PRINCIPLES OF DESIGN. 



51 



terms of the ratio of weights of conductor and insulator as 

 equal to 



1,400 logy/- + 1 megohms per naut. . (11) 



Curve B (Fig. 29) is plotted from this formula, and shows 

 the insulation resistance per naut for any ratio (n) of weight 

 of copper to gutta-percha between 0-6 and 1-6. 



From this formula and curve it will be seen that, if the 

 insulation resistance is not to be less than a certain fixed value, 



. 2-5 



2-3 



§2-1 

 fe 2-0 



1'9 



1-8 



1-7 



1-6 



0-6 0-7 0'8 0-9 1-0 1-1 1-2 1-3 1-4 



Ratios of Weights of Copper to Gutta-Percha. 



1-5 1-6 



Fig. 28. 



-Curve A, showing Relation between Dimensions and Weights 

 of Conductor and Insulator. 



the weights of copper and gutta-percha must not exceed a 

 definite ratio. As shown above, the insulation of the cable 

 under consideration is not to be less than 356 megohms per naut 

 at standard conditions, and by curve B it is seen that the ratio of 

 weight of copper to gutta-percha must not exceed 1*35. That 

 is, the ratio may be lower than this if the financial estimates 

 allow for a more expensive cable of longer life, but must not 

 exceed this figure in the direction of a cheaper cable. The 



E 2 



