Mi*. R. Phillips on Volta-Electric Induction. 265 



and opened the circuit ten times, when the point of the needle 

 was its marked magnetic end. 



I substituted for the zinc in the two last experiments an- 

 other plate of copper jTj tn °* an mcn thick » m one experi- 

 ment there was obtained a scarcely appreciable amount of 

 magnetism and in the other none ; in both instances the cir- 

 cuit was made ten times. 



I could produce no effect in the indicating helix even with 

 the aid of the file when a plate of glass ^th of an inch thick 

 was placed on one side of B, and a corresponding interval 

 filled of course with air, maintained on the other side. 



I thought it interesting to ascertain what relation two similar 

 plates of one metal, but having different thicknesses, bear to 

 each other in limiting the action of the primary coil upon the 

 secondary coils. I therefore placed a plate of zinc y^ydth of 

 «n inch thick, together with a plate of glass ^th °f an i nc K 

 thick, between A and B, and between B and C a plate of zinc 

 ^th of an inch thick ; the glass was merely employed to pro- 

 duce a suitable space between A and B ; the battery circuit 

 was closed and opened twenty times, which made the eye end 

 of the indicating needle to become as the marked end of a 

 magnet. When the thin zinc and glass were placed between 

 B and C, and the thick zinc between A and B, then on com- 

 pleting and breaking the battery circuit as before, the indica- 

 ting needle showed a reverse magnetism. I made some ex- 

 periments to ascertain if any difference arose by putting the 

 glass plate on different sides of the thin metal plate, but could 

 obtain no result to countenance such a notion. 



Like results were obtained by using for the two zinc plates 

 of the last experiments two plates of copper, one yyjth of an 

 inch and the other ^th of an inch thick. Nor did it appear 

 to occasion any difference if the place of the thicker metal 

 plate was supplied by two thinner ones. 



From these experiments I conclude, that a thick mass of 

 metal plate stands to a thinner mass of the same metal as cop- 

 per does to lead. 



The question now arose in my mind, Does a metallic plate 

 effect similarly, as to degree, the first and second induced 

 currents? This was answered negatively by the following 

 experiments. 



Having removed the upper coil C, I united that end of the 

 indicator, with which C had previously communicated, to the 

 end of A which had been joined to C, and placed two plates 

 of copper, each ^th of an inch thick, between A and B; 

 after having completed and broken the battery circuit once 

 through B, the point of the indicating needle was as the marked 



Phil. Mag. S. 3. Vol. 33. No. 222. Oct. 1848. T 



