280 BP.I.L SYSTEM TECHNICAL JOURNAL 



cable coils were niaiuifactured for the Hell System in 1925, and ap- 

 proximately 1()(),{)()0 exchange area cable coils. Recent estimates of 

 the loading coil requirements for the next five years indicate an annual 

 demand at a rate which would double the total number of loading 

 coils in service about 1930. 



As regards the field of application for cable loading in terms of cable 

 lengths, the entrance and intermediate cables represent the minimum 

 lengths; for instance, pieces as short as 500 feet when present in carrier 

 telephone systems may require loading. In the local exchange areas, 

 toll switching trunks as short as two miles may require loading. On 

 the other hand, as illustrating the longest circuit now entirely in cable, 

 a connection between Boston and Milwaukee^via New York, Pitts- 

 burgh, Cleveland, and Chicago — typifies the possibilities in the exist- 

 ing repeatered loaded cable plant. The over-all length of such a circuit 

 is approximately 1200 miles. There is no technical obstacle to the use 

 of repeatered loaded cables for distances several times as great; i.e., 

 in the present state of the art, this is primarily a question of economics 

 rather than of development. 



IX. Conclusion 



It will be appreciated from the foregoing account that the invention 

 of coil loading was the beginning of an era of intensive development 

 which has been marked by enormous advances in the design of tele- 

 phone transmission lines, and that there has been no slackening of 

 the inventional or development activity devoted to this subject. It is 

 significant that at present more engineers and physicists in the depart- 

 ments represented by the authors are engaged on loading development 

 problems than at any previous time. 



In this account of the progress of the loading art during the past 

 quarter century, the authors have endeavored to point out the relation 

 of the loading developments to other phases of telephone development 

 such as cables, repeaters, telegraph working, and carrier telephone 

 and telegraph systems. In the space that is available, it would be 

 impracticable to assign full credit to the many individuals who have 

 been engaged in the development work on loading and the related 

 problems. The final accomplishments should be regarded as the result 

 of well coordinated efforts along many lines. 



In conclusion, it may be of interest to note what the development 

 and use of loading has meant to the telephone using public from an 

 economic standpoint. Leaving out of consideration altogether loading 

 on long toll cables — where the interdependence of repeaters and load- 



