396 BELL SYSTEM TECHNICAL JOURNAL 



close accuracy. The principal factors were found to be the velocity of 

 transmission, the number and spacing of repeaters, the line attenuation, 

 and the reflections at significant points of irregularity; i.e., at the line ter- 

 minals, and at the repeaters. These studies and line tests clearly demon- 

 strated the importance of the transmission velocity in echo current phe- 

 nomena, and the limitations on transmission performance imposed by the 

 echo currents. 



To achieve transcontinental transmission on non-loaded 165 mil lines 

 became an objective in the repeater development work. Although this 

 development was started in good time, the pressure of war work interfered 

 so that not much progress occurred until late in 1918, when arrangements 

 were made for a trial of the improved repeaters on a non-loaded circuit 

 between New York and Chicago. The success of this tryout in 1919 led 

 to arrangements being made for unloading the transcontinental circuits 

 west of Chicago. At the new repeater points west of Chicago entirely new 

 repeaters were installed; those used at other points and in the new non- 

 loaded Chicago-New York circuits were modified to have equivalent trans- 

 mission features, including 3000-cycle filters, which became a characteristic 

 feature of the 22-repeaters for non-loaded lines. Different sections of the 

 transcontinental line became available on a non-loaded basis at intervals 

 during the spring of 1920. 



The Un-Loaded Transcontinental Line 



The through circuits had a total of twelve intermediate repeaters. The 

 net loss was about 11 db, or 9 db below that of the loaded line, and the effec- 

 tive transmission band was twice as wide. The expected improvements in 

 stability under varying weather conditions were realized in full. The 

 better repeater balances that were obtainable with the inherently more 

 uniform non-loaded lines were factors in the greatly reduced net loss. 

 The factor of fundamental importance, however, was the approximately 

 3.5 to 1 increase in the velocity of transmission, which shortened the time 

 interval between the direct transmission and echoes from points of im- 

 pedance irregularity and thereby reduced the disturbing effects of the 

 echoes. When the unloaded transcontinental circuits were demonstrated 

 to a conference of Bell System presidents held at Yama Farm, N. Y., on 

 May 25, 1920, the ''sense of nearness" made possible by the high transmis- 

 sion speed was especially commented upon as a component of the improved 

 transmission quality. 



Following the unloading of the transcontinental line, other 165 mil 

 circuits on important routes were unloaded, and soon there was a complete 

 backbone of non-loaded 165 mil circuits operated with the improved re- 

 peaters. 



