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THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1951 



cap" ends were used in potting the smallest complements of coils. For all 

 cases, lead-sheathed stub cables contained the coil terminal leads. 



During the late 1920's, new case designs using thick, copper-bearing, steel 

 plate (f inch thick) with welded seams were introduced for reasons of econ- 

 omy and to simplify manufacture. Because of the very great current demand 

 for loading, and the extensive use of the large-size loading coil cases re- 

 quired by the larger loading complements, the foundry problems had become 



Fig, 19 — Cast iron vs. welded steel loading coil cases. Complements of 84 



P-type phantom loading units. At left: Cast iron case; In center: welded 



steel aerial case; At right: welded steel underground case. 



very formidable; also, the production schedules had become somewhat ir- 

 regular because of the great difficulties encountered in getting enough satis- 

 factory castings of the largest sizes. The use of heavy machinery for shearing 

 the steel plates and forming them to obtain cases of rectangular cross-section 

 gave adequate dimensional design-flexibility. The welded steel designs, how- 

 ever, are not generally so satisfactory as the cast iron designs with respect 

 to resistance to corrosion of accidentally exposed steel surfaces. Accordingly 

 different types of protective coatings were provided for cases intended for 

 underground cable and buried cable installations, and those intended for 



