PBINCIPLES OF DESIGN. 57 



where 0'084:35 is the constant for the resistance of a 12-wir6 

 strand. This would mean that each wire of the strand should 

 be 50 mils, or a little larger than No. 18. 



The capacity has been found above to be 0*426 mfd. per 

 naut, and the copper resistance at 40°F. to be 2*03 ohms 

 per naut. Therefore we have the kr per naut = 0"426 x 2'03 

 = 0"865, which agrees with the specified value. 



The core dimensions of the proposed 2,100 naut cable to 

 satisfy the speed conditions, and calculated from the minimum 

 insulation resistance specified, have now been determined and 

 are : — 



Conductor: 5481b. copper per naut. Strand of 12 No. 18 

 {full) wires. Resistance at sea temperature 2*03 ohms per 

 naut and 4,263 ohms for the whole cable. 



Insulator: 3521b. gutta-percha per naut. Capacity,0"426mfds. 

 per naut and 895mfds. for the whole cable. 



KR of laid cable, 895 x 4,263 = 3-81 millions. 



Secondly, having given the maximum weight per naut of the 

 conductor, we should proceed to find, by curve D or formula 

 {16), the weight ratio for this weight of copper per naut. 

 Prom this ratio all other constants of the core can be easily 

 found, as shown above. 



Thirdly, having given the minimum thickness of dielectric, 

 the weight ratio could be found as follows : — 



The thickness t of insulation is 



t = — - — mils, 

 2 



or, expressed in terms of weights, 



«=^V69^ry(^ + l)-l1mils. . (17) 



From this formula and curve D, curve E (Fig. 31) is plotted, 

 and shows the variation of thickness of dielectric for diflferent 

 weight ratios for the hr per naut required in the cable under 

 consideration (0'865). 



The points determining this curve are found by taking certain 

 weight ratios {n) and finding, by reference to curve D, the 

 respective weights of conductor corresponding to them for the 

 kr required (0*865), and then working out the formula with 



