00 SUBBIAEINE CABLE LAYING AND REPAIRING. 



That is, the resistance of a dielectric of gutta-percha in which 

 the outer and inner diameters are as 10 : 1 is equal to 1,400 

 megohms per naut. 



It is sometimes required to calculate the resistance per 

 naut-cube from that per naut (/•) as measured on the cable. 

 From the above it is clear that 



T 



Resistance per naut cube = . megohms. 



0-366 xlog^ 



Returning now to the cable, it is convenient to express the 

 ratio of diameters in terms of the relative weights of copper 

 and gutta-percha. 



The weight of a seven-strand copper conductor is 



— lb. per nautical mile (7) 



The weight of compounded gutta-percha is, approximately, 



J)2 _ ^2 



lb. per nautical mile, ... (8) 



207 ^ ' 



in each case D and d being in mils (thousandths of an 

 Inch). 



Combining these we have 



-°-y^ + l (9) 



where W is the weight of gutta-percha and w of copper. It ia 



convenient to express the ratio of weight of copper to weight 



of gutta-percha by one letter n, thus 



w 

 n= — 

 W 



Putting this in formula (9) we obtain 



5=\/-+i (10) 



d y n 



Curve A (Fig. 28) is plotted from this formula, and shows 

 the relation between the ratios of outer and inner diameters 

 and the ratios of weights of conductor and insulator within the 

 limits generally in use. 



Substituting this value for the ratio of diameters in formula 

 (6), we have the insulation resistance at standard conditions in 



