130 Contributions to Electricity and Magnetism. 



case, could not screen the magnetic influence. Now if we sub- 

 stitute for the magnet a coil through which a galvanic current is 

 passing, the effect should be the same. The experiment was 

 tried by attaching the ends of the helix to a galvanometer,* and 

 the result was as I expected : when the coil was suddenly brought 

 down on the plate, the needle swung in one direction, and when 

 lifted up, in the other ; the amount of deflection being the same, 

 whether the plate was interposed or not. 



35. It must be observed in this experiment, that the plate was 

 at rest, and consequently did not partake of the induction produ- 

 ced by the motion of the helix. From my previous investiga- 

 tions, I was led to conclude that a different result would follow, 

 were a current also generated in the plate by simultaneously mo- 

 ving it up and down with the helix. This conclusion, however, 

 was not correct, for on making the experiment, I found that the 

 needle was just as much affected when the plate was put in mo- 

 tion with the helix as when the latter alone was moved. 



36. This result was so unexpected and remarkable, that it was 

 considered necessary to repeat and vary the experiment in several 

 ways. First, a coil was interposed instead of the plate, but 

 whether the coil was at rest or in motion with the helix, with its 

 ends separated or joined, the effect on the galvanometer was still 

 the same ; not the least screening influence could be observed. 

 In reference to the use of the coil in this experiment, it will be 

 recollected that I have found this article to produce a more per- 

 fect 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 mov- 

 ing the battery attached to the coil up and down in the acid. 

 But in this case, as in the others, the eff'ect 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 



The arrangement will be readily understood by supposing in Fig. 3, the han- 

 dles removed, and the ends of the helix joined to the ends of the wire of a galva- 

 nometer; also, by a plate of metal interposed between the helix and the coil. 



