366 



SUBMAEINE CABLE LAYING AND BEPAIRING. 



galvanometer, as in Fig. 217, because this sets up extra damp- 

 ing and sluggishness during the period of return of the spot 

 from its deflected position to zero, and takes up appreci- 

 able time when balancing on the bridge. The key should be 

 connected across the universal shunt or the combined shunt 

 and high resistance, as explained in the note on the " Universal 

 Shunt," and as shown in Fig. 227. 



(3) Polarisation Current from a Fault or Break. — Under the 

 action of current from a testing battery the fault or break be- 

 comes polarised or charged like a secondary cell, and an opposing 

 difference of potential is set up in it called the E.M.F. of polari- 

 sation. The potential active in charging the fault is not the 

 full potential of the battery, but something less, depending on 



Fig. 217. — Galvanometer Short Circuit Key. 



the distance of the fault. On releasing the battery key (which 

 disconnects battery and puts cable to earth), the charge stored 

 at the fault is discharged through the cable, producing what 

 is known as the polarisation current. Its chief characteristics 

 are that, from the moment it commences to flow, it falls 

 quickly and steadily in strength, owing to the fall in the 

 E.M.F. of polarisation, and that it is always in a direction 

 opposite to the testing current. 



The eflect of the polarisation current can be eliminated by 

 the same methods as for earth current. The two currents, in 

 fact, combine and form one for the time. If reversals are 

 used, the zinc current can generally be kept on; but the 

 positive current can only be put on for a second or two, as 

 it may run the resistance of the fault up to something very 



