PRINCIPLES OF CONSTRUCTION. 89; 



distinct — viz., the core department, which embraces the copper- 

 testing and stranding shops, gutta-percha covering shop, tem- 

 perature tanks and testing room, and the cable, or sheathing 

 department, where the core is served, sheathed, and compounded, 

 coiled as manufactured into tanks, tested and shipped for 

 laying. 



In the core department the wires are put through the stranding 

 machine, which lays six, ten or twelve wires as required round 

 one central wire. For the purpose of making the gutta-percha 

 adhere, the stranded conductor passes through a tank of hot 

 Chatterton's compound, which coats it and fills in the interstices 

 of the wires. Passing onwards, the conductor enters a receiver 

 containing molten gutta-percha, whence it passes out with the 

 first coating. A second coating of compound is then followed 

 by a second of percha and a third of compound by a third of 

 percha, when the core passes out through a die under pressure 

 which makes a perfectly cylindrical compressed covering. The 

 finished core is then led through a long trough of cold water 

 to cool and harden it. It is then wound on drums in 

 lengths of about two and a-half nauts per drum and weighed- 

 Each drum length is numbered, and records are kept of its 

 length, weight, resistance and final position in the cable. The 

 drums are then placed in tanks, under water kept at the 

 standard temperature of 75°F. These tanks are compara- 

 tively small, each holding not more than two or three drums, 

 and are generally fixed in rows close together, the tops flush 

 with the floor. Water and drain service is laid on so that 

 the tanks can be filled or emptied at will. 



After 24 hours' immersion the resistance of the conductor 

 and dielectric and the electrostatic capacity are measured. 

 Some companies repeat the insulation test in water at a lower 

 temperature, as the existence of any small fault is then more 

 readily detected owing to the higher resistance of the dielectric. 

 As a further test of the insulator the drums are put in a pressure 

 cylinder containing water at 75°F. to which hydraulic pressure 

 up to three or four tons per square inch, corresponding to that 

 at 2,500 to 3,300 fathoms depth, can be applied. If the coat- 

 ings are perfectly sound the insulation improves under pressure 

 and the tests have to show a certain percentage improvement. 

 At the same time this test is intended to open up any weak 



