SUBMARINE TELEGRAPHY 197 



copper is covered to the specified thickness by a succession of 

 alternate layers of gutta-percha and compound. Three or four 

 coats of each material are generally used ; the largest wires with 

 their insulating cover are nearly half-an-inch in diameter, the 

 smallest in practical use for cables are about a quarter of an inch 

 in diameter ; but it is quite possible to cover in this way copper 

 wire no thicker than a hair. The dangers encountered in this 

 part of the manufacture are, impurities in the gutta-percha ; 

 eccentricity of the conductor in the insulator, leaving a danger- 

 ously thin coating of the latter ; and, lastly, air-bubbles which 

 may lodge in the insulator unperceived, and do serious injury. 

 In time, water is certain to penetrate to these air-bubbles ; it 

 becomes partly decomposed, the gas generated bursts the bubble, 

 and exposes the copper to the water. The slight leak thus 

 formed is, by the action of the battery used in signalling, easily 

 developed into a very serious fault. Fortunately, the manu- 

 facturers have been able almost, if not wholly, to prevent the 

 occurrence of these dangerous cavities. 



If the cable is to have only one conductor, as is the case in 

 most long lines, the insulated wire is served or wrapped with 

 hemp or jute, which acts as a padding between the gutta-percha 

 and the outer iron wires used to give strength. This serving 

 used to be tarred, but Mr.W. Smith pointed out that the tar was 

 occasionally squeezed into small faults, and was a sufficiently 

 good insulator to prevent their detection during manufacture, 

 though not sufficiently good to prevent these flaws, under the 

 action of the battery, from developing into serious faults. Since 

 then, wet tanned hemp has been generally used. Outside the 

 hemp serving come the iron wires, laid round and round the 

 core, so as to give the whole the appearance of a simple wire 

 rope. 



These iron wires are very generally galvanised to prevent 

 rust. In many cases they are further covered by a double 

 serving of hemp, and a bituminous compound of mineral pitch, 

 Stockholm tar, and powdered silica, patented by Messrs, Bright 

 and Clark. This compound is used in the Persian Gulf Cable, 

 the Lowestoft-Xorderney (Hanover) Cable, and several less im- 

 portant lines, and seems to answer well. In other cases, as in 

 the present Atlantic Cables, each iron wire is separately covered 



