334 BELL SYSTEM TECHNICAL JOURNAL 



May not, however, the same thing happen in respect to the two 

 protons, so rendering the experiment hopeless? It turns out that for 

 these the spins are anti-parallel in some molecules but parallel in 

 others. Molecules of the former type, which is called para-hydrogen, 

 are indeed useless for the experiment; but molecules of the latter type, 

 which is called ortho-hydrogen, are available, and in them the magnetic 

 moments of the two protons collaborate so that the magnetic moment 

 of the molecule-as-a-whole is twice as great as that of the single 

 proton, a welcome assistance! In ordinary gaseous hydrogen at room 

 temperature, about three-quarters of the molecules are ortho-hydrogen. 



When the experiment was at last achieved by the school of Stern, 

 it was found that the foregoing inference as to the ^u-value of the 

 proton is roughly but not exactly correct! The latest information is, 

 that the magnetic moment of the proton is close to 2\ times ehjAirMc. 

 Measurements with another method by Rabi and his school have 

 confirmed these results; and we are definitively debarred from believing 

 that for the proton and the electron, the magnetic moments stand in 

 the inverse ratio of the masses. Perhaps this signifies that the proton 

 is itself a composite particle, a notion for which there is some support 

 from other sources. 



I mention briefly the characteristics of a few other nuclei. After 

 the proton, the next simplest is the deuteron or nucleus of the heavy- 

 hydrogen atom. It is composed of a proton and a neutron, the latter 

 being a neutral particle of about the same mass as the proton. We 

 can observe free neutrons wandering about in space, but we cannot 

 determine their spins nor their magnetic moments. The deuteron, 

 however, has three permitted orientations (this we discern from the 

 spectrum of heavy hydrogen) and consequently an angular momentum 

 of {hjlir). It is inferred that the neutron has |(///27r) for its angular 

 momentum, and that in the deuteron these two constituent particles 

 — proton and neutron — are oriented with their equal spins parallel 

 to one another. The magnetic moment of the deuteron is less than 

 that of the proton, and accordingly the neutron must have its magnetic 

 moment oppositely directed to that of its companion in the system, 

 even though their angular momenta be similarly directed — a strange 

 complication ! 



After the deuteron, the next simplest among the nuclei (except two 

 which are much too rare for investigation) is the alpha-particle or 

 helium nucleus." It is composed of two neutrons and two protons. 

 We find that its angular momentum and its magnetic moment are 

 zero, a clear indication that the four spins of its components are 

 cancelling each other two by two, and the four magnetic moments 



