THE FUTURE OF TRANSOCEANIC TELEPHONY 13 



types of tubes based on these principles were developed and put on life 

 tests more than five years ago. As yet they have shown no evidence of 

 deterioration, and one now may be reasonably sure from their behavior and 

 from physical considerations of a life of at least ten years. There is good 

 reason to think that they should last several times that long, but further 

 observation will be required before a life of as much as 20 years steady 

 operation can be confidently predicted. The tubes must also be more 

 rugged than ordinary vacuum tubes since the cable will be subjected to 

 considerable vibration and perhaps to heavy blows in the course of laying 

 and lifting, though the tubes can be protected to some degree by resilient 

 mountings. 



Other elements of the repeater structure such as coils and condensers 

 are also subject to special requirements both electrical and mechanical. 

 These requirements have been met in a preliminary way and the assembled 

 repeater in its housing subjected to mechanical tests in the laboratory. 



Although the electrical requirements of such a cable are very severe, 

 there are some respects in which the submarine telephone repeater is 

 simpler than a land-line repeater. The temperature at the bottom of the 

 ocean is nearly constant; consequently, the repeater does not have to be 

 regulated to compensate changes of cable characteristics with temperature. 

 Also, once the cable is laid, it is in a very quiet place, and except in shallow 

 water near shore is not likely to be disturbed. True, the electrical char- 

 acteristics of the cable may show effects of aging, but over a long period 

 of time changes are not great, and they can be allowed for by providing 

 some margin in the electrical design. 



In the circuit of the repeater the heating filaments of the amplifying tubes 

 are placed in series with the central cable conductor. The fall of potential 

 through the heater filaments provides the plate potential for the tubes. 

 Appropriate networks compensate for variation of cable attenuation with 

 frequency. A negative feedback circuit gives a high degree of stability 

 over a wide band of frequencies and minimizes the effect of variations of 

 tube characteristics. It is interesting to note that the amplification pro- 

 vided by a single tube could drop to a tenth its normal value with scarcely 

 appreciable effect on the performance of the repeater. 



The number and spacing of repeaters depends of course on the length and 

 design of the cable. For a cable 2000 miles long to connect Newfoundland 

 and Great Britain there was calculated a core comprising 516 pounds of 

 copper per mile insulated with 370 pounds of paragutta, surrounded by a 

 return conductor of 600 pounds. This is like the core of the 1930 Key 

 West-Havana telephone cable but somewhat smaller. Paragutta was 

 assumed as the insulating material because of extensive experience with it. 

 By using for the calculation the characteristics of one of the newer synthetic 



