6&4 BELL SYSTEM TECIISICAL JOURNAL 



direction of its maKnelic moment and the direction of the field, and 

 is given by 



AU= Mil cose (77) 



According to equation (73), the observed stationary states of hydro- 

 gen atoms in a magnetic field ha\e specific discrete energy-\alues. 

 These must correspond to specific discrete values of the angle 8; 

 the orientation of the atom in the magnetic field must be constrained to 

 certain particular directions, an extraordinary idea! We ascertain 

 these "permissible directions" by equating the two \alues of AU 

 figuring in (73) and (77), obtaining 



seh -iirmc = M cos e (78) 



into which we then insiTt ihe expression for .1/ in terms of p: 



sh 2t:c = p cos 6 (79) 



We have experinicnted at length with the notion that the angular 

 momentum p of the electron in its orbit is constrained to assume 

 only such values as are integer multiples of h/2-K; let it be intro- 

 duced here also. If p = kh '2w, then 



i- = k cos (80) 



The angle 6 nia\' assume only such values, as will gi\c to the quan- 

 tity 5 = ^ cos d two or more values, differing by one unit. I'or 

 instance, if k=\. the values 6 = 60" and 120^ will suffice. 



This, the most spectacular of all the remarkable consequences 

 of Bohr's interpretation of the stationary states, is also the only 

 one w^hich has ever been directly verified. 



The verification has not been made upon h\'drogen nor upon 

 ionized helium, but upon the atoms of certain metals '^. I shall there- 

 fore reserve the account of it for the following sections of tiie article, 

 where also there are certain other reasons for desiring to put it. Xe\er- 

 theless, the reader should be aware of it at this point. 



" I gave an account of the earliest of these experiments in the first article of this 

 series (This Journal, Z, October, 1923; pp. 112-114). The subsequent experiments 

 have added nothing fundamentally new. 



{To be continued) 



