installation, including a series of delicate electrical tests on the cable 

 and portions of the cable during the process of installation. 



With these developments, it became possible to use repeaters to 

 provide a large improvement in the transmission efticiency of telephone 

 circuits both in open wire and in cable, a number of repeaters where 

 necessary being used at different points on the same circuit. The 

 repeater element itself could be made so free from distortion that a 

 very large number of them could be used in succession on the same 

 circuits. The limitation in the use of repeaters, however, was deter- 

 mined largely by the degree of balance practicably obtainable through 

 the more uniform construction of the telephone lines. While it was 

 practicable to have such a degree of balance that a single repeater 

 would amplify the telephone current often six or sevenfold, it was not 

 generally practicable even with the use of a number of repeaters to 

 extend the length of circuit using a given size of conductor over eight 

 to tenfold. For transmission over very long distances or for the use of 

 very small conductors in long toll cables, another principle was 

 developed which was applied to toll cables and to the use of carrier 

 systems on open wire, and will be discussed in connection with those 

 subjects. 



Toll Cable System 



The success achieved in the general application of repeaters to 

 telephone toll circuits opened the way for a great extension in the use 

 of toll cables. Toll cables have the obvious advantage of providing a 

 high degree of security and continuity for telephone circuits due to the 

 fact that they are nearly immune from the effects of bad weather, 

 particularly of sleet and of high winds which sometimes seriously 

 interrupt open-wire telephone service by placing on the conductors and 

 on their supporting structures loads greater than those for which they 

 can economically be designed. Also, cables form the practical means 

 for providing the very large numbers of circuits required to take care 

 of the demand on very heavy routes by making it possible to crowd 

 into one route a much greater number of circuits than could be pro- 

 vided by the ordinary open-wire technique. 



Before the general use of repeaters became practicable, toll cables 

 had the inherent disadvantage that, with the small conductors neces- 

 sary to place a large number of circuits in one cable (until recently 

 maximum outside diameter 2^ inches), the cable circuits, even when 

 equipped with loading coils, had a very high attenuation loss per mile 

 compared with the open-wire circuits. Even when very large con- 

 ductors were used in the cable at the sacrifice of the number of circuits 



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