PRINCIPLES OF DESIGN. 



59 



ness of dielectric with not too light a conductor, is the one 

 which will have the longest life. The question of durability 

 is kept in view when considering cost, all other conditions 

 being complied with. 



In the core worked out above the weights came out — copper 

 5481b. and gutta-percha 3521b. per naut. The minimum thick- 

 ness of insulation by curve in Fig. 31 for this cable is seen to 

 be 77 mils (for a weight ratio of 1'35). This core is shown in 

 full-size section at C (Fig. 32). The sections A, B and D in the 

 same figure represent full-size dimensions of cores having the 



1-35 

 "Weight Ratios 0-7 0-8 0-9 10 M 1-2 1-3 ; 1-4 1-5 l-B 



Lbs Copper _ 385 463 548 607 



Lbs G.P. ' 550 463 352 380 



Fig. 32. — Various Core Dimensions for kr per Naut = 0"87, 



same Tcr as C, but with different weight ratios. All these and 

 the cores intermediate between them would be of equal Tcr. 

 These sections to scale show the different proportions in which 

 a core can be constructed to fulfil specified speed conditions. 



Core D is for a weight ratio of 1*6 having a dielectric of 

 71 mils thickness (by curve E) and 320 megohms insulation 

 per naut (by curve B) at standard temperature and pressure, or 

 5,850 megohms per naut of the laid cable. The diameter of 

 conductor is 206 mils. The weights are — copper 6071b., 

 gutta-percha 8801b. per naut. The insulating covering is too 

 thin in this core and is mechanically weak. 



