PRESENT STATUS OF FERROMAGNETIC THEORY 



65 



or elliptical orbits around the nucleus. To this picture now must 

 be added the idea that each electron itself is spinning about an axis 

 that passes through its center. Thus, there is circulation of electricity 

 in an atom, both around the nucleus and within each electron— and 

 the latter motion is called the "electron spin" because of its similarity 

 to a spinning ball. Each electron in an atom is then a small gyroscope, 

 possessing a definite magnetic moment on account of its moving elec- 

 trical charge and a definite angular momentum on account of its 

 moving mass. The ratio of these two quantities is known from various 

 independent lines of reasoning and evidence to possess a particular 

 value. Electrons revolving in orbits also exhibit both magnetic mo- 

 ments and angular momenta due to their orbital motions, but for 

 these the ratio is just half what it is for the spinning electron. 



The Barnett experiment ^^ shows in a very direct way the existence 

 of these magnetic and mechanical moments of the electron and con- 

 firms the ratio between them in ferromagnetic materials (Fig. 1). A 



V/////////////////////A 



(_) 



(B) 



Fig. 1 — ^Gyroscopic action (left); force F produces rotation R. Gyromagnetic effect 

 (right); field H produces rotation R. 



rod of iron is hung from a fine suspension and then is magnetized 

 suddenly, whereupon the rod is observed to turn, twisting the suspend- 

 ing fiber a minute but measurable amount. The spinning electrons 

 responsible for ferromagnetism have been turned by the applied field 

 so that they are more nearly parallel to it; but the mechanical moment, 

 which is also a property of those same electrons, causes the whole rod 

 to rotate in just the way that a gyroscope would. Or, 'to put it dif- 

 ferently: When the elementary magnets, pointing originally in all 

 directions, are turned more nearly into parallelism with the axis of 



