MAGNETIC MATERIALS IN RELATION TO STRUCTURE 13 



reactions do not go to completion, and residual impurities remain. 

 Another obvious disadvantage is the fact that it would be only acci- 

 dental if oxygen and carbon were present in the iron in the right pro- 

 portions to eliminate one another. If the oxygen and carbon contents 

 of the raw materials are known, adjustments can theoretically be made 

 by adding either carbon or iron oxide, but if such a process is in com- 

 mercial operation, the writers are not aware of it. 



Yensen ^ reports a laboratory experiment in which this method of 

 purification was used. A number of samples were prepared in high 

 vacuum from electrolytic iron. To successive samples, increasing 

 amounts of carbon were added to a maximum of 0.5 per cent. The 

 maximum permeability increased with additions of carbon from ap- 

 proximately 25,000 for the sample with no addition to a maximum of 

 61,000 for a 0.06 per cent addition. From this value, the permeability 

 dropped rapidly with further additions of carbon. The maximum 

 value of 61,000 is explained as due to the fortuitous coincidence that the 

 correct amounts of carbon and oxygen were present for optimum 

 elimination. 



Vacuum melting and vacuum purification present some interesting 

 possibilities in connection with the preparation of magnetic materials. 

 Commercial vacuum melting has been developed in Germany at the 

 Heraeus plant, ^° where furnaces with capacities of 5 tons are in opera- 

 tion. It is stated that capacities of 20 to 30 tons can be achieved 

 without large departures from the present designs. Vacuum melted 

 metal is mechanically softer and works more readily than metal pre- 

 pared by ordinary melting procedures. For magnetic alloys, the com- 

 bination of vacuum melting and casting provides facilities for preparing 

 material without contamination by the atmosphere. There seems to 

 be no reason why one cannot go further and actually carry on refining 

 operations in the furnace. The future of vacuum melting and casting 

 appears extremely rich in possibilities, particularly for producing 

 magnetic alloys where high purity is a primary consideration. 



The third method of purification of iron is by treatment with an 

 element which reacts with the interstitial elements to form gases which 

 are removed. Hydrogen has been used for this purpose by Cioffi,^^ 

 and the method has been described recently. The treatment is carried 

 out at high temperatures between 1300° C. and the melting point of 

 iron. Ciofifi states that ordinary amounts of carbon, oxygen, nitrogen, 

 sulfur and phosphorus are reduced to very small quantities by hy- 

 drogen treatment. The excess hydrogen is either liberated as the metal 

 cools, or, if it remains in the metal, is without harmful effects on the 

 magnetic characteristics. A value of maximum permeability of 280,000 



