90 SUBMAKINE CABLE LAYING AND REPAIRING. 



places In the insulation such as might exist by minute quantities 

 of air imprisoned in the layers during manufacture, but it is 

 just as likely that such defects if deep-seated are sealed up 

 instead of being opened out by mechanical pressure. The pro- 

 cess is also long and tedious, as the presses only take a couple 

 of drums or so at one test. 



The insulation is sometimes tested at 5,000 volts for five 

 minutes, but this is open to the objection that the insulation 

 may suffer permanent strain with so high a voltage. The elec- 

 trification test at 250 to 500 volts is quite sensitive enough to 

 show up any defects that may be present and at the same time 

 is perfectly harmless to the core. This test must be taken after 

 30 minutes' charge, one-minute results being of no value what- 

 ever. The increase in apparent resistance depends on the con- 

 stituent parts of the insulator, and the rise may be 100 per 

 cent., but in good gutta-percha compounds the rise is often less 

 than this. Where the material is perfectly good the core im- 

 proves in insulation with time as it goes through the shops, and 

 if a drum were to show no such improvement it would point to 

 defects being present. The insulation of each drum is there- 

 fore carefully tested at intervals for this purpose. The insula- 

 tion is principally a guide to the durability, which, after all, is 

 the main thing, and an excessively high insulation is not 

 desirable. When they have passed the tests the drum lengths 

 of core are transferred to the cable shops for serving and 

 sheathing. To prevent any possibility of damage to the core 

 the drums are cased in during their transit from one department 

 to the other. 



When the cable for which the core is being prepared is to be 

 laid in depths affected by boring insects the next process is to 

 serve over the gutta-percha a layer of cotton tape, then one of 

 brass tape, followed by another of cotton tape. These layers 

 are put on in one operation by Mr. Heory Clifford's process. 

 The core is then served with ordinary or proof tape before 

 receiving the jute yarn, and for this purpose is put through a 

 machine with two or more taping heads revolving in opposite 

 directions. The core is then passed through the serving 

 machine (Fig. 43), where it receives the inner serving, consisting 

 of two layers of tanned jute or yarn in opposite lays, the object 

 being to form a cushion between the sheathing wires and the 



