66 SUBMAEINE< CABLE LAYING AND EEPAIRING. 



forming a solid homogeneous and seamless covering, whereas 

 india-rubber has to be laid on in strips. Vulcanised rubber, 

 first brought out for electrical purposes by William Hooper, 

 Is more of the nature of gutta-percha — that is, in being close- 

 grained, homogeneous, and non-porous, and, like all india-rubber 

 compounds, its inductive capacity is less than that of gutta 

 percha in the proportion of 100 to 137. It has also a much 

 higher specific resistance than gutta-percha, and not so large a 

 variation with temperature. The resistance of vulcanised 

 rubber is, according to its purity, reduced from 30 to 60 times 

 with a rise of temperature from 0°C. to 38°C., whereas the 

 resistance of gutta-percha, with the same rise of temperature, is 

 reduced at least 70 times. It will be seen, therefore, that 

 vulcanised rubber has much to recommend it. 



The variation of resistance of gutta-percha with temperature 

 was found by Messrs. Bright and Clark to be expressed approxi- 

 mately by the formula 



r = 0-89% 



where E. is the resistance at any given temperature. The 

 number of degrees Centigrade rise of temperature is expressed 

 by t and the resistance at the higher temperature by r. 

 Further, the insulation resistance of gutta-percha is improved 

 under sea pressure, while that of india-rubber is decreased, and 

 that of vulcanised rubber very slightly affected. India-rubber 

 decreases in resistance by pressure at the rate of about 16 per 

 cent, per 1,000 fathomsdepth. At the low temperatures prevalent 

 in great depths both gutta-percha and india-rubber considerably 

 improve in resistance, and both are practically imperishable 

 under water at low temperatures, but in shallow water, where 

 the temperature is higher and the pressure not very great, the 

 insulating properties of india-rubber can be more relied upon. 



The probable physical nature of the changes produced by 

 pressure and temperature in the above insulating materials and 

 in the copper conductor was first advanced by Mr. Charles 

 Bright in the year 1888, in an interesting communication to 

 the Institution of Electrical Engineers {Journal, Vol. XVII., 

 No. 75, p. 679). 



Supplies of gutta-percha come from the forest tracts of the 

 Malay Peninsula, Borneo, Java and Sumatra, and are delivered 

 here in lumps, many in fantastic shapes, just as they leave the 



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