388 BELL SYSTEM TECHNICAL JOURNAL 



cables ^ have been fully discussed elsewhere and only a brief summary 

 will be given here for the purpose of indicating the importance of the 

 measurements that will be described. On account of the fact that 

 for a given value of sending voltage the amplitude of the signals 

 received over a submarine cable diminishes rapidly as the speed of 

 signalling is increased, there is a practical limit to the speed of operation 

 of any cable. This limit depends on the electrical characteristics of 

 the cable and the magnitude of extraneous interference encountered 

 at the receiving terminal. The criterion for legibility of signals is, 

 in general, that the attenuation constant of the cable at a value of 

 frequency which may be termed the critical frequency shall not 

 exceed a given value, the attenuation constant as being defined by 

 the relation 



\Vs\ ' ^^ 



where | Vr \ is the amplitude of voltage arriving at one end of the 

 cable when a sinusoidal voltage of amplitude \Vs\ is impressed at 

 the other terminal. The value of this critical frequency depends 

 mainly upon the method of operation, and it usually lies somewhere 

 between the signal frequency and one and one half times the signal 

 frequency. 



Given the values of the four fundamental parameters of the cable, 

 resistance {R), inductance (L), capacity (C) and leakance (G), the 

 attenuation constant at the frequency pjii: can be computed by means 

 of the formula 



2^2 = ^(7^2 + piu^){Gi + p^O) + RG - p'LC, (2) 



which to a close approximation reduces to the form 



a = -iirfCR (3) 



in the case of a non-loaded cable, where R is large compared with 

 lirfL, and to the form 



in the case of the loaded cable, where R is small compared with 27r/L 

 at the critical frequency. In all cases it is assumed that G is very 

 small compared with lirfC, which is strictly true for the insulating 

 materials employed on submarine cables. 



The manner in which the attenuation constant varies with frequency 



