SUBMARINE CABLES AND THEIR MANUFACTURE. 



211 



as a preservative. This is to act as a padding or 

 cushion for the iron sheathing or protector next 

 to be applied. This serving is applied in the fol- 

 lowing way: A circular disk or frame, carrying 

 on one face a series of bobbins which hold the 

 threads of the yarn, is kept revolving. The core 

 is made to pass through a hole in the centre of 

 this disk, and the threads are wound spirally 

 round it as the disk revolves. 



The iron wires of the sheathing, which com- 

 pletely inclose and cover the served core, are 

 wound on by the powerful " cable-machines," 

 whose operation is so interesting a feature in a 

 visit to a cable-factory. The great revolving 

 disk, seven or eight feet in diameter, is set round 

 with iron bobbins filled with the iron sheathing- 

 wires. These bobbins are suspended on the face 

 of the disk, so that as the disk revolves they 

 always preserve their fixed position with respect 

 to the earth. In this way the wires themselves 

 are not twisted round their own axes as they are 

 laid on the core. These wires are generally of 

 the best homogeneous iron wire — that is, a wire 

 intermediate in quality between iron and steel, 

 and uniting some of the toughness of the former 

 to the strength of the latter. They are some- 

 times themselves covered with a serving of the 

 best tarred Manila hemp ; sometimes laid on in 

 single wires abutting against each other, so as to 

 form a smooth and complete casing for the cable ; 

 and sometimes they are applied in strands of 

 three wires, each abutting against each other. 

 The composite sheathing of hemp and iron is 

 usually applied to the deep-sea portion of a cable 

 where, in laying, a union of lightness and strength 

 is demanded, and where, when once laid, the 

 cable is not likely to be molested. The single- 

 wire sheathing is applied to cables to be laid in 

 shallower depths, such as coast-waters ; and the 

 heavy-strand sheathing is for protecting the cable 

 in anchorages and on sea-beaches. The light- 

 sheathed cable is called " main " or " deep-sea 

 cable ; " the medium is called " intermediate ; " 

 and the heavy-sheathed cable is called " shore- 

 end." There is seldom more of the last than ten 

 or twelve miles, to carry the cable well out of 

 reach of the abrasion of storm-shifted bowlders 

 and coast-anchorage. The intermediate usually 

 extends until deep water has been reached, where 

 the deep-sea portion takes its place. These three 

 types of cable are connected together by "taper- 

 pieces." The core is, of course, uniform through- 

 out the entire length of the cable ; but the taper- 

 pieces serve to connect the different types of 

 sheathing artisticallv and soundlv with each oth- 



er. The intermediate cable generally has its 

 sheathing-wires covered with a serving of min- 

 eral pitch, silica, and hemp of a coarse quality, 

 in order to ward off as long as possible the dis- 

 solving action of the sea-water. 



The cable being thus finished at the manufac- 

 tory, it is coiled into large iron tanks, and there 

 immersed in brine until it is shipped for transport 

 and laying. 



All the materials of a submarine cable are 

 carefully watched and tested — the iron wire for 

 stretching, twisting, and breaking stress, and the 

 core for all its electric properties. The special 

 properties of every knot or mile of the core are 

 chronicled, so that a complete account of every 

 part of the cable is preserved during its progress 

 of manufacture. And after it is made, it is test- 

 ed electrically every day, to see that no change 

 takes place in its electric qualities. These elec- 

 tric tests are three in number : For resistance — 

 the resistance of the copper wire to the passage 

 of the current. For inductive capacity — the 

 amount of charge or quantity of electricity which 

 the cable will take up. For insidation resistance 

 — the insulating power of the gutta-percha coat- 

 ing. 



These tests are made by direct comparison 

 with units, just as bodies are weighed by com- 

 parison with a pound or unit of weight. The 

 unit of electric resistance is the ohm ; so called 

 after the celebrated German physicist and elec- 

 trician Ohm. The ohm is the resistance of a cer- 

 tain length of platinum-silver wire determined by 

 a committee of the British Association. Multi- 

 ples of the ohm are readily obtained, and these 

 are arranged and made up into what are called 

 resistance-boxes — the practical tool of the elec- 

 trician. A resistance-box usually contains coils 

 of platinum-silver wire of from five thousand 

 ohms downward to single ohms or fractions of 

 an ohm. It is with this finely-graduated tool 

 that the electrician compares the resistance; or, 

 in other words, ascertains the conducting power 

 of the cable. » 



The unit of measurement of the insidation re- 

 sistance of the cable is a very high multiple of the 

 ohm, called the meg-ohm or million-ohms; for, 

 inasmuch as the insulator is, technically speak- 

 ing, a non-conductor, its office is to exercise the 

 necessary resistance to the escape of the current. 

 The unit for capacity is called a micro-farad or 

 millionth part of the farad, which derives its 

 name from Faraday, and represents a certain 

 quantity of electricity. The practical tool for 

 the micro-farad is a contrivance called a condenser, 



