266 Mr. R. Phillips on Volta- Electric Induction. 



end of a magnet. One only of the copper plates was insuffi- 

 cient to produce this effect. 



I now used instead of the copper plates two zinc plates each 

 ^\jth of an inch thick ; the magnetism of the needle, after the 

 completion and disruption of the battery circuit as before, 

 was much weaker than if no plates had been interposed be- 

 tween the coils, and in the same direction, that is, the eye end 

 of the needle was as the marked magnetic end. Having re- 

 moved these zinc plates, and substituted a pair of zinc plates 

 each 8 inches by i:5 inches square, their combined thickness 

 being *3 of an inch, and having closed and opened the battery 

 circuit once, I found the point of the needle strongly as the 

 marked end of a magnet; these plates had however been 

 amalgamated and a little used for galvanic purposes. Two 

 plates of zinc and two of tin, each J^th of an inch thick and 

 of the usual size, acted the same as the two thick zinc plates 

 in the last experiment ; the tin plates alone were quite in- 

 effective. In these experiments the induced current always 

 passed in the directions pointed out by Dr. Faraday. 



Here, then, we have the first induced current the more 

 powerful, which is the contrary to that obtained in any of the 

 foregoing experiments. The superiority of copper in produ- 

 cing this effect evidently arises from its great power of electric 

 conduction. When a mass of glass was used instead of the 

 copper, the eye end of the indicating needle became of course 

 as the marked end of a magnet. 



I made some experiments on the transmission of the mag- 

 netic force by the metals with the aid of two small electro- 

 magnets, the coil of one being connected with the indicator 

 and that of the other with the battery; the poles of each were 

 placed near to each other, so that the battery current being 

 sent through, or checked in the coil of one magnet, induced 

 a current in that of the other. The interposition of masses of 

 metal between the magnetic poles occasioned no difference in 

 the intensity of the magnetism imparted to the indicating 

 needle when the battery circuit was closed or opened. I ac- 

 count for this by considering the slowness with which iron 

 varies its magnetic state, and by supposing the action of 

 such a metal as copper, when placed in the lines of magnetic 

 force, retards their transmission more than air, and my ap- 

 paratus not being sufficiently delicate to distinguish these two 

 effects. Dr. Faraday first supposed such might be the action 

 of interposed substances, and his experiments on this subject 

 much confirm this view*. 



* See Experimental Researches in Electricity, vol.i.p.545 and following. 



