METHOD OF DISCHARGES. 427 



indicated by the sign of v t it is advantageous to withdraw this couple 

 from the battery. 



1021. METHOD OF DISCHARGES. When two points in an elec- 

 trical system which has attained a permanent state are connected 

 with the coatings of a condenser, the charge of the condenser is 

 proportional to the difference of potential V of the two points in 

 question. Further, if the permanent state is kept up by constant 

 electromotive forces, and the losses of the condenser may be neg- 

 lected, this difference of potential is the same as if the condenser 

 had not been brought in. 



The capacity of the condenser being C, the charge m is equal to 

 CV. A comparison of the discharges measured by a ballistic gal- 

 vanometer (883) will give the ratio of the electromotive forces. 

 The method is equivalent to employing an electrometer or gal- 

 vanometer of high resistance. 



In order to avoid complicated corrections, it is advantageous 

 that the swings to be compared be of the same order of magnitude, 

 even when the electromotive forces are very different. Instead 

 of using shunts, the inconveniences of which have already been 

 noted (884), it is better to use standard capacities, such a value 

 being used in each experiment that the quantities are of the same 

 order. 



1022. The experiment may be arranged so as to determine the 

 resistance r of a battery. The poles of the battery are connected 

 up with a condenser through a ballistic galvanometer. The swing 8 

 of the needle is proportional to the electromotive force e of the 

 battery in open circuit. Leaving this connection, the poles are 

 joined by a shunt of resistance /o. The swing 8 of the needle is 

 in the opposite direction, and it is proportional to the excess e - e 

 of the electromotive force over the difference of potential of the 

 poles in the new state. We deduce 



e-e' 8' 



By means, of two keys the changes are rapidly made, and 

 polarization has not time to establish itself. 



We may work in the opposite direction. The battery being 

 first closed by a resistance />, the poles are connected with the 

 condenser by the galvanometer. The shunt is then cut. Swings 

 are thus obtained in the same direction, which are respectively 



