The Evolution of Inductive Loading for Bell System Telephone Facilities

General ONTINUOUS loading, i.e., the addition of uniformly distributed inductance, 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. Patent (575,275) describing a "bi-metallic" wire cable. Later on, when the commercial development was authorized, cost considerations 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 advantages 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 suspended for some time. During the next two decades, coil loading was found to be economically suited to all Bell System needs for inductive loading, even on short intermediate submarine cables required at shallow water crossings of rivers and bays.