity battery, discovered the law by which their 

 union was effected, and rendered their divorce forever impos- 

 sible." The intensity magnet is that which is to-day in use 

 in every telegraph system. 



With the discovery of these two agents began a new epoch 

 in science and in the arts. They brought the force of electric- 

 ity, hitherto only in part subdued, fully under the control of 

 man. Before Henry, the only electro-magnet which had been 

 made, though under the influence of a battery of 125 plates, 

 was incapable of lifting more than nine pounds ; but he, after 

 a few months of experiment, produced one which, with one 

 pair of plates, sustained 39 pounds, or fifty times its own 

 weight; in 1830, 750; in 1831, 2300; and in 1834, 3500 



