On Electro-Dynamic Induction. 131 
magnetic, by suddenly bringing in contact with its two ends the 
different poles of two magnets, a current, of course, was genera- 
ted in the wire, and this, as before, was found to affect the galva- 
nometer to the same degree, when the copper cylinder was inter- 
posed, as when nothing but the paper intervened. 
39. The last experiment was also varied by wrapping two cop- 
per 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 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 ef- 
fect of interposed coils and plates of metal. But it will be ob- 
served 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 arrangement of apparatus 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 inten- 
sity, 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. 
1. In reference to this surmise, it became important to exam- 
ine 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 t 
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 cur- 
rents, yet they were very different 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 
* See Contributions, No, Il. 
