The Evolution of Inductive Loading for Bell System 

 Telephone Facilities 



By THOMAS SHAW 



{Concluded from July J 95 J issue) 



PART VI: CONTINUOUS LOADING 



General 



CONTINUOUS loading, i.e., the addition of uniformly distributed in- 

 ductance, was studied theoretically in the Bell System several years 

 before theoretical work started on coil loading. This early work of John 

 Stone Stone, then a member of the headquarters technical staff of the 

 American Bell Telephone Company, resulted in the issue to him on March 

 2, 1897 of a U.S. Patenl (575,275) describing a "bi-metallic" wire cable. 



Later on, when the commercial development was authorized, cost con- 

 siderations made it desirable to start with laboratory experiments on an 

 "electrically equivalent" artificial line using small lumped inductances. 

 In planning these experiments, it soon became apparent that only a small 

 amount of distributed inductance could be obtained with the best magnetic 

 material then available, namely, iron. Recognition of the important ad- 

 vantages inherent in the use of large amounts of inductance, and of the 

 absence of limitations regarding the magnitude of inductance that could 

 be provided in coil form, then shifted the development emphasis to the as 

 yet unsolved problem of spacing inductance coils in relation to wavelength. 

 This theoretical problem was quickly solved by G. A. Campbell, who was 

 then in charge of the project, and accordingly the laboratory artificial line 

 was designed to demonstrate the practicability of coil-loading (early in 1899). 

 The Bell System development work on continuous loading was then sus- 

 pended for some time. 



During the next two decades, coil loading was found to be economicaUy 

 suited to all Bell System needs for inductive loading, even on short inter- 

 mediate submarine cables required at shallow water crossings of rivers and 

 bays. Shortly after the First World War, however, it became necessary to 

 undertake the development of continuously loaded cable to meet an urgent 

 demand for telephone communication with Cuba. Exploratory theoretical 

 studies and laboratory investigations had been started shortly before the 

 war, but were discontinued during the war. The exploratory work included 

 consideration of the possible use of a new nickel-iron magnetic alloy which 



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