MAGNETIC MATERIALS IN TELEPHONE SYSTEM 



457 



coils, used as network elements, are generally equipped with com- 

 pressed powder cores, and the same improvements are indicated as 

 for loading coils noted above. Laminated permalloy cores are often 

 used for low-frequency applications (ringing, telegraph), where high 

 inductance is desired, and a-c. losses are naturally low. 



1.0 

 0.9 

 0.8 



0.6 



0.35 



0.30 

 0.28 

 0.26 

 0.24 

 0.22 

 0.20 

 0.18 

 0.16 



0.14 



0.8 



1.0 1.2 1.4 1.6 2.0 2.5 3 



e 



7 8 9 10 



Fig. 7 — Effect of eddy current shielding on apparent permeability and on eddy 

 current resistance/reactance ratio of sheet core material. 



Coils designed for direct currents must have cores with high a-c. 

 permeability in the presence of superposed field, i.e. they must be 

 made of high permeability magnetic materials having high saturation 

 values. Reference to Fig. 5 shows permendur outstanding in this 

 regard. In practice, the low costs of silicon iron or magnetic iron are 



