PRESENT STATUS OF FERROMAGNETIC THEORY 



85 



higher temperature the one phase dissolves completely in the other to 

 form a solid solution. In making the material, it is quenched rapidly 

 from a high temperature and then reheated to 700 degrees centigrade, 

 at which point the second phase precipitates slowly in very finely 

 divided form. When the optimum amount has precipitated, the 

 material is cooled to room temperature, no more changes occurring. 

 Each submicroscopic precipitated particle is a center of strain, and it 

 is the presence of these unusually large internal strains that is respon- 

 sible for the good quality of the permanent magnet. 



350,000 



300,000 



250,000 



200,000 



M 



150,000 



100,000 



0.02 0.04 0.06 0.08 



0.10 

 H 



0.12 0.14 0.16 0.18 0.20 



Fig. 22 — Permeability curves of ordinary iron and of iron purified by heat treatment 

 in hydrogen at 1,500 degrees centigrade (Cioffi). 



Going now to the other extreme, where ease of magnetization is 

 required, it is known, of course, that thorough annealing and a homo- 

 geneous structure are beneficial. Still there are at least two sorts of 

 strains that annealing will not relieve. One is that attributable to the 

 non-metallic chemical impurities that do not fit into the regular 

 arrangement of atoms in a pure metal or alloy. It has been found 

 recently that by heat treating iron in hydrogen at about 1,500 degrees 

 centigrade the non-metallic impurities are largely removed, and that 

 what are called "chemical strains" are much reduced. As a result it is 

 found (Fig. 22) that the maximum permeability increases from 10,000 

 to 340, 000, '2 and a large reduction in mechanical hardness occurs 

 simultaneously. 



