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BELL SYSTEM TECHNICAL JOURNAL 



the rod by the applied field, they acquire a net angular momentum 

 parallel to that axis. By the principle that action must be balanced 

 by a corresponding reaction, the rod itself now must recoil with an 

 equal and opposite momentum; it is this last that manifests itself by 

 the sudden twist of the rod and may be calculated from the measured 

 value of the twist. Its sign shows that the spinning magnetic particle 

 is charged negatively, and its magnitude is what would be expected 

 from the hypothesis that that particle is a spinning electron. Thus 

 a change in magnetization is fundamentally a change in the direction 

 of the spin of the electrons in the atom, and not a change in orientation 

 of the whole electron orbit. 



The next question is: Why is not every substance ferromagnetic? 

 The picture of the atom of iron as now envisioned by the experts in this 

 field, is represented by the diagram in Fig. 2. The twenty-six elec- 



Fig. 2 — Electron shells in the iron atom. 



trons in iron are divided into four principal "shells," each shell a more 

 or less well defined region in which the electrons move in their orbits, 

 and some of these shells are subdivided. The first (inner) shell con- 

 tains two electrons, the next shell eight, the next fourteen, and the 

 last two. As the periodic system of the elements is built up from the 

 lightest element, hydrogen, the formation of the innermost shells 

 begins first. When completed, the number of electrons in the first 

 four shells are two, eight, eighteen, and thirty-two, counting outward, 



