392 SUBMARINE CABLE LAYING AND REPAIRING. 



suitably sub-divided and traversed by the sliding contacts 

 h and c. This pair of contacts move together so that if one is at 

 100 the other is at 0, or if one is at say 70 the other is at 30, 

 always including 100 ohms between them. Therefore, in any 

 position of & and c there is always 100 + 9,900 = 10,000 ohms as 

 a permanent shunt on the galvanometer. The shunt part (S) 

 namely, from a to & is the slide reading. For instance, a 

 reading of 6,578 would be 65 on the a and 78 on the b 



<I^ 



b 

 100 . .1 S R-S I" 100 



9900 



Fig. 233a. — Rymer-Jones' Universal Shunt. 



pointers. The remaining part (R — S) or 10,000 - S is from a to 

 c. The multiplying powers and joint resistance are the same 

 as already given for these shunts (pp 382 and 389). The 

 complete shunt is made up in the form of two resistance dials 

 like the Kelvin- Varley slides but contained In one case, and 

 can be used for all tests for which the Wheatstone bridge and 

 the Kelvin- Varley slides are suitable. 



BREAKS AND SIMILAR EXPOSURES. 



When a cable is fractured there is generally a large area of 

 copper exposed where the conductor has parted, and if not 

 buried the resistance of the end will vary in accordance with 

 laws established experimentally by Kennelly and Schaefer 

 within well defined limits of current. Faults which are not 

 due to actual breaks in the cable are sometimes of sufficient 

 exposure to bear treatment by break methods. If, for example, 

 a fault polarises with positive current, runs down with nega- 

 tive and tests lower on increasing the strength of testing 

 current, it is pretty certain that the exposure is clean, unob- 

 structed and large enough to be treated by break methods. If 

 carbon is higher than zinc the end is generally of quite normal 

 exposure, say, about a quarter of an inch, and a few hundred 



