The Bell System Technical Journal 



April, 1926 



Development and Application of Loading for 

 Telephone Circuits ' 



By THOMAS SHAW and WILLIAM FONDILLER 



Synopsis: A review of the art of loading telephone circuits as practised 

 in the United States. The introductory section briefly reviews the theory 

 of coil loading, and summarizes the principal characteristics of the first 

 commercial standard loading coils and loading systems, thereby serving as 

 a background for the description of the various improvements of outstanding 

 importance which have been made in the loading coils and loading systems 

 during the past fifteen years to meet the new or changing requirements m 

 the rapidly advancing communication art. 



These major improvements are described in detail under the appropriate 

 headings (1) Phantom Group Loading, (2) Loading for Repeatered Circuits, 

 (3) Incidental Cables in Open Wire Lines, (4) Cross-talk, (5) Telegraphy 

 over Loaded Telephone Circuits, (6) Loading for Exchange Area Cables, and 

 (7) Submarine Cables. The discussion of these various developments sets 

 forth the relations between the loading features and the associated phases 

 of telephone development, such as the cables, repeaters, telegraph working, 

 and carrier telephone and telegraph systems. 



The concluding part of the paper gives some general statistics regarding 

 the extent of the commercial application of loading in the United States, 

 and a brief statement indicative of the large economic importance of loading 

 to the telephone using public. 



Introduction 



THE year 1926 marks the fiftieth anniversary of the birth of the 

 telephone, and the completion of the first 25 years of the com- 

 mercial application of loading to telephone circuits by means of in- 

 ductance coils inserted at periodic intervals. The present time is thus 

 peculiarly appropriate for a survey of loading developments. 



The purpose of this paper is to present a review of the art of loading 

 telephone circuits, as practised in the United States. In a paper^ pre- 

 sented before the Institute in 1911 Mr. B. Gherardi described the de- 

 velopments in loading up to that time and gave a comprehensive 

 statement of the results obtained. In the present paper, therefore, 

 references to the early developments in loading may be confined to 

 matters that are necessary to the treatment of the subsequent develop- 

 ments in the art. 



During the period under consideration many improvements of out- 

 standing importance have been made in the characteristics of the load- 



1 Presented at the Midwinter Convention of the A. I. E. E., New York, N. Y., 

 February 9, 1926. 



2 "Commercial Loading of Telephone Circuits in the Bell System," B. Gherardi, 

 Trans. A. L E. E., Vol. 30, 1911, p. 1743. 



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