MEASUREMENT OF A CAPACITY. 565 



and the guard-ring; the condenser is charged. P is then imme- 

 diately separated from F, and the key K is raised ; the plate N 

 and the box S are then connected with B. F is touched by the 

 branch Q of the fork ; the charge of the disc runs to the earth by 

 the galvanometer. 



The difference of potential E of the two points A and B remains 

 to be measured. A greater resistance p is put in the galvanometer 

 circuit, and one end of the resistance AB is attached at A, and 

 the other at a point D, such that the resistance AD is a fraction m 

 of the resistance AB, so that the difference of potential between 

 A and D is equal to mE. Moreover, the galvanometer being 

 shunted, the current /, measured by the deflection 8, is a fraction 



of that which the electromotive force ;;/E would give, in a re- 



sistance P + = . This, then, is the current which the 



g + S S+g P(x+s} + rs 



electromotive force E would produce in the resistance - 



ms 



and from equation (i) we have 



MS Ta 



The galvanometer was a Thomson's astatic one, in which, in 

 order to increase the time of oscillation and diminish the damping, 

 the ordinary needles had been replaced by two systems of 20 bars 

 each, mounted in a kind of small leaden shell, so as to represent 

 a small sphere. The time of oscillation attained 39*5 s., and the 

 logarithmic decrement was reduced to 0-1565. 



1136. Instead of a single discharge, a series of discharges may 

 be used. If /3 is the permanent deflection due to the discharges, 

 5 that which corresponds to the current produced in a resistance R, 

 by the electromotive force established between the armatures, and 

 n the number of discharges in a second, we have 



If the conditions are chosen so that the deflections 8 and ft are 

 equal, we have simply a 2 = nc~R. In these conditions, it -is un- 

 necessary that the apparatus should have as large dimensions as 

 for a single discharge, and the battery may be reduced to a small 

 number of cells. 



