RESISTANCE OF A GALVANOMETER. 327 



may be passed through them, and their ends be alternately connected 

 with the quadrants of an electrometer, or with the galvanometer of 

 very high resistance ; the ratio of the electromotive forces observed is 

 equal to that of the resistances. 



The electrometer may be used with alternate currents, if the 

 coefficient of self-induction is not appreciable, and if the circuit is 

 sufficiently insulated that the condition is not altered when one point 

 is put to earth. If this is not the case, the case of the electrometer 

 must be insulated. 



934. RESISTANCE OF A GALVANOMETER OR OF A BATTERY. 

 This method enables us to determine the resistance of the battery 

 and that of the galvanometer itself, without having recourse to 

 another galvanometer. Equation (8) gives, in fact, 



12 r = - - -r. 



Knowing r, the second member represents sensibly the resistance 

 of the battery, or that of the galvanometer, when one is very small 

 compared with the other. 



The resistance of the battery, and that of the galvanometer, 

 may be determined separately by making use of shunts. The current 

 / is first measured with a shunt .$, of power m on the galvanometer ; 

 then removing the shunt a resistance r is added, and the total current 

 I is observed. 



We have then 



(13) 



This second equation between p and g combined with the preceding 

 (12), which gives rQ = p+g, enables us to calculate the two values 

 sought. 



If the resistance r has been chosen so that the two intensities / 

 and I are equal, we get simply 



rs 



or 



(13)' Pg=rs. 



