PRESENT STATUS OF FERROMAGNETIC THEORY 89 



Summary 



In concluding the author wishes to go back from here to summarize 

 what is known about the origin of the forces responsible for the various 

 magnetic properties and about the sizes of the various units. This 

 information is summarized in Table II. 



TABLE II 



Summary of D.\ta Regarding Origin of Forces Responsible for Various 



Magnetic Properties 



Unit Concerned 



Property 



Origin of Property 



Size of Magnetic Unit 



Electron 



Paramagnetic atom 



Domain 



Single crystal or 

 region of homo- 

 geneous strain 



Polycrystal 



Magnetic moment 



Magnetic moment 



Ferromagnetism. 

 Change in proper- 

 ties at Curie point 



Cr>'stal anisotropy. 

 Magnetostriction. 

 Strain sensitivity 



Orientation-aver- 

 age of single crys- 

 tals and strain 

 units 



Electron spin 



Uncompensated 

 spins and orbital 

 motions of elec- 

 trons 



"Exchange" be- 

 tween electrons in 

 neighboring atoms 



Magnetic forces be- 

 tween atoms 



Sum of effects of 

 single crystals and 

 strains 



One unit of spin per 

 electron 



4, 3, and 2 uncom- 

 pensated spins per 

 atom in Iron, Co- 

 balt and Nickel, 

 respectively 



Volume of domain 

 isabout (0.001 

 inch)' 



10* domains per 

 cubic centimeter 



Size of specimen 



Acknowledgment 



I take pleasure in acknowledging the detailed criticisms given by 

 Dr. K. K. Darrow, and also the more general criticism given by Dr. 

 O. E. Buckley during the preparation of this paper. 



Referenxes 



Most of the material used in this article can be found in more technical form in 

 the following books and summarizing articles. For some topics reference must be 

 made to the original works, as indicated in the text. 



Books 



1. "Magnetism and Matter," E. C. Stoner. Methuen. London, 1934. 



2. "Magnetic Induction in Iron and Other Metals," J. A. Ewing. Electrician, 



London, 3d ed., 1900. 



3. "Magnetic Properties of Matter," K. Honda. Syokwabo, Tokyo, 1928. 



4. "Die ferromagnetischen Legierungen," W. S. Messkin and A. Kussmann. 



Springer, Berlin, 1932. 



