46 SUBMAEINE CABLE LAYING AND KEPAIEING. 



An example will now be worked out showing how the dimen- 

 sions of a core may be determined, having the quantities given 

 as specified below : — 



The core to be constructed for a working speed of 210 letters 

 per minute. 



Length, after allowing for depth and slack, 2,100 nauts. 

 Mean depth, 1,800 fathoms. 

 Mean bottom temperature, 40°F. 



Speed constant, 800 (from observations on a similar cable). 

 Firstly, having given that the minimum insulation resistance 

 after laying is 6,500 megohms per naut. 



By formula (4j, the KR of this cable when laid must not be 

 greater than 



80O X 10« 



210 



= 3"81 millions. 



In calculating the weights of copper and gutta-percha to 

 satisfy the conditions specified, it is most convenient to work 

 to the Tcr per naut, which in this case at sea temperature and 

 pressure by formula (2) must not exceed 



■3-81x^Q" = 0-865. 



(2,100f 



As the determining factor in this case is the insulation resis- 

 tance of the laid cable, we have to consider the variation of 

 resistance of gutta-percha with sea temperature and pressure. 



The low sea temperatures generally, especially in great 

 depths, and the pressure of sea water cause a remarkable 

 increase in the insulation resistance of laid cables as compared 

 with that at standard temperature and pressure. If means 

 are provided in the factory for subjecting cores as manu- 

 factured to the same pressures and temperatures as exist ia 

 the sea the pass tests can be made direct without calculation, 

 otherwise the tests will be made at standard temperature 

 (75°F.) and atmospheric pressure, in which case the insulation 

 resistance at these standard conditions must be calculated 

 from that specified on the laid cable. It follows that the 

 temperature and pressure coefficients for the particular com- 

 position of dielectric proposed to be used must be known, and 

 these Constanta are determined with great oare by faoftory ^Qit^ 



