398 



SUBMARINE CABLE LAYING AND REPAIRING. 



line. It is sometimes more convenient to use this instrument 

 in the battery circuit in order to get larger deflections on a 

 given range, and when that is done the deflections must be 

 halved (on an even bridge) because the current to line is only 

 half that coming from the battery. It is only necessary to 

 know the actual current to see that the current limit of 

 25 milHamperes to line is not exceeded. For the purpose of 

 the test the ratio between the two currents is all that is 

 required, not their actual values. The diagram also shows the 

 galvanometer unshunted, but with a high resistance in series 

 to reduce its sensitiveness as required. This arrangement 

 makes the galvanometer indications quicker, and is sometimes 



High Besistance 



Left hand Key 

 Zinc to Line 



]ili|i|i|i|i|i|ifi|i|ili|i!i|iM 



Carbon Zinc 



Fig. 235. — Two-current Kennelly Test. 



an advantage in false zero work. If a universal shunt is used 

 it should be set at or near the position for maximum sensitive- 

 ness, and a high resistance put in series for adjustment of the 

 deflections as required. The short-circuit key should be con- 

 nected right across the bridge ratio points, as in Fig. 227, so 

 that the return of the spot to true zero is not damped. The 

 test is taken as follows : — 



Even bridge ratios 1,000/1,000 or 100/100. Arrange battery 

 commutator or switch to give two battery powers on key 

 approximately in the proportion of 4 to 1, the higher current 

 being about 20 milliamperes to line, as indicated on 

 the milammeter. First put the higher battery power 

 on key and put zinc current to line for a few minutes to clean 



