42 SUBMARINE CABLE LAYING AND REPAIRIKa. 



speed constants of each are correct indications of their relative 

 speeds. If, for instance, the speed constant of one cable was 

 600 and the other 660 (both having the same KR), their speeds 

 would be in the same proportion, one being 10 per cent, better 

 than the other. In a case like this it would show that the 

 instruments, battery and condensers were not so well adjusted 

 to the line in the case of the cable with 600 speed constant as 

 in that with 660, and it would be possible by improving these 

 conditions to attain 660. In all cables of the same KR it Is 

 possible by suitable apparatus and adjustment to attain the 

 highest speed constant that has been attained on any one of 

 them. But in cables of different KR the speed constants are, 

 of course, no guide to tjieir relative speeds. For the same 

 speed constant the speeds on different cables are In inverse 

 proportion to their respective KRs, but, as the speed constant 

 becomes rather greater for higher KRs and lower for lower 

 KRs, owing to the better adaptability of the receiving instru- 

 ment to take advantage of theoretical possibilities at high KRs 

 than at low KRs the speeds on different cables are not as 

 different as would be indicated by the inverse proportion of 

 their KRs. 



For instance, if in two cables of 3 millions and 7 millions 

 KR respectively the speed constants were assumed to be equal, 

 say 780, the speeds would be 



780^3 = 260 letters per min. 

 and 780-4-7 = 111 letters per min. 



This would be in inverse proportion to their respective KRs. 

 But in practice bhe speed constant would be somewhat lower at 

 the lower KR and somewhat higher at the higher KR, say, 720 

 and 840 respectively, giving speeds of 



720-^3 = 240 letters per min. 

 840^7 = 120 letters per min. 



showing less difference in the speeds than obtained by con- 

 sidering only the inverse ratio of the KRs. 



A cable core can be designed for a given speed when the speed 

 constant is specified. The conditions under which the cable 

 will be worked must be known, and the more completely they 

 are known the more closely can the required dimensions be got 



