518 



SUBMARINE CABLE LAYING AND EEPAIEING. 



may be 30 or 40 milliamperes, which on an even bridge must be 

 doubled to obtain the working current from the battery. For 

 this purpose it is convenient to put a milammeter in the shunt 

 circuit as shown in the diagram and adjust K to get, say, 

 80 milliamperes indicated. This adjustment should be made 

 by gradually reducing E, from a high value to its proper 

 amount so that the current does not exceed what is intended. 

 When the current is adjusted the charge and discharge throws- 

 can be taken. 



H'H'I'H' 



Fig. 289.— Distribution of Potentials when Battery Shunted. 

 (Muirhead's Test.) 



Referring to the diagram of potentials Fig. 289 — 

 The charge throw D is proportional to E. 

 The discharge throw d „ „ „ E — e. 



That is, ^ ^: 



(/"E-tJ 



and by proportionality of resistances to potential?, 



E _B+h 

 E-e b ' 



where b is the battery resistance and E is the external resis- 

 tance or shunt, including the milammeter. 



Hence 



D 



d' 



R + b 



and 



6 = R 



d 



D-cl 



The charge throw D should be taken again after the dis- 

 charge to see whether the total E.M.F. has dropped at all 

 owing to current having been taken from the battery. The 

 shunt key is in the ordinary way released as soon as the dis- 



