288 BELL SYSTEM TECHNICAL JOURNAL 



same spectroscopic object sometimes as a line and sometimes as a 

 pair or group of lines, and the corresponding confusion about states, 

 are very bad impediments to clarity of exposition, but there is simply 

 no way of getting around them.) Such is in fact the case. The two 

 members of a pair have a common value of the index n and a common 

 value of the orbital angular momentum of the valence-electron, and 

 yet there must be something physical which distinguishes them. To 

 leap at once to the conclusion: in the atom-model, this something is 

 orientation. 



But, orientation of what with respect to what? We have as yet 

 introduced only one outstanding direction, only one vector, into the 

 atom-model. The outstanding direction is that of the normal to the 

 orbital plane of the valence-electron ; the vector is the orbital angular 

 momentum. To these it is necessary to supply a second vector, — as 

 it turns out, a second angular momentum. 



Since it may occur to some reader that the natural place to seek 

 this angular momentum is among the electrons of the cage — that we 

 should begin by assigning a net or resultant angular momentum to the 

 ten electrons which we have hitherto so much neglected — I will recall 

 that such was actually the first suggestion. It prevailed during the 

 early twenties, and was generally accepted; but it suffered from 

 certain disadvantages, which now there is no particular reason for 

 retelling at length. Yet it was of the greatest assistance in preparing 

 the ground and the technique for the suggestion which superseded it 

 in the middle twenties, that the second angular momentum is to be 

 ascribed to the electron itself; the electron is to possess, like the earth, 

 not only a motion of revolution but also a motion of rotation. (As 

 for the ten electrons of the cage, their angular momenta both of rota- 

 tion and of revolution are so oriented as to balance one another out, 

 and we have made no error in neglecting them.) 



Now there are two vectors and two directions in the model of the 

 sodium atom: that of the axis of the rotating electron, and that of the 

 normal to the orbit — the spin momentum and the orbital momentum. 

 To speak of different orientations of the one with respect to the other 

 is now sensible. But different orientations must correspond to differ- 

 ent energies if they are to explain the data, since the two lines of a 

 principal-series doublet are separate and distinguishable because and 

 only because the two members of a pair of P-states differ in energy. 

 Why should they? 



This happens to be the easiest question of the lot, or at any rate 

 the one which can be answered from classical physics. The rotating 

 electron is a magnet, by virtue of its whirling charge. Also it behaves 



