

A/A' WILLIAM SIEMENS, fi.R.S. 73 



alerted by its thickness, the inductive power diminishing as the 

 thickness was increased. A thicker coating of the insulating 

 covering therefore offered less resistance by induction to the 

 passage of an electric current, and allowed of more rapid speaking. 

 In this respect also india-rubber and its compounds had an 

 advantage over gutta-percha, its inductive power being about 

 three quarters that of the latter. 



In the use of gutta-percha as the insulating material, a great 

 amount of care was necessary in the process of coating the wire, 

 and there was great risk of imperfection in the covering. In the 

 submarine telegraph between Rangoon and Singapore, the cable 

 was veiy good for many miles, but a point was then found to exist 

 where the insulation failed from a defect in the original construc- 

 tion of the gutta-percha coating ; and such defects were liable to 

 arise in the manufacture from various causes. In covering the 

 wire the gutta-percha was squeezed forwards in a semifluid state 

 through the die, by means of a piston in a cylinder ; and air 

 bubbles were liable to get enclosed within its substance, which 

 were so minute as not to be detected at the time of manufacture, 

 though the cable was tried under a pressure of GOO to 1000 Ibs. 

 per square inch ; but they were sufficient to impair the insulation 

 at the part where they occurred, and ultimately cause the failure 

 of the cable. Moreover, the manufacture was a hot process, as 

 the gutta-percha had to be kept soft in coating the wire ; and if a 

 slight delay took place in the operation, the gutta-percha was too 

 much softened at that part, and the weight of the wire cable itself 

 made the coating thinner on one side than the other, so that the 

 insulation was defective ; the electric current afterwards sent 

 through the wire was constantly leaking out more or less at the 

 imperfectly protected parts, and caused a chemical action on the 

 gutta-percha, gradually decomposing it at the leak and increasing 

 the amount of leakage. If the finished cable were allowed to lie 

 for only a quarter of an hour exposed to a hot sun, it would be 

 completely spoiled, as the heat would soften the covering and the 

 core would take an eccentric position by sinking through the 

 gutta-percha by its weight ; and in the event of a strain coming 

 on the cable in laying it, the copper core being non-elastic, would 

 remain permanently stretched, while the gutta-percha would be 

 constantly endeavouring to regain its original length, forcing the 



