402 



SUBMABINE CABLE LAYING AND EEPAIRING. 



plug commutator may be conveniently used, as in Fig. 237, 

 for changing over from cable to known resistance. The 

 currents are changed by means of a commutator or switch for 

 altering the number of cells in approximately the proportion 

 of 4 to 1, always using the higher current first in a pair. 



The deflections in this case must be taken from true zero. No 

 short-circuit key need be used, as the shunt on the galvanometer 

 is so low. The polarisation and earth currents combined is read 

 immediately before and after each observation, and added to 

 or deducted from the cable deflections, according as they are 

 against or with them respectively. Immediately on applying 

 the battery current the spot takes up its deflection, and every 

 change in the fault can be followed with accuracy and a great 

 saving of time= If the cable deflections are reproduced the 



Low-shunted 

 Sullivan Galv. 



y y 9 Commuta tor 



Hl'I'I'I'lSl'I'M 



d^-o 



□— E 



Cable 



Break 



Adjustable 

 Resistance 



Fig, 237. — Kennelly's Two-current Test by Reproduction Method. 



resistance of the battery and shunted galvanometer do not 

 come into the calculations. It is not necessary, however, 

 to exactly reproduce the deflections ; the resistances they re- 

 present can be calculated in terms of a constant, as in an insu- 

 lation test, allowing for the resistance of the battery and 

 shunted galvanometer in the usual way. The ratio of the 

 deflections will correctly represent the ratio of currents to line^ 

 and for the working out it is not necessary to know the actual 

 currents. But the larger current to line should be deter- 

 mined in order to see that it does not exceed 25 milliamperes. 

 It is a simple matter to note the constant of the instrument 

 and work out the limits of current strength expressed in scale 

 divisions. Mr. C. W. Schaefer has given a good deal of atten- 

 tion to the construction of a divided low-resistance shunt in 

 combination with a Sullivan galvanometer on the potentio- 



