2 BELL SYSTEM TECHNICAL JOURNAL 



moments, but it has now been established that when the magnetization 

 is altered all that changes is the direction or '.'sense" of the spin of 

 certain of the electrons in the atoms — the orbital motions remain prac- 

 tically unchanged. 



The electrons that are responsible for the magnetic properties of 

 iron, cobalt, nickel and their alloys lie in a definite "shell" in the 

 atom. As shown in Fig. 1, there are four shells or regions, more or 



Fig. 1 — Electron shells in an atom of iron. The arrow indicates the incomplete 

 sub-shell that is responsible for ferromagnetism. The numbers specify how many 

 electrons with each spin are in the corresponding sub-shells. 



less well defined, into which all the electrons circulating about the 

 nuclei of these atoms may be divided when the atom is separated 

 from its neighboring atoms, as it is, for example, in a gas. Some of 

 these shells are subdivided as shown. When the atoms come closer 

 together as they do in a solid, the fourth or outermost shell of each 

 becomes disrupted, and the two electrons which comprised it wander 

 from atom to atom and are the "free" electrons responsible for 



