HALL. — ELECTRIC CONDUCTIOX AND THERMOELECTRIC ACTION. 93 



In Figure 4, M 2 , disconnected from the electrometer but still con- 

 nected with the grounded A/\ by the wire ic 2 , is brought very near to Mi, 

 so that the two act as the plates of a condenser. The difference of 

 potential between them is still <5I\ but M 2 now acquires a considerable 

 charge, Q. The two plates are next 

 disconnected, and M 2 is then re- 

 moved to its first position, out of 

 range of the condenser action of Mi, 

 and is connected with the quadrants, 

 now insulated from the ground, as * 

 in Figure 5. The charge Q dis- . 

 tributes itself over Mo and the quad- /To r.5. 



rants, .but, as the capacity of this 



combination is much less than that of M 2 when near M u the potential 

 now exceeds that of Mi by the amount AT = 8 a I' + N 5 C V, the 

 value of N being greater the less the distance between M 2 and Mi in 

 Figure 4. 



If we assume, as we shall do for convenience here, that the capacity 

 of the quadrants is negligible compared with the capacity of M 2 by 

 itself, we take the capacity of M 2 when near M h as in Figure 4, to be 

 A 7 times the capacity of M 2 by itself. 



After grounding M 2 and the quadrants for a moment, thus reducing 

 their excess of potential over Mi to the original 5F, we bring Mo again 

 to the same position as in Figure 4 and again connect it with Mi by 

 means of wire of metal (2) ; but now through a part of this wire flows a 



H 





M, 



M, 



F,g.Z 



current, so that by a potentiometer arrangement, indicated in Figure 6, 

 we can make the Mi end of the wire exceed the M 2 end of it by a differ- 

 ence of potential 8P. If we make 5P such that the difference of 

 potential between M 2 and Mi in Figure 6 is 8 a V + (8 C V 4- N), the 

 charge on Mo will be precisely what it was after M 2 and E were grounded 

 from the condition shown in Figure 5; that is, when M 2 is separated 

 from Mi in Figure 6 and is afterward brought to the position and con- 



