EVOLUTION OF INDUCTIVE LOADING 1239 



Toll Cable Loading 



The pattern of the commercial evolution of loading practices for long- 

 distance cable systems has been generally similar to that for open-wire 

 loading, but with important quantitative and qualitative differences, and 

 especially in the relative time-elements. These various differences have 

 been mainly due to the previously mentioned inherent differences in the 

 basic transmission properties of non-loaded cables and non-loaded open-wire 

 lines. 



Prior to the availability of vacuum-tube repeaters, loading was an essential 

 factor in the establishment of a very important expanding network of 

 storm-proof, intercity, toll cables; coarse-gauge conductors and expensive 

 coils were used for distances ranging up to about 250 miles, 16 ga. con- 

 ductors and less expensive coils being satisfactory for terminal business 

 over short distances. Without using loading, these early toll cable systems 

 would not have been economically feasible. 



In the early uses of repeaters on toll cables the cable circuits also used 

 loading. These combinations permitted improved transmission performance 

 and important extensions in transmission range. In this general connection, 

 it is of interest to note that it was not economical to use non-loaded con- 

 ductors for toll cable transmission until cable carrier telephone systems 

 became available about two decades after the commercial introduction of 

 the vacuum-tube repeater. For voice-frequency transmission, the use of 

 repeaters without loading would have been unduly expensive, due to the 

 high costs of the additional repeaters and the much more expensive distortion- 

 correcting networks and regulating networks that would have been required. 



In the early part of the period that intervened between the introduction 

 of vacuum-tube repeaters and of cable carrier systems, the substantially 

 continuous development of improved loading, and of improved repeaters 

 and auxiliary equalizing and regulating networks, provided improved fa- 

 cilities of several different types especially proportioned on a minimum 

 cost basis to meet the transmission-service needs of different geographical 

 distances. 



High- velocity, four-wire, H 44-25 19 gauge circuits were very extensively 

 used for long-haul facilities ranging up to about 2000 miles in length. It is 

 of interest that the timely completion of the development of the first cable- 

 carrier system stopped the contemporary efforts to make additional im- 

 provements in the H 44-25 voice-frequency loaded four-wire circuits so that 

 they would be suitable for transcontinental distances. These improvements 

 would have involved the use of velocity distortion corrective networks. 



Nineteen gauge two-wire circuits having lower-velocity, higher-im- 

 pedance, loading than that employed on the above mentioned four-wire 



