PERMALLOY, A NEW MAGNETIC MATERIAL 111 



With the remarkable ferromagnetic behavior of permalloy in mind 

 one naturally looks for analogous peculiarities in its other properties. 

 As has been shown, however, the equilibrium diagram does not point 

 accurately to the composition exhibiting highest initial permeability. 

 The conductivity curve is even less indicative of a peculiarity at this 

 point, its minimum lying at about 35 per cent nickel. The crystal 

 structure is that of nickel and its type does not change until the nickel 

 content is made less than 35 per cent. Even the mean spacing between 

 adjacent atom-centers, and with it the density, varies continuously 

 throughout the entire range. Our experience in working these alloys 

 also indicates that the series has no mechanical peculiarities at or 

 near 80 per cent nickel. Not only do these characteristics indicate 

 no abnormality as the nickel content is increased beyond 70 per cent, 

 but, what is more surprising they are little affected by the heat-treat- 

 ments which so profoundly change the magnetic properties. So far 

 as has been determined, therefore, it is only in connection with its 

 magnetic properties that permalloy is unusual. 



To the engineer the discovery of permalloy means the realization of 

 plans long impossible of accomplishment for lack of a suitable material. 

 For the scientist the principal interest in these materials may well 

 lie in the large response of their magnetic properties to simple external 

 controls. Without alteration of composition these properties may be 

 adjusted through extraordinary ranges by strain, by magnetization, 

 or by heat-treatment. This allows a more definite study of the 

 way in which these factors are related to magnetic properties than 

 has been possible with materials hitherto available in which their 

 effects are comparatively small and may be associated with com- 

 plicated and irreversible changes in other properties. The behavior 

 of permalloy demonstrates that ferromagnetism is associated with 

 material structure in a different way than are the ordinary physical 

 and chemical properties and its extreme sensitiveness to control 

 gives us a powerful method for use in magnetic investigations. 



