PRINCIPLES OP DESIGN. 55 



of seven-strand copper conductor of high-conductivity copper 

 (Committee standard) at 60°F. 



Eeducing this to 40°F., as required for the cable under 

 consideration, we have 



r=- (14) 



w 



Multiplying this by (13) we obtain a useful expression for the 

 h' per naut 



7 120-4 ,, „, 



icr= (15) 



*^ log a/ -4-1 



The hr required in the cable under consideration is given 

 above as 0'865, and, therefore, the weight of copper per naut 

 required is 



^'''' . . . (16) 



V n 



From this formula curve D (Fig. 30) has been plotted, showing: 

 the weight of conductor per naut required for any given ratio 

 of weight of copper to gutta-percha between the limits of 0-& 

 and 1*6 for the Jcr per naut required in the cable under con- 

 sideration (0'865). In this case the maximum ratio of weights 

 permissible to satisfy the conditions of insulation was found 

 above by curve B to be 1*35, and curve D shows that the 

 weight ratio being fixed, the conductor must not exceed a 

 certain weight per naut. It will be seen by inspection of this 

 curve or calculation by formula (16) that the weight of conduc- 

 tor must not exceed 



120-4 



■■ = 5481b. per naut, 



0-865 log /j_+i 

 V 1-35 



and the maximum ratio of weights given above being 1 -35, the 

 weight of gutta-percha must not be less than 



——- = 3521b. per naut. 



The diameter of conductor (if of seven strands) corresponding to 

 this weight of copper would be, by formula (7), 



769x548 = 195 mils, 



