60 SUBMABINE CABLE LAYING AND EEPAIKING. 



Core A is for a weight ratio of 0*7 having a dielectric of 

 106 mils thickness (by curve E) and 506 megohms insulation 

 resistance per naut (by curve B) at standard temperature and 

 pressure, or 9,250 megohms per naut of the laid cable. The 

 diameter of conductor is 163 mils. The weights are — copper 

 3851b., gutta-percha 5501b. Core A is unnecessarily costly, 

 on account of the excessive amount of insulation material. 



Core B has a weight ratio of unity, the weights of copper 

 and gutta-percha being each 4631b. per naut. The curves 

 show that the thickness of dielectric is 89*5 mils, insulation 

 at standard temperature and pressure 420 megohms per naut, 

 or 7,660 megohms laid, and diameter of conductor 179 mils. 

 This core is satisfactory, but there is still more gutta-percha 

 than necessary for mechanical strength. Core C has the best 

 proportions of conductor and insulator. 



The best proportions of copper and gutta-percha, that is, 

 having a sufficient thickness of insulator without unnecessarily 

 raising the cost, varies with the size of cable and the speed 

 required. For instance in small cores the weight ratio is 

 generally below unity and descends to 0*7. In large cores it is 

 practicable to make this ratio greater than unity, and it rises 

 to 1 "6 or more in long cables of low Tcr for high speeds. 



Referring to the foregoing curves it should be remembered 

 that they apply only for the particular composition of gutta 

 for which the constants are given. 



In Fig. 33 curves have been plotted for different sizes and 

 weights of conductor, showing how these vary for different ^r's 

 and different weight ratios. These curves also show the fact 

 mentioned above, that cores having several different weights of 

 conductor at correspondingly different weight ratios can be 

 constructed to satisfy a given kr condition. The curves show 

 the usual limitations for core dimensions. The upper curves, 

 with small cores as used in short cables, are shown produced to 

 lower weight ratios, and the lower curves, representing large 

 cores as used on long cables, to higher ratios in accordance with 

 well-established practice. For further assistance in calculations 

 on cores the notes written by Mr. Arthur Dearlove, entitled 

 "Tables to find the Working Speed of Cables with various 

 Cores, and other Data" (Spon), are of exceptional value, 

 the particulars and constants given being derived from actual 



