THE BELL SYSTEM 



TECHNICAL JOURNAL 



VOLUME XXXII MARCH 1953 number 2 



Copyright, 1952, American Telephone and Telegraph Company 



Ferrite Core Inductors 



By H. A. STONE, JR. 



(Manuscript received November 19, 1952) 



This paper describes the use of ferrite materials as cores for inductors 

 and develops methods for taking maximum advantage of their properties in 

 the design of inductors for communication circuits. 



INTRODUCTION 



The extent to which the theoretical capabihties of wave filters and 

 networks can be realized in practice usually depends on how high a Q, 

 ratio of reactance to resistance, can be obtained in the inductors. In 

 the voice and carrier telephone frequency ranges the dissipation in mica 

 capacitors is so small compared to that in inductors that it can generally 

 be neglected. Even paper capacitors, in the lower frequency ranges, 

 compare favorably in Q with the best available coils. Consequently, 

 there has been considerable incentive to develop improved magnetic 

 materials that will permit the realization of higher Q inductors for filter 

 and network use. 



Work along this line has resulted in the development of the permalloys, 

 and later the molybdenum permalloys which, powdered, insulated and 

 pressed into shapes, have become the standard materials for wave filter 

 coils in the voice and carrier frequency ranges.^' ^ 



Although the permalloys represent a vast improvement over the soft 

 iron that preceded them they share the fundamental disadvantage of 

 all metals, that they are good conductors. Any conductor in the vicinity 

 of an alternating magnetic field has eddy currents induced in it, and, if 

 the conductor is the core of a coil, the power loss associated with these 

 currents appears as added resistance in the coil windings. To restrict 

 the eddy current paths it is customary to powder the material and in- 

 sulate the particles with some kind of inorganic filler. However, there 



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