166 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



tions, (3) reduced delay-distortion because of the more uniform velocity- 

 frequency characteristics at the upper speech frequencies, (4) complete 

 elimination of telegraph-flutter impairments, and (5) better quality of 

 speech transmission by the effective transmission of a much wider frequency- 

 band. In this latter connection, it is of interest that the transmission band 

 which was effectively transmitted over the loaded, repeatered, transcon- 

 tinental circuits ranged from about 350 to 1250 cycles, defining the band 

 as that between the lowest and highest frequencies whose transmission was 

 not more than 10 db higher than that of the 1000-cycle transmission. At 

 the higher frequencies, the line losses and the loading coil losses piled up so 

 as to effectively suppress transmission. The excess losses at the low voice- 

 frequencies were due to the line terminal apparatus and the repeater auxiliary 

 apparatus. 



The rapidly growing appreciation of these advantages led initially to a 

 curtailment in the installation of new loading on 165-mil circuits, and 

 subsequently to the removal of the existing loading and the installation of 

 additional repeaters. ^^^ By this time, the vacuum-tube repeater had been 

 accepted in its own right as an independent instrumentality for improving 

 transmission. 



On 104-mil lines, the economic competition between loading and repeaters 

 was much closer than that on 165-mil line, and for a period of several years 

 the aggregate mileage of loaded 104-mil lines increased substantially while 

 the mileage of loaded 165-mil lines decreased at an accelerating rate. In 

 this connection the transmission disadvantages of loading on repeatered 

 104-mil lines were not so serious as those on repeatered 165-mil lines, partly 

 because of the much shorter lengths involved and partly because of their 

 more stable transmission performance under varying weather conditions. 



During the early 1920's, the commercial exploitation of open- wire carrier 

 telephone and carrier telegraph systems became an increasingly important 

 factor in the removal of loading from open-wire lines. 



About 1924, the practice of installing new loading on 104-mil lines was 

 stopped in order to increase the plant flexibility for the more extensive use 

 of repeaters and of carrier systems, and accordingly the production of new 

 open-wire loading coils was discontinued. The removal of the existing load- 

 ing, however, was not completed until about 1934. 



(5) High Stability Type Coils for Coarse-Gauge Toll Cables 



The use of improved telephone repeaters started on a small-scale basis 

 on loaded coarse-gauge circuits along the Boston-Washington route even 



<•>) The unloading of the original transcontinental circuits was completed early in 1920. 

 The net loss was reduced from about 20 db to 1 1 db, and the width of the effective trans- 

 mission band was doubled. 



