1840] WRITINGS OF JOSEPH HENRY. 163 



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 induc- 

 tion produced by the motion of the helix. From my pre- 

 vious 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 experi- 

 ment in several ways. First, a coil was interposed instead 

 of the plate, but whether the coil was at rest or in motion 

 wath the helix, with its ends separated or joined, the effect 

 on the galvanometer was still the same ; not the least screen- 

 ing 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 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 cur- 

 rent was of course generated in the wire, and this as before 



