286 ELECTROSTATICS. [PT. II. CH. VI. 



Taking differences we obtain 



In this manner we can accurately bring the ratio to unity. 



C 2 



Whether the adjustment be made or not, and without the 

 necessity of making F 3 large, if we can reverse the sign of V 

 we may by observing 1 and 2 get the correct value, since 



so that V is directly proportional to the difference of the two 

 deflections, or to the arithmetical mean of their absolute values. 



149. Induction Electrical Machines. As a further 

 example of induction in 'a system of conductors, we shall con- 

 sider the action of a class of electrical machines typified by Lord 

 Kelvin's Replenisher. 



This consists essentially of two semi-cylindrical conductors 

 A and B called the inductors, and two smaller conductors G 

 and D called the carriers, which may be rotated as a rigid system 

 about the axis of symmetry. If V l be the (positive) potential 

 of A at any time, F 2 that of B, supposed 

 negative, then if C and D be put in conduct- 

 ing communication with the earth while in the 

 position shown, G will have a negative, and 

 D a positive charge induced upon it. Now on 

 insulating G and D, and turning them until G 



Tji nn 



is opposite B and D opposite A, if C be put into 

 communication with B, being nearly surrounded by B, it will give 

 up its charge, thereby increasing the absolute value of the negative 

 potential of B. D being put into communication with A gives up 

 its positive charge, and increases the positive potential of A. 

 The connections of G and D with each other and with A and 

 B are made automatically by contact springs once in each half 

 revolution. 



If F t (n) and F 2 (7l) are the potentials of A and B after n half- 

 revolutions, KI and K z the capacities of A and B and whatever 



