THE PHYSICAL BASIS OF FERROMAGNETISM 7 



The regular pattern observed ^ is similar in nature to the familiar one 

 obtained when iron filings are sprinkled near a permanent magnet; 

 the fine colloidal particles are necessary in this case because the whole 

 scale is small. This micro-pattern changes when the applied field 

 changes, and the difference is attributed to the redistribution or 

 reorientation of groups of domains. These patterns are obtained only 

 on magnetic materials and are found on them even when the material 

 is unmagnetized ; such a one is shown in Fig. 4, 



Fig. 4 — The powder pattern produced b\" colloidal iron oxide on the surface of 

 a demagnetized silicon-iron crystal, showing the presence of inhomogeneous mag- 

 netic fields. Magnification about 1000. 



Magnitudes of Magnetic Forces 



Ferromagnetic theory has been made difficult by the fact that the 

 magnetic forces between the electrons in an atom are small compared 

 to the electrostatic forces. The latter force between two electrons of 

 charge e (in e.s.u.), a distance a apart, is equal to 



The magnetic force between the same electrons depends on the speed 

 of the charges as well as on their magnitudes, and, when the direction 

 of motion is perpendicular to the line joining them, is equal to 



7^ ' 72 ' 



where v\c is the ratio of the speed of each electron to the speed of 



light. Since y/c is usually of the order of 0.01, these magnetic forces 



1 L. W. McKeehan and W. C. Elmore, Phys. Rev., 46, 226-228 (1934). See also 

 the earlier experiments by F. Bitter, Phys. Rev., 41, 507-515 (1932). See also the 

 account by Elmore in F. Bitter's Introduction to Ferromagnetism, McGraw-Hill, 

 New York, 55-66 (1937). 



