The Evolution of Inductive Loading for Bell System 

 Telephone Facilities 



By THOMAS SHAW 



(Continued from April 1951 issue) 



PART IV: CABLE LOADING COIL CASES 

 General 



Up to this point this review of coil loading has been primarily in terms of 

 transmission features of the loading systems and the coils, and development 

 economics, and for this reason the references to potting developments have 

 been brief, so as to minimize diversions from the main theme. A complete 

 chronological review of all important aspects of the potting development 

 work would require much more space than is available for this present ar- 

 ticle. On the other hand, because of the substantial importance of the pot- 

 ting developments in the economics of loading, more should be said than 

 was included in the brief references in Parts II and III of the story. 



This particular part of the review accordingly describes the more im- 

 portant high spots of the potting developments. It is limited to cable load- 

 ing coil cases because of the early obsolescence of open-wire loading. The 

 discussion is in terms of the changes from time to time in the various impor- 

 tant design features, as indicated by the side headings and paragraph head- 

 ings, and is thus a departure from the individual project-description pro- 

 cedure followed in other parts of the review. 



At this point it should be emphasized that the work on the cases, which 

 has been more nearly continuous than that on the coils, has kept pace in 

 design ingenuity with the work on the coils, and has been very much more 

 than the mere accommodation of the case designs to the changing sizes of 

 the loading coils and of the loading complements.^*^ 



Casing Materials 



Until the late 1920's, cast iron casings were used for housing the coils. 

 The moisture-proof seal between the case top and the main casing was ob- 

 tained by "tongue and gutter" details, supplemented by metal gaskets. 

 For the first decade or so, short lengths of wrought iron pipe with "pipe 



(*) Additional information on potting developments is given in Bibliography items (6), 

 (8), (26) and (30). 



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