246 ELECTRO-TELEGRAPHY 



continuity is greatly reduced. It is scarcely to be expected that 7 weak pianos would 

 occur at the same spot, and, if a single wire wore- broken, it would not interfere, to 

 any sensible degree, with the passage of the current. In constructing the Atlantic 

 cable, however, the utmost care prevailed in every department. Every portion of the 

 copper-wire was submitted to exact examination, and no wire was used which possessed 

 a lower conducting power than 85 per cent, of that of pure copper. The insulation of 

 this conductor was thus effected. The centre wire of the copper strand was first 

 covered with 'Chatterton's compound' (that is, gutta-percha rendered viscid with 

 Stockholm tar). This first coating was so thick that, when the other six wires forming 

 the strand wore laid spirally around it, every interstice became filled and air excluded. 

 The mechanical solidity of the core thus formed was very great, This cable is nearly 

 3 times as heavy as the old Atlantic conductor, its weight being 300 Ibs. to the nautical 



820 



819 



Section of the Atlantic Cable, 

 18C5. 



The Atlantic Cable Shoro-End, 1865. 



mile against 107 Ibs. The complete exclusion of air, which is a most important object, 

 is believed to be effected by this arrangement. The conductors of some cables have 

 suffered from their being to some extent loose with their insulating covering. The 

 core being thus formed, next received another coating of Chattorton's compound, which 

 quickly solidified, and thus thoroughly protected the whole. It was then surrounded 

 with a coating of the purest gutta-percha, which was pressed round it in a plastic state, 

 by means of a very accurate die, and sheathed the core in a continuous tube. Over 

 this was again laid another coating of Chatterton's compound, for the purpose of 

 effectually closing up any pores or flaws which may have escaped detection in the gutta- 

 percha tube. To this covering succeeds a second tubo of gutta-percha, then another 

 coating of Chatterton's compound, and so on alternately until the wire was covered 

 with 4 coatings of gutta-percha and 4 of the compound. This being completed, it 

 weighs 400 Ibs. to the nautical mile. It is well known that thu insulating power of 

 gutta-percha sensibly decreases by heat. Tho whole of the conductor was immersed 

 in water of a temperature of 75 Fahr., in which it remained for 24 hours, and during 

 that time it was constantly submitted to electrical testa. The core, after having been 

 subjected to this and other tests to prove the perfect character of its insulation, was 

 then submitted to the process of being protected by its iron-wire armour. 



Jute, which had boon submitted to the action of catechu, in order to preserve it 

 from decay, was very carefully wrapped round the core, and as fast as the wrapping 

 proceeded, it was coiled in water. If the water at any time penetrated to the wire, it 

 would at once conduct away the electric current to the earth, and the loss of insulation 

 would be at once detected. This wrapping of juto is surrounded by 10 wires manu- 

 factured from homogeneous iron ; ench separate wire bring itself, in the first instance, 

 covered with tarred Manilla yarn, by which the iron was protected, and the specific 

 gravity of the mass lessened. * 15y a very ingenious arrangement, which our space will 

 not allow us to describe, the 10 wires covered with the tarred Manilla hemp are spirally 

 wound round the core, and the deep-sea cable is completed. The section and side-view 

 of the electric cable will be fully understood by reference to^s. 819 and 821. As 



