THE LOCALISATION OF BREAKS AND FAULTS. 519 



charge throw is obtained, and it is on for so short a time that 

 the battery has hardly time to polari-e. On the other hand, it 

 ia desirable sometimes to see if the battery E.M.F. will stand 

 up to a sustained current, and this can conveniently be tried 

 on these connections by keeping the shunt key down for 15 or 

 30 seconds, then releasing it, and immediately afterwards 

 taking the charge throw. 



The author has suggested that when on these connections 

 the battery resistance may be checked by taking a second, 

 observation with the resistance increased to R^ (Inclusive of 



the milammeter), so reducing the current to, say, -th part of 



n 



its first value. 



We have then the ratio of curreats inversely proportional 



to the total resistance in each case, from which it follows that 



whence J= — t — . 



n— 1 



This formula assumes the E.M.F, of the battery to remain 

 constant during the test. The current ia the second observa- 

 tion should be reduced to about half to obtain good apprecia- 

 tion. The internal resistance of a battery falls somewhat as 

 the current taken from it is increased, and therefore if great 

 accuracy is required the resistance should, be measured when 

 giving approximately the same current as the battery will be 

 required to give on duty. 



Munro's Method. — With the same connections as for the 

 preceding test this method is different in the manner of taking 

 observations, and slightly simpler in the calculation. The shunt 

 is first put on the battery by closing the shunt key, followed by 

 closing the galvanometer and condenser circuit. The throw d-^ 

 obtained is proportional to the potential e at the battery ter- 

 minals — that is, to D - f^ in the last test. The shunt is opened 

 as soon as this throw is observed, keeping the galvanometer key 

 closed. The charge is thus completed to the full potential of 

 the battery. As soon as the galvanometer is steady the shunt 

 key is closed again, and a reverse throw obtained proportional 



