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at different stages of purification. Table V shows that impurities of a few 

 thousandths of a per cent are quite effective in depressing the maximum 

 permeabiUty of iron. 



Carbon and nitrogen, present as impurities, are known to cause "aging" 

 in iron — that is, the permeability and coercive force of iron containing these 

 elements as impurities will change gradually with time when maintained 

 somewhat above room temperature. As an example, a specimen of iron was 

 maintained for 100 hours first at 100°C, then 150°C, then 100°C, and so on. 



Table V 



Maximum permeability of Arnico iron with different degrees of purification, effected by heat 



treatment in pure hydrogen at 147 5° C for the times indicated {P. P. Cioffi). 



Analyses from R. F. Mehl (private communication to P. P. Cioffi). 



The corresponding changes in coercive force are given in the diagram of 

 Fig. 28. A change of about 2-fold is observed. 



Some Important Physical Properties 



There are many physical characteristics that are important m the study 

 of ferromagnetism from both the practical and the theoretical point of view. 

 These include the resistivity, density, atomic diameter, specific heat, ex- 

 pansion, hardness, elastic limit, plasticity, toughness, mechanical damping, 

 specimen dimensions, and numerous others. In a different category may be 

 mentioned corrosion, homogeneity and porosity. Most of these properties 

 are best discussed in connection with specific materials or properties; only 

 the most important characteristics will be mentioned here. A table of the 

 atomic weights and numbers, densities, melting points, resistivities and 

 coefficients of thermal expansion of the metallic elements, is readily avail- 

 able in the Metals Handbook. 



Dissolving a small amount of one element in another increases the re- 

 sistivity of the latter. To show the relative effects of various elements, the 

 common binary alloys of iron and of nickel are shown in Figs. 29 and 30. 

 From a theoretical standpoint it is desirable to understand (1) the relatively 



