ON ELECTRO-DYNAMIC INDUCTION. 13 



might be produced in such a way as not to be affected by the interposition of 

 a plate of metal. To understand this, suppose the end of a magnetic bar placed 

 perpendicularly under the middle of a plate of copper, and a helix suddenly 

 brought down on this; an induced current would be produced in the helix by 

 its motion towards the plate, since the copper, in this case, could not screen the 

 magnetic influence. Now, if we substitute 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 produced by the motion of the 

 helix. From my previous investigations, I was led to conclude that a different 

 result would follow, were a current also generated in the plate by simultaneously 

 moving 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 motion 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 gal- 

 vanometer 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 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 pur- 

 pose, about forty feet of copper wire, covered with silk, were wound around a 



* The arrangement will be readily understood by supposing in Fig. 3, the handles removed, and 

 the ends of the helix joined to the ends of the wire of a galvanometer; also, by a plate of metal 

 interposed between the helix and the coil. 

 VIII. — D 



», 



