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BELL SYSTEM TECHNICAL JOURNAL 



The temperature also is effective in changing permeability and other prop- 

 erties, even when no change in phase occurs. Figure 7 shows the rapidity 

 with which the initial permeability decreases as the Curie point is ap- 

 proached. For this material, Ferroxcube III, a zinc manganese ferrite 

 (ZnMnFe408), the Curie point is not far above room temperature. 



The effect of impurities may be illustrated by the B vs H curves for iron 

 containing various amounts of carbon. Curve (a) of Fig. 8 is for a mild 



5 



iiJ 1000 



20 40 60 80 100 120 



TEMPERATURE IN DEGREES CENTIGRADE 



Fig. 7 — Variation of initial permeability of Ferroxcube 3, showing maximum at tem- 

 perature just below the Curie temperature. 



lo^xie 



O 6 



I 23456789 10 11 12 



FIELD STRENGTH, H, IN OERSTEDS 



Fig. 8 — -Effect of impurities on magnetic properties of iron. Annealing at 1400''C in 

 hydrogen reduces the carbon content from about 0.02 per cent to less than 0.001 per cent. 



steel having 0.2 per cent carbon, (b) is for the iron commonly used in elec- 

 tromagnetic apparatus — it contains about 0.02 per cent carbon and is an- 

 nealed at about 9()0°C. When this same iron is purified by heating for several 

 hours at 1400°C in hydrogen, the carbon is reduced to less than 0.001 per 

 cent and other impurities are removed, and curve (c) is obtained. 



Finally, Fig. 9 shows that large differences m permeability may be found 

 by simply varying the direction of measurement of the magnetic properties 

 in a single specimen. The material is a single crystal of iron containmg about 

 4 per cent silicon, and the directions in which the properties are measured 



