504 SUBBIAKINE CABLE LAYING AND REPAIRING. 



softened ia the spirit lamp. As soon as the wax has cooled, 

 the guard ring is drawn tightly over it so that the hardened 

 wax entirely fills the space between the core and the ring. 

 In the absence of paraffin wax a lapping of pure rubber may 

 be put on to form a slight taper seating, over which the 

 Ting is screwed on. This eflfectually prevents leakage along the 

 surface of the core where it is covered by the guard ring, and 

 if a little paraffin or other insulating oil be put into the cup no 

 leakage takes place through the oil however long the test, 

 even should the ebonite surface become damp owing to unusual 

 moisture in the air. 



Although rubber or gutta may be used as an insulating 

 packing paraffin wax is preferable, because in the event of the 

 paraffin oil In the cup being upset, it dissolves the rubber strip. 



The ring has been tested by playing a steam jet on it during 

 a D.R. test on a section of cable in tank. No increase in the 

 surface leakage was ob3ervable as indicated by the deflection 

 when the test readings were resumed. 



These rings are found of great service for tests on short 

 pieces of cable in which any leakage shows up to a greater 

 extent than on long lengths. 



Conductor Resistance Tests on a Good Cable : Correction 

 for Earth Current. — All calculations for distance of faults and 

 breaks depend for their reliability upon the resistance of the 

 conductor being accurately known. And as cables are from 

 time to time repaired the length becomes altered, and the 

 resistance undergoes changes when different types are pieced 

 in. Consequently, the records of the CR absolute (that is 

 for the whole line) and the CR per naut must be kept up to 

 date to be of use when required. 



To obtain the correct CR the test employed must eliminate 

 the effects of earth currents, and correction for temperature 

 must also be applied to the bridge reading. 



For elimination of the earth current effect in a CR test the 

 false zero method is, in practised hands, the most reliable 

 under all conditions of earth current, but the reversals test 

 and the Mance method are also applicable, and as both these 

 are to scale or true zero they can readily be carried out without 

 the special skill or experience required in false zero work. 



