Engineering Wonders of the World
Volume III

THE CONSTRUCTION OF SUBMARINE CABLES. 359 cables is very similar to that of a lead pencil, the wire conforming closely to the lead and the dielectric to the wooden case of the pencil. It only remains to be said that the cover- ing of the conductor with a suitable insulating dielectric is the most important feature in the manufacture of a submarine cable, besides representing the largest proportion of the total cost of the line. The conductor and dielec- ©Fig. 4.—TYPICAL ATLANTIC CABLE CORE (actual size). This is made up of 650 lbs. copper and 400 lbs. gutta-percha per nautical mile. trie combined are commonly termed “ the core.” The core of a modern Atlantic cable pro- vides for a speed of fifty words per minute by ordinary manual transmission, and, in effect, some 100 words a minute by the duplex-automatic system of sending signals in both directions simultaneously. For teredo-ridden waters the core is pro- tected by metal taping, applied helically. Inasmuch as no insulated conductor, such as we have described, could be up from any substantial for the purposes of sub- sequent repair, or even withstand the abrasion involved by some portions of the sea-bottom, the core is always covered with a sheathing of galvanized iron or steel wires, with a packing of jute between the core and the wires. Mechanical Protection. depth The jute yarns are served round the core by machinery of the same type , i . -i , r , Inner Ser vine;, as that employed tor laying up the copper conductor strands, but set hori- zontally instead of vertically. The sheathing of iron or steel wires is applied in a similar helical fashion, by gear like that associated with the of ordinary wire-ropes. There may be anything from ten to twenty of these wires, and the each may be anything between 0'07 of an inch and 0'4 of an inch, according to the depth and nature of the bottom for which the cable is intended. manufacture Armour. diameter of Galvanizing iron wires is an insufficient guard against rust in salt-water, and mainly on this account the sheathing is covered with a mixture of mineral pitch, tar, and silica— commonly known as Bright and Clark’s Com- pound—which is again applied after the cable has been enveloped in an outer serving of hemp, the latter constituting a firm binding and further preservative. The silica in the compound serves as an additional protection against incursions by the teredo ; and in modern practice each wire is either separately compounded in advance, or—for “ main ” types—enveloped in compounded cotton tape. Fig. 5 below gives a general view of the simultaneous serv ng and sheathing of a cable. On the upper floor of the factory may be seen the drum of insulated con- ductor, with two jute serving Cable Manu = , . „ b facture. machines tor applying separate layers of yarn, each in opposite directions. From here the served core is drawn down, as Jfig. 5.—GENERAL VIEW OF CABLE MANUFACTURE.