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BELL SYSTEM TECHNICAL JOURNAL 



tributed air-gaps, affords a means for constructing magnetically 

 stable cores without the production of poles and their attendant 

 magnetic leakage. 



Fig. C gives photographs of a standard compressed iron powder 

 core ring such as is used in the cores of toll cable loading coils; a 



'» j»TO|B i^a8gi»^^^- 



Fig. 6 — Compressed powdered Iron core loading coil 



completely assembled core with part of the core taping removed; a 

 completely wound coil of the side circuit type; and a coil in cross- 

 section. Table IX gives general data regarding typical coils. 



The first application of powdered iron cores was to replace some 

 of the 95-permeability wire core loading coils in 16 and 19-A.w.g. 

 cable. The effective permeability of the 95-permeability wire cores, 

 making correction for air spaces and insulation, was approximately 

 60, and accordingly, the replacing powdered iron cores were designed 

 to have the same effective permeability. 



As a result of further developments in the direction of applying 

 vacuum tube repeaters to loaded cable circuits, it became necessary 

 with the extension of the length of these circuits to improve the 

 characteristics of the loading coils. This led to the development of 

 an improved grade of powdered iron core having an initial permeability 

 of 35 which corresponds closely to the effective permeability of cores 

 using iron wire having a permeability of 65. It was decided that 

 for circuits such as interoffice trunks and short cables which would 

 not be operated with superposed telegraph, the 60-permeability 

 compressed iron core coils should be used; while for toll cable work 



