1234 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



The 3000-ft. spacing has been used much less extensively on toll cable 

 circuits than in the exchange plant, on which basis the weighted average 

 coil-spacing for quadded toll cable loading is somewhat longer than the 

 weighted average value for exchange area loading. Within the accuracy- 

 required for the present general estimates, 5500 ft. seems to be a reasonable 

 estimate for the average coil spacing in quadded toll cable loading. On 

 this basis, and assuming a production total of about 9,500,000 side circuit 

 and phantom coils, the aggregate loaded toll cable circuit-mileage is of the 

 order of 9,900,000 miles. Keeping in mind the substantially universal use of 

 quadded cables and of phantom group loading for long-distance and inter- 

 urban toll cables, the aggregate mileage of loaded toll cable quads is of 

 the order of 3,300,000 miles. Because of the extensive installation of loaded 

 H 44-25 four-wire repeatered circuits during the period 1925-1931, the 

 loaded "facility** mileage-aggregate is considerably less than the loaded 

 "circuit" mileage-figure above given. Meanwhile, much of the loaded H 44- 

 25 4-wire circuit mileage has been converted for short haul two-wire circuit 

 usage, and much has been unloaded to permit the operation of Type K 

 carrier systems. The available data on these plant changes do not permit 

 accurate estimates regarding the mileage of loaded four-wire and two-wire 

 types of toll cable circuits now in commercial use. It is again appropriate, 

 however, to call attention to the important part in the growth and im- 

 provement of the telephone service which the displaced loading coils played 

 m their own period of commercial use. 



Economic Significance 



Since loading has been used only when it permitted the use of cheaper 

 facilities than would otherwise have been feasible, the great economic value 

 of loading in the growth of the Bell Telephone System is indicated by the 

 circuit mileage-figures given above. Other factors, however, would have to 

 receive consideration in a complete appraisal, namely, the contributions of 

 loading to nation-wide customer satisfaction that have resulted from im- 

 proved transmission performance and higher speed of service. In turn, these 

 factors themselves have been greatly influenced by the unit plant-cost 

 reductions made possible by the use of loading. 



For example, if loading had not been available when new or additional 

 facilities became desirable, it is highly questionable as to whether it would 

 have been economically feasible to work to the high-grade transmission- 

 performance standards that have been readily achieved at reasonable costs 

 with the cheaper loaded facilities. Moreover, it is even more questionable 

 whether it would have been economically feasible to provide as many 

 facilities without loading as were actually installed on a loaded basis. 



