of Varying Currents in Inductive Circuits. 



257 



be measured by the ordinary compensation or potentiometer 

 method ; but when it is varying a modification of the method 

 is required. The usual galvanometer is replaced by one pro- 

 vided with stops, so that the needle system is free to turn in 

 one direction only. Let now the strength of the current rise 

 continuously, the slider G meanwhile making contact with the 

 potentiometer wire at such a point that u balance " occurs for 



Fig. 1. 



Non-ln&ucbive resistance 



B„ ^ /■ v vvw\aaaA_ 



7* 



Stopped galvanometer 



a particular value of the current. There will then be a current 

 in the galvanometer which changes sign at the instant of 

 potentiometer balance ; and if the galvanometer terminals are 

 so connected that the initial current in it forces the needle 

 against the stops, the needle will begin to fly off just as this 

 current passes through zero. Therefore if the galvanometer 

 circuit be broken at or before this instant no throw of the 

 needle is produced ; it is clear then that either by altering 

 the time interval during which the current is permitted to 

 flow through the galvanometer, or by adjusting the position 

 of the slider, the throw can be reduced to zero; and the fall of 

 potential (e) between the points C and D (fig. 1) would be 

 equal to the fall down the non-inductive resistance if it were 

 not for the self-inductance A of the galvanometer. We have 

 in fact for each balancing position of the slider 



at 



y=o 



(i) 



