494 ANNUAL REPOKT SMITHSONIAN INSTITUTION^ 1915. 



LOADING. 



Although Oliver Heaviside had shown mathematically in the early 

 days of telephone development that material improvements in the 

 transmission efficiency of circuits would result from increasing their 

 self-inductance in a uniform fashion, it was not until the latter part 

 of the last century that any scheme was suggested for the practical 

 application of Heaviside's work. During the two or three years 

 immediately preceding 1900, Prof. M. I. Pupin, of Columbia Uni- 

 versity, and Mr. George A. Campbell, of the American Telephone & 

 Telegraph Co., working independently, showed that by the insertion 

 of suitable inductance coils at intervals regularly spaced over the 

 length of the line the effect of a distributed inductance could be simu- 

 lated to an}'' desired degree of precision. 



This invention of so-called " lumped " or " coil " loading came at a 

 most opportune time in the development of the telephone industry 

 in the United States. By 1900 telephone service requirements within 

 the larger urban areas and on the long-distance toll lines were taxing 

 the then known methods of transmission to the limit. In cities such 

 as New York the number of circuits required for interoffice trunk 

 purposes had become so gTeat that it was no longer feasible to carry 

 them on open-wire pole lines in the streets. The only known alter- 

 native was the employment of heavy-gauge conductor cables, whose 

 use would entail vast expenditures for copper and conduit space and 

 which even then would provide an inferior grade of transmission. 

 For this service the benefits to be derived from loading offered a most 

 welcome relief by insuring the possibility of obtaining the necessary 

 grade of transmission through cables with small-gauge wires. 



The problem in the long-distance toll line field was somewhat dif- 

 ferent. Here it was not so much a question of securing more circuits 

 but of extending the range of transmission or bettering the service 

 over existing circuits. With the large size of copper conductors then 

 in use on the longer lines it was clear that no practical transmission 

 benefits would be derived from a further increase in the weight of 

 metal, while such an increase would necessitate a large addition to 

 the item of line costs. It was seen at once that if coil loading could 

 be applied to open-wire lines the effective range of the circuits could 

 be very greatly increased. 



Under the economic spur of these two requirements, the engineers 

 of the Bell system attacked vigorouslj^ the problem of producing in- 

 ductance coils which would fulfill the requirements set forth by 

 Pupin and Campbell. Although the mathematical solutions had been 

 obtained and their accuracy demonstrated in the laboratory, the prac- 

 tical problem of physical application to existing telephone circuits 

 had yet to be completed. 



