216 



ELECTRICAL MEASUREMENTS 



resistances of two insulators which are used in series; for 

 instance, a trolley wire insulator and its accompanying strain 

 insulator. 



Northrup Method. If the insulation resistances are too 

 high for the voltmeter method, a galvanometer may be used 

 according to a scheme due to Northrup. 11 



The connections are shown in Fig. 123. 



A fairly high resistance, db, which is provided with a sliding 

 tap, s, is placed across the circuit. The galvanometer G is 

 shunted, preferably by an Ayrton shunt as shown, so that its 

 sensitivity may be varied to suit the conditions. 



X 2 



FIG. 123. Northrup connections for measuring resistance to ground when 



power is on. 



When the key K is against the back stop, connection is es- 

 tablished between the ground and s. By moving s, a position 

 may be found where the galvanometer will stand at zero, the 

 potential of s being that of the ground. It will be seen that 

 this is a Wheatstone bridge arrangement. Consequently 



1 = 6 

 x 2 a 



and 



x l 



b 

 a l + V 



i + 



If the key be depressed, any deflection will be due to the current 

 from the battery B which flows to s and there divides, part going 



