328 Transactions. — Miscellaneous. 



The insulator of a telegraph cable, whether rubber or 

 guttapercha, must be carefully purified before use, as an ad- 

 mixture of impurities would seriously interfere with its insu- 

 lating properties. In the case of guttapercha, indeed, that 

 substance after purification is sometimes purposely mixed 

 with other substances whereby its insulating-power is lessened, 

 but which enables signalling to be carried on more quickly. 

 After rubber has been purified it is stretched into long tapes. 

 Great care is needed here, as the manipulation of it renders it 

 highly electrical, and light bodies, such as feathers, are readily 

 attracted by it. If some substance adheres which would 

 become charred on subsequent heating, the insulation would 

 be destroyed, as charcoal is a good conductor of electricity. 



If rubber could be used as an insulator it would have a 

 great advantage over guttapercha, in that for a cable of the 

 same dimensions it \vould allow of quicker signalling, or, in 

 other words, a cable as effective could be made with a smaller 

 amount of material. 



Eubber is sometimes vulcanized — that is, mixed with 

 sulphur and the two substances melted together. This is 

 the common grey-coloured rubber. The mixture of these 

 two insulating bodies forms another valuable insulator. Pure 

 indiarubber is not suitable for cables, as when immersed for 

 any long period the sea-water softens it and destroys the 

 insulation. Vulcanized rubber resists the action of sea- 

 water, but the sulphur attacks the copper, forming copper- 

 sulphide. A consideration of these facts produced what w^as 

 known as Hooper's core, a copper-insulated rubber, which at 

 one time threatened to rival the guttapercha-covered wire. 

 In this the conductor was covered with pure rubber, w^iile 

 the outside of the core, which was exposed to the action of 

 salt-water, was composed of vulcanized rubber. Between 

 these two layers was a third, the composition of which was 

 a trade secret. It was claimed that this layer prevented the 

 sulphur of the outer layer from penetrating to the copper. 

 I cannot learn that any rubber cables have been made for 

 very many years. Previous to that time Hooper's core was 

 used for cables in the Persian Gulf, on the coast of China, for 

 a length of two or three thousand miles on the coast of Brazil, 

 and for torpedo purposes in the neighbourhood of the fortifi- 

 cations of Portsmouth. 



3. The Jute Padding. — This need not detain us. Its use 

 is simply to prevent the outside sheathing from injuring 

 the core. 



4. The Sheathing of Iron Wires. — This consists of a 

 number of iron or steel wires bound round with hemp, 

 which are twisted spirally round the core. These wires are 

 simply for the protection of the cable, and are not used in 



