DEVELOPMENTS IN COMMUNICATION MATERIALS 



251 



in first cost resulted in appreciable savings in annual cost of the device, 

 considered from the operating companies' standpoint. 



Ferro- Magnetic Metals 



Up to about 15 years ago, telephone engineers used the magnetic 

 materials in their designs which had been originally developed for the 

 power industry, viz., magnetic iron and silicon steel. An exception was 

 the use of 4. mil hard drawn steel wire for loading coil cores where 

 extremely low permeability was desired. 



The increasingly severe requirements imposed by compositing and 

 phantoming of telephone circuits and the introduction of vacuum tube 



Fig. 14 — Loading coils showing core rings of liighly compressed powdered iron. 



repeaters, made necessary the development of materials which would 

 more adequately meet the new requirements. It was in 1915 that the 

 Western Electric Company first produced compressed powdered 

 electrolytic iron cores for loading coils. The construction of such 

 powdered iron core coils is illustrated by Fig. 14. Electrolytically 

 deposited iron is ground to a fine powder; the particles are covered with 

 an insulating film and then compressed at a pressure of 200,000 lbs. 

 per square inch to form rings as shown in the figure. This material 

 was sensational in the improvements which it afforded over the core 

 materials theretofore available as it combined with extremely high 

 resistivity, high stability of A.C. permeability under conditions of 

 powerful superposed or residual D.C. magnetization. The change in 



