240 BELL SYSTEM TECHNICAL JOURNAL 



New York-Bay Roberts- Penzance cables, on both ends of the Horta- 

 Emden cable, and it has also been used in the loaded cables of the 

 Pacific Cable Board. 



With the maximum sending voltage determined and with the 

 received voltage necessary to work through interference known, the 

 cable can be designed to give the desired speed of operation. More 

 specifically it is necessary to provide that the attenuation for fre- 

 quencies essential to the formation of the signal shall be materially 

 less than the attenuation corresponding to the ratio of the sending 

 voltage to the interference at the receiving end. This condition 

 can be met by establishing the attenuation of the cable for one par- 

 ticular frequency related to the speed of signalling. The relation 

 between this frequency and the speed in letters per minute depends 

 of course on the code and method of operation used. In the case of 

 the New York-Horta cable the fundamental frequency of a series of 

 alternate dots and dashes of the cable code, that is, one half the center 

 hole frequency, was used as a basis for design. For this frequency a 

 voltage attenuation of e"^", corresponding to 87 TU, can be safely 

 assumed for recorder operation under conditions of interference such 

 as are encountered on the New York-Horta cable. With the Baudot 

 type of code and using the most improved apparatus, that is including 

 a synchronous vibrating relay, a voltage attenuation of e~^-^, corre- 

 sponding to 82 TU, may be assumed for the frequency resulting 

 from assigning 1.25 cycles to a character of the Baudot code. 



The computation of the attenuation of a loaded cable requires, 

 of course, only the substitution in the ordinary telegraph equation of 

 the specific values of inductance, capacity, resistance, leakance and 

 frequency which apply to the particular cable in question. The 

 method of calculation of these electrical quantities has been discussed 

 in previous papers and need not be repeated here. 



The design of the cable is thus reduced to proportioning the elements 

 of its construction so as to obtain the most economical cable of a 

 given attenuation at a given frequency. The thickness of insulating 

 material is, as has been noted above, determined practically by 

 mechanical considerations. The electrical characteristics of the insu- 

 lating material are effectively limited by the quality of gutta-percha, 

 account being taken of its dielectric leakance which is of considerable 

 effect on the behavior of the loaded cable though usually of almost 

 negligible effect on non-loaded cables. With the possibilities of 

 insulating materials thus limited the problem of electrical design 

 reduces practically to determining the size of the conductor and the 

 composition, size and shape of the loading material. 



