164 WRITINGS OF JOSEPH HENRY. [1840 



was found to affect the galvanometer to the same degree, 

 when the copper cylinder was interposed, as when nothing 

 but the paper intervened. 



39. The last experiment was also varied by wrapping two 

 copper wires of equal length around the middle of the 

 keeper of a horse-shoe magnet, leaving the ends of the inner 

 one projecting, and those of the outer attached to a galva- 

 nometer. A current was generated in each by moving the 

 keeper on the ends of the magnet, but the effect on the 

 galvanometer was not in the least diminished by joining the 

 ends of the inner wire. 



40. At first sight, it might appear that all these results 

 are at variance with those detailed in my last paper, relative 

 to the effect of interposed coils and plates of metal. But it 

 will be observed that in all the experiments just given, the 

 induced currents are not the same as those described in my 

 last communication. They are all produced by motion, and 

 have an appreciable duration, which continues as long as 

 the motion exists. They are also of low intensity, and thus 

 far I have not been able to get shocks by any arrange- 

 ment of apparatus from currents of this kind. On the 

 other hand, the currents produced at the moment of sud- 

 denly making or breaking a galvanic current, are of con- 

 siderable intensity, and exist but for an instant. From these 

 and other facts presently to be mentioned, I was led to sup- 

 pose that there are two kinds of electro-d^mamic induction ; 

 one of which can be neutralized by the interposition of a 

 metallic plate between the conductors, and the other not. 



41. In reference to this surmise, it became important to 

 examine again all the phenomena of induction at suddenly 

 making and breaking a galvanic current.* And in connec- 

 tion with this part of the subject, I will first mention a fact 

 which was observed in the course of the experiments given 

 in the last section, on the direction of the induced currents 

 of different orders. It was found that though the indica- 

 tions of the galvanometer were the same as those of the 

 spiral, in reference to the direction of the induced currents, 



*See my last paper. (No. III.) 



