150 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



of telephone repeater did not become available for general use until 1915, 

 about 15 years after the invention of loading. 



The commercial limit in the economical use of loading to extend the 

 transmission range of open-wire lines was reached in 1911, several years 

 before the vacuum-tube repeater became available. Then, repeaters and 

 loading had to be used as teammates to conquer the transcontinental 

 distances. Subsequent improvements in repeaters, auxiliary equipment and 

 circuits eventually made it advantageous to discontinue open-wire loading, 

 and simultaneously opened an important new field for impedance-matching 

 loading on the entrance and intermediate cables that unavoidably occur in 

 open-wire lines. When different types of carrier systems became available 

 for open-wire facilities, several new types of impedance-matching loading 

 suitable for these carrier systems were developed. 



The early efforts to extend the transmission range of long-distance cables 

 in competition with open-wire lines, so as to obtain increased stability of 

 service and lower facility costs, reached a climax during the period 1911- 

 1915 in the use of composite, quadded, 10 ga. and 13 ga. cables, and of load- 

 ing coils nearly as large as the open-wire loading coils. This trend slowed 

 down shortly after vacuum-tube repeaters became commercially available. 

 During the next fifteen years or so, intensive development work on improved 

 repeaters, on equalizing and regulating networks, and on higher velocity, 

 higher cut-off loading, made it feasible to use 19-gauge conductors and 

 loading coils no larger than the initial standard cable coils for distances 

 ranging up to about 1500 miles. 



In the exchange area cable plant, coil loading has made it possible at a 

 low cost to meet the needs imposed by geographical factors, with as yet 

 very little competition by telephone repeaters. Large reductions in the 

 costs of the trunk plant have resulted from the extensive utilization of 22- 

 and 24-gauge cables, made feasible by the use of inexpensive loading. The 

 substantially continuous transmission developments in exchange area serv- 

 ices also made possible important improvements in the intelligibility of 

 transmission by using higher cut-off loading to transmit wider speech- 

 frequency bands. 



The important loading apparatus developments in the period covered 

 by the review have taken full advantage of the development at fairly even- 

 spaced intervals of a series of successively better magnetic core-materials 

 to improve the transmission service performance or reduce loading costs, 

 sometimes combining these features. The loading coil cost-reductions which 

 resulted from the large size-reductions made possible by the standardization 

 of compressed permalloy-powder core loading coils during the late 1920's 

 were especially important in influencing the growth of the long distance and 



