THE LOCALISATION OF BREAKS AND FAULTS. 



521 



The advantage in this test is that balance is obtained to one 

 -deflection. In observing that a given deflection does not alter 

 on pressing a key it is not possible to make any mistake, as the 

 very slightest movement of the spot from the fixed deflection 

 can be detected. It might be called a null method to false zero. 



Simultaneous Battery Resistance and E.M.F. Test. — A 



reliable null method which in two readings would give both the 

 E.M.F. and resistance of a battery appeared to the author to be 

 desirable, and he has suggested the following simple form of test. 



LjmsismsisiSL 



MSiSL 



Fig. 291.— Simultaneous Battery Resistance and E.M.F. 



The battery, whose E.M.F. (E) and resistance (6) is to be 

 found, is connected direct to two resistances, R and r (Fig. 291). 



Fig. 292. 



A standard cell of e volts is connected through a key and gal- 

 vanometer across the fixed resistance r. A variable resistance 

 (R) is joined in series and adjusted till there is no deflection 

 on the galvanometer when the key Is closed. We have then 

 the distribution of potential shown in Fig. 292. 



