14 CONTRIBUTIONS TO ELECTRICITY AND MAGNETISM. 



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 aifect 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 dimi- 

 nished 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 commu- 

 nication. 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 arrangement of appa- 

 ratus 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 con- 

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

 presently 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 phenomena of induction at suddenly making and breaking a galvanic current. 

 And in connexion 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 indications of the galvanometer were the same as those of the spiral, 

 in reference to the direction of the induced currents, yet they were very dif- 

 ferent in regard to the intensity of the action. Thus, when the arrangement 

 of the apparatus was such that the induction at making the battery circuit 

 was so feeble as not to give the least magnetism to the needle, and so powerful 



