330 BELL SYSTEM TECHNICAL JOURNAL 



magnetic field. I write down " 2 " to indicate this number of separated 

 beams; but I will call it by preference the "number of orientations in 

 the field," because that is the fundamental point. The spinning 

 electron always sets itself in one or the other of two orientations, with 

 respect to whatever field it happens to be traversing. We call them 

 the "parallel" and the "anti-parallel" orientations, though according 

 to quantum mechanics these terms are a little too strong. Here 

 then is the list of the properties of the electron-spin: g equal to 2 — 

 angular momentum equal to h(hl2ir) — magnetic moment equal to 

 ehlAirnic — two permitted orientations in any field. 



It has doubtless struck you as rather odd that I began by talking 

 about the angular momenta and the magnetic moments of electron- 

 orbits, and then carefully picked out a couple of special cases in which 

 these neutralized each other altogether and there was nothing left 

 over except what I ascribed to the electron-spin. Is there no point 

 at all, then, in talking about the electron-orbits? Oh, very much so! 

 Indeed there are cases in which the electron-spins neutralize each 

 other altogether, and we have nothing left over except what is at- 

 tributed to the orbits. To do this I may choose an atom like mag- 

 nesium, which has a nuclear charge of -\- \2e, a cloud of ten inner 

 electrons which neutralize one another completely as to angular 

 momentum and magnetic moment (just as in sodium), and two valence 

 electrons instead of one. In some of the states of the magnesium 

 atom — not in all of its states, but in some of them — the spins of the 

 two valence electrons are oriented opposite to each other in the atom, 

 and cancel each other out. When the atom is in a state of this kind, 

 then nothing is left over except the angular momenta and the magnetic 

 moments of the orbits of the two valence-electrons; and then, all the 

 statements of the orbital theory (page 326) are applicable — g is equal 

 to unity, the angular momentum takes one of the values nh/lir and 

 the magnetic moment takes one of the values n(eh/4:Trmc). Moreover, 

 there is another theorem derived from quantum mechanics which 

 turns out to be valid : the number of orientations of such an atom in a 

 field, the number of separated beams which appear in the Gerlach- 

 Stern experiments, is chosen from among the members of this sequence : 

 1, 3, 5, 7. . . . (It is a most interesting historical fact, that Gerlach 

 and Stern were moved to undertake their difficult experiment by the 

 wish to test this remarkable assertion of quantal theory.) You notice 

 that the number 2 does not appear in the sequence; were it not for 

 the electron-spin, we never could obtain it; it is distinctive of the 

 spinning electron. 



