PBIKCIPLES OF DESIGN. 43 



out, but an allowance must be made if the conditions are not 

 completely known. 



To determine the dimensions of a core to comply with cer- 

 tain speed conditions, the KR of the cable must be specified or 

 such data given as will permit of this factor being derived. 

 For instance, if the speed only is specified, the speed constant 

 on a cable of similar length must be known ; or, failing that, 

 the speed constant must be derived from the working on other 

 cables of the same system. If, for example, a speed of 200 

 letters per minute is specified, and the speed constant is given 

 as 750, the KE, required will be by formula (4) 



1^0^^ = 3-75 millions. 



200 



But the KE alone is not sufficient data from which to 

 determine the dimensions of the core, the reason being that 

 cores of widely different dimensions can be constructed to have 

 the same KR. For instance, a core in which the weight of 

 copper is greater than the weight of gucta-percha can be con- 

 structed to have the same KR for the same length as one 

 having a greater weight of gutta-percha than of copper ; but 

 the insulation resistance will be lower in the former case. It is 

 not possible to carry the increase in weight of conductor and 

 decrease in weight of insulator beyond a limit determinable 

 from the conditions on account of the necessity of keeping up 

 the mechanical strength of the insulator, and for manufacturing 

 reasons. In long cables for high speeds this principle can be 

 carried further than in short cable cores because a mechanically 

 Bafe thickness of insulator can be maintained while the ratio of 

 weight of copper to weight of insulator is considerably increased, 

 and it is on these lines that most of the cores for modern high 

 speed long cables have been designed. 



In reducing the weight of gutta-percha in a core of given hr 

 per naut, the cost does not, of course, fall in the same propor- 

 tion, because the weight of copper must at the same time be 

 increased ; but gutta-percha being the more expensive con- 

 stituent, the total cost is reduced. As the thickness of dielectric 

 and insulation resistance is thereby reduced, it is important to 

 fix a safe limit to one or other of these conditions. On the 

 other hand, the cost must not be unnecessarily raised by 

 •using an excessive weight of gutta-percha with a conductor of 



