244 BELL SYSTEM TECHNICAL JOURNAL 



always agrees with the "theoretical" value derived from the spectrum 

 of that substance when a gas. This is indeed the case with gases and 

 even with a certain number of solids, a large enough number to inspire 

 confidence in the theory. There are, however, numerous exceptions 

 among solids — a circumstance not to be wondered at, since an atom 

 incorporated in a solid is usually in a very different condition from an 

 atnm frppl vwandprincr ahniit in a frpR The like is true about that num- 



ULiici ciCLUiuiia. i lie dLwiiia iii i^ucauiuii uciicivc, >\ ncii v^uiiipciv- tcu 



into solids, as though this angular momentum of individual spinning 



electrons were the only one left outstanding. 



This striking inference is greatly strengthened by measurements 



upon the one phenomenon in which that angular momentum, which 



according to atomic theory is always the companion of magnetic 



moment, comes to light. Imagine a cylinder of some paramagnetic or 



ferromagnetic substance, hanging freely from a suspension attached 



to one end. Suppose it to be unmagnetized at first; this signifies 



that the atoms (whether or not they are grouped into domains) are so 



oriented that the resultant of all their angular momenta, as well as 



* The reasons furnished by spectroscopy for making this postulate are much too 

 complex to be interpolated in this article: I refer to the first fourteen pages of "Con- 

 temporary Advances in Physics," XXIX, this Journal, 14, 285-321 (April 1935). 



