560 RATIO OF UNITS. 



of the needle is observed at the moment the electrostatic measure 

 is made, we shall have 



G 



Instead of working in this way, Shida determined then the 

 deflection 8 obtained by closing the battery the resistance of which 

 is R , and the electromotive force E , by an auxiliary resistance r. 

 The intensity of the current gives 



TT 



E = /(R + r) = - 



If the electromotive force of the battery is unchanged, the 

 product (R + ^) tan 8 is constant, when the external resistance is 

 made to vary, and the conditions can be chosen so that the de- 

 flection is near 45, which corresponds to the maximum sensitiveness 

 (833). We have then 



This mode of arranging the experiments does not appear to be 

 very advantageous, because the observation for the two systems of 

 measurements are not simultaneous, and the resistance of the battery 

 comes into the calculations, the determination of which always pre- 

 sents some difficulties. 



1133. In the preceding experiments two forces are in reality 

 compared the attraction between the two plates of the electrometer 

 and the mutual action of the two coils of the electrodynamometer. 

 Instead of measuring these two forces separately, Maxwell* arranged 

 the experiments so as to balance one by the other. Two systems 

 were arranged, each consisting of a conducting disc and an opposed 

 coil ; one of the systems is fixed and the other movable. 



The two plates being charged with opposite electricities with a 

 constant difference of potential, attract each other ; the two coils, 

 which are traversed by currents in contrary directions, repel. The 

 first force is sensibly inversely as the square of the distance of the 

 plates, while the second varies according to a less rapid law ; there 



* MAXWELL. Transactions of the Royal Society for 1868, p. 643. 



