The Introduction of the Loading Coil: George A. Campbell and Michael I. Pupin

The introduction of loading coils into telephone circuits was probably the most important single technical innovation in telephony during the forty-year period between the time of Bell's original inventions and the introduction of electronic amplifiers. By 1900 longdistance telephony had reached what seemed to be a practical limit in a 1,200-mile circuit between Boston and Chicago. The loading innovation, in effect, doubled the practical distance and also led to substantial reductions in the construction costs of underground cable circuits used in urban areas. Exploitation of the loading coil saved the American Telephone and Telegraph Company an estimated one hundred million dollars during the first quarter of the 20th century.1 Despite its great economic importance and the fact that it has been frequently cited to illustrate the impact of science on technology, the early history of the loading coil has been subject to considerable misunderstanding. This is due not only to the somewhat esoteric nature of the innovation but also to a persistent myth regarding the role of Michael I. Pupin in its introduction. The coil-loaded telephone circuit may seem deceptively simple, since it consists of inductance coils connected in series with the