236 BELL SYSTEM TECHNICAL JOURNAL 



just west of Denver there are today one hundred and forty through 

 telephone circuits and about the same number of telegraph circuits 

 carried by four open-wire routes, the last of which was completed 

 during 1937. While open wire was used almost exclusively as a matter 

 of necessity in the first transcontinental telephone lines, cable is now 

 used for about half of the circuit mileage. This is a striking illustra- 

 tion of the large-scale changes which have taken place in the interest 

 of more reliable toll telephone service. 



Continued Importance of Open-Wire Lines 



The open-wire line seems destined to continue to play an important 

 part in long-distance telephone communication, particularly where 

 distances are great and circuit requirements on any one route are 

 relatively small. Improvements in the usage to which the wires may 

 be put have made this increasingly so. The three circuits on the 

 first transcontinental line were operated at voice frequencies and were 

 obtained from two pairs of line conductors, the third circuit being 

 derived by means of phantom circuit arrangement of these two pairs. 

 The development of carrier telephone systems made it possible to ob- 

 obtain three additional circuits on some pairs of wires, using frequencies 

 above those required for existing voice-frequency circuits. Carrier 

 telephone systems were first installed on a transcontinental route in 

 1926 and were quickly followed by others, so that today ninety-six of 

 the one hundred and forty circuits mentioned earlier are obtained by 

 means of these three-channel carrier telephone systems. Develop- 

 ment work, however, has been continued, and it is now expected that 

 it will be possible, by means of carrier telephone systems using still 

 higher frequencies, to obtain as many as twelve more telephone circuits 

 on some pairs of wires. It has been with a view toward using such 

 systems and obtaining a total of sixteen telephone circuits on a pair of 

 wires that the latest of the four transcontinental routes has been 

 designed. 



Construction of New Transcontinental Line 



Early in 1937, it became clear from a study of loads carried on 

 existing transcontinental routes that additional circuits would be 

 required in the near future. Circuits in cable were available as far 

 west as Omaha, Kansas City, Oklahoma City, and Dallas. After 

 consideration of all the factors, it was decided to construct the new 

 facilities west from Oklahoma City to Los Angeles on the route shown 

 in Fig. 1. It was also decided to carry out the work in such a way 

 that the route could be utilized for the future addition of a relatively 



