494' Prof. J. Henry's Contributions 



found this article to produce a more perfect neutralization 

 than a plate, 



37. Next, the apparatus remaining the same, and the helix 

 at rest during the experiment, currents were induced in it 

 by moving the battery attached to the coil up and down in 

 the acid. But in this case, as in the others, the effect on the 

 galvanometer was the same, whether the plate or the coil was 

 interposed or not. 



38. The experiment was also tried with magneto-electricity. 

 For this purpose, about forty feet of copper wire, covered 

 with silk, were wound around a short cylinder of stiff paper, 

 and into this was inserted a hollow cylinder of sheet copper, 

 and into this again, a short rod of soft iron ; when the latter 

 was rendered magnetic, by suddenly bringing in contact with 

 its two ends the different poles of two magnets, a current, of 

 course, was generated in the wire, and this, as before, 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 galvanometer. 

 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 in- 

 ner 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 in- 

 duced currents are not the same as those desci'ibed in my last 

 communication. They are all produced by motion, and have 

 an appreciable duration, which continues as long as the mo- 

 tion exists. They are also of low intensity, and thus far I 

 have not been able to get shocks by any arrangement of ap- 

 paratus from currents of this kind. On the other hand, the 

 currents produced at the moment of suddenly making or 

 breaking a galvanic current are of considerable intensity, and 

 exist but for an instant. From these, and other facts pre- 

 sently to be mentioned, I was led to suppose that there are 

 two kinds of electro-dynamic 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 phsenomena of induction at suddenly 



