CONTEMPORARY ADVANCES IN PHYSICS, XXIX 313 



got values for the magnetic moments of proton and deuteron ^^ which 

 are of the order of one one-thousandth of those associated with elec- 

 trons. These values are stated (for a reason which may be obvious 

 but will be set forth presently) as 3.25 and as 0.75 times the quantity 

 {eh/Airmpc), where Mp stands for the mass of the proton; the uncer- 

 tainty is given as 10 percent for the former, about 25 percent for the 

 latter. 



There remain the experiments whereby M was directly determined, 

 for protons and for deuterons, from the force exerted by an inhomo- 

 geneous magnetic field on the nuclear moment itself. One cannot use 

 the bare proton for such an experiment, since owing to its charge it 

 would suffer, as it flies through the magnetic field, an electrodynamic 

 force and a deflection by comparison with which the others would be 

 trivial. One cannot use the isolated hydrogen atom, since just as in 

 the case of sodium which we considered at such length, the force ex- 

 erted by the field upon the magnetic moment of its electron would 

 far outweigh that exerted on the nucleus. There remains the hydrogen 

 molecule, which in its normal state has the convenient feature that the 

 spins of its two electrons are oriented anti-parallel (in the loose sense 

 of the term) and cancel one another out, while the angular momenta 

 and the magnetic moments of their orbital motions likewise vanish. 

 This seems to remove all the possible competitors to the nuclear 

 moments, but there arises another which does not occur in individual 

 atoms: the rotation of the molecule-as-a-whole, which has an angular 

 momentum and a magnetic moment. This magnetic moment, 

 however, is of the same order as those of the nuclei, and its contribu- 

 tion to the net magnetic moment of the nuclei can be estimated and 

 subtracted from theirs. As for the nuclei, they may set themselves 

 with their spins either parallel or antiparallel;^^ in the latter case their 

 magnetic moments cancel one another, and observations on such 

 molecules teach us only about the rotation, knowledge which is useful ; 

 in the other case their magnetic moments add, and the data of the 

 experiment yield a value which is double the moment of the individual 

 proton — or of the individual deuteron, according as the molecule is 

 formed of two light atoms or two heavy atoms of hydrogen. 



'2 This substitute for the names deuton and diplon, by which the nucleus of the 

 H^ atom (deuterium or "heavy hydrogen") has usually been known in America and 

 England respectively, was recommended at a recent meeting of the American Physical 

 Society by Dr. Urey, the discoverer of deuterium. 



1' This implies that nuclei conform to rules of quantization in direction relative to 

 one another similar to those for electrons. This is true, and is superbly demonstrated 

 by observations on band-spectra and by the chemical separation of the two kinds of 

 hydrogen molecules ("ortho-hydrogen" and "para-hj'drogen") here mentioned; 

 but the stor)' is much too long for this place. 



