394 BELL SYSTEM TECHNICAL JOURNAL 



(c) Better quality of speech transmission obtainable by the effective 

 transmission of a much wider frequency band. 



(d) A reduction in costs. 



Additional advantages of non-loaded lines for voice-frequency telephony 

 became apparent from subsequent studies and experiments:^ 



(1) The practicability of securing materially lower net losses, in con- 

 sequence of the effect of the higher velocity of transmission in reducing 

 disturbances caused by echo currents. 



(2) A reduction of delay distortion, resulting from the fact that the 

 velocity of transmission of the upper speech frequencies in the non- 

 loaded line is approximately constant with frequency, whereas in 

 the loaded line the velocity decreases substantially with rismg fre- 

 quency, especially near the cut-off. 



In September 1915, quantitative line tests were made to verify the 

 theoretical expectations per items (b) and (c), above. The tests involved a 

 comparison of the transmission over a non-loaded New York-Denver 

 circuit, via Pittsburgh and St. Louis using six intermediate repeaters, with 

 that over the New York-Denver section of the loaded transcontinental line 

 using three intermediate repeaters. The non-loaded circuit had a somewhat 

 lower net loss and noticeably better quality, and, of course, a complete free- 

 dom from transmission distortion by telegraph flutter. As a result of these 

 tests, plans for using loaded lines in certain parts of the proposed backbone 

 network were quickly modified to call for non-loaded lines and additional 

 repeaters. In some instances where the lines were already loaded with old 

 types of coils that were susceptible to magnetization by lightning surges, 

 the change in plans avoided the expense of installing new high-stability type 

 loading. 



Change in Engineering Attitude Towards Loading 



This decision to use non-loaded 165 mil lines instead of loaded 165 mil 

 lines for parts of the continental backbone network was of great significance 

 in that it marked the beginning of a new engineering attitude with respect 

 to the fields of use for loading and for repeaters. Up to that time, loading 

 had been accepted as the dependable and indispensable method of improving 

 transmission in long distance circuits and extending their range, and re- 

 peaters had been regarded primarily as auxiliary devices for stretching 

 the transmission benefits obtainable with loading. It will be recalled that, 

 prior to the transcontinental development project, the type of repeater 



5 The non-loaded lines also had important possibilities in the application of carrier 

 telephone systems, the commercial development of which got well started during the 

 1915-1920 period under consideration in Chapter IV. 



