Method of Measuring Battery Resistance. 523 



mean may be taken instead of their geometric, as being easier 

 to calculate. 



Use of a Double-wire Galvanometer as an Electrometer. 



When a differential galvanometer, or a galvanometer with 

 two long fine wires wound side by side, is employed, a sepa- 

 rate condenser is not absolutely necessary ; for the galvano- 

 meter itself has a certain capacity, and in order to charge one 

 of its wires up to the potential B, and the other down to the 

 potential D, a certain quantity of electricity must flow into 

 the one wire and out of the other, and any variation in this 

 quantity will affect the needle (though the galvanometer-con- 

 stant has only half its ordinary value). Even when a separate 

 condenser is used the capacity of the differential galvanometer 

 may be taken advantage of, by connecting the terminals of the 

 condenser to its two middle screws (instead of joining them di- 

 rectly to each other by a wire and inserting the condenser as in 

 fig. 3), so that both condenser and galvanometer get charged 

 instead of only the condenser. The defect of this method is, 

 that the insulation between two silk-covered wires is not very 

 perfect, and there is a slight leakage, which maintains a slight 

 continuous deflection of the needle when the two outer screws 

 are joined up to a battery ; moreover the statical capa- 

 city of an ordinary fine-wire differential galvanometer is not 

 very great. 



But I think it maybe often convenient to use a double-wire 

 galvanometer as an electromoter in this way. For instance, 

 rapidly to compare the electromotive force of any number of 

 cells, join them up to the outer screws of the galvanometer 

 with disconnected wires one after the other ; the kick in each 

 case measures the electromotive force of the cell. It might 

 also be used to measure very high resistances. It is quite 

 possible, and indeed very probable, that this method has been 

 suggested before. 



Elimination of Variations in Electromotive Force. 



It has been stated above that if only momentary variations 

 are made in the resistance r, or in the value of u, we can con- 

 sider e, the electromotive force of the battery, constant. The 

 plan I adopt is to make the effective variation of r, or the va- 

 riation which is to have any influence on the galvanometer, 

 very short indeed. And this is done by arranging that the 

 key m which closes the circuit of r shall break the galvano- 

 meter circuit g, the instant after, at the point n, as shown in 

 figs. 1 and 4. For an instant, then, u is varied ; and if the 

 resistances are not balanced so that ac = bd, a certain quantity 

 of electricity will enter or leave the condenser through the 



