106 WRITINGS OF JOSEPH HENRY. [1838 



INDUCED CURRENTS FROM ORDINARY ELECTRICITY. 



(Proceedings of the Americim Philosophical Society, vol. i, pp. 54-56.) 

 November 2, 1838. 



Professor Henry read a paper entitled " Contributions to 

 Electricity and Magnetism, No. 3. On the Phenomena of 

 Electro-dynamic Induction." 



The primary object of the investigation undertaken by 

 the author was the discovery of induced currents from ordi- 

 nary electricity similar to those produced by galvanism. 

 Preparatory to this, a new investigation was instituted of 

 the phenomena of galvanic induction, and the result of this, 

 forms perhaps the most important part of the communica- 

 tion. 



The first section of the paper refers to the conditions 

 which influence the induction of a current on itself, as in 

 the case of a long wire and a spiral conductor. These are 

 shown to depend on the intensity and quantity of the bat- 

 tery current, and on the length, thickness, and form of the 

 conductor. 



The next section examines the conditions necessary to the 

 production of powerful secondary currents, and also the 

 changes which take place in the same when the form of the 

 battery and the size and form of the conductor are varied. 



The important fact i^ shown that not only a current of 

 intensity can be induced by one of quantity, but also the 

 converse — that a current of quantity can be produced by 

 one of intensity. 



The third section relates to the effect of interposing dif- 

 ferent substances between the conductor which transmits 

 the current from the battery, and that which is arranged to 

 receive the induced current. All good conducting sub- 

 stances are found to screen the inducing action, and this 

 screening effect is shown, by the detail of a variety of ex- 

 periments, to be the result of the neutralizing action of a 

 current induced in the interposed body. This neutralizing 



