CONTEMPORARY ADVANCES IN PHYSICS 333 



about m? Formerly it represented the mass of the electron; now we 

 are dealing with a different sort of particle, having a mass which 

 (as many other kinds of experiments show us) is about 1835 times as 

 great as the electron-mass. I denote this mass by M. It seems 

 natural, then, to expect for the magnetic moment of the proton the 

 value ehj^irMc, or about 1/1835 of that of the spinning electron. 



This is a formidably small magnetic moment to hope to measure, 

 nay even to detect! yet Stern and his pupils undertook to measure it, 

 and they succeeded. Of course, modifications had to be made in the 

 technique which worked so well for sodium. Hydrogen being a gas 

 at room-temperature, no heated oven was required; nevertheless 

 they used an "oven," but it was refrigerated instead of being heated — 

 a sort of super-ice-box; this was in order to obtain slow-moving atoms, 

 for the slower the atoms traverse the field, the more accurately the 

 experiment can be made. I just said "atoms"; but as most people 

 know, the particles of gaseous hydrogen are not atoms, but diatomic 

 molecules — systems composed of two protons and two electrons apiece. 

 This is a circumstance which in many desirable tests of modern 

 theoretical physics is a great inconvenience, for usually our simplest 

 theoretical affirmations refer to hydrogen atoms and we should like 

 to be able to experiment on them directly. Here, however, it turns 

 out to be a great convenience, indeed perhaps the only thing that 

 makes the experiment possible. F"or if we had an isolated hydrogen 

 atom, the magnetic moment of its electron would so far exceed that 

 of its proton that the latter would be indetectable. (Perhaps it is 

 not superfluous to mention that bare protons could not be used in the 

 experiment either, as the magnetic field would exert so large a force 

 upon their moving charges that the forces upon their magnetic poles 

 would be insignificant by comparison.) 



But if in a single hydrogen atom the magnetic moment of the 

 electron swamps that of the proton, how shall this fate be avoided for 

 a system composed of two electrons and two protons? Here enters in, 

 and in a very important and significant way, that law of the permitted 

 orientations. Just as a spinning particle of angular momentum 

 \{hl2ir) can take only two permitted orientations in a field, so it can 

 take only two with respect to another particle of its kind — the parallel 

 and the anti-parallel. It chances — or rather it does not chance, it 

 follows from the underlying laws of Nature — that in the hydrogen 

 molecule the two electrons are oriented anti-parallel to each other. 

 Their magnetic moments cancel each other, and do not trouble the 

 experimenter. 



