Contemporary Advances in Physics, XXXI — Spinning Atoms 

 and Spinning Electrons ^ 



By KARL K. DARROW 



NO doubt you are all accustomed to thinking of atoms as objects — 

 very small objects, of course — which are endowed with weight. 

 I can say that with perfect safety to an audience of engineers and 

 physicists; but indeed it can be said with safety to any audience — 

 I mean, of course, any audience literate enough to attach any meaning 

 at all to such a word as "atom." It may be that philosophers of the 

 past have imagined weightless atoms— I am not historian enough to 

 deny that, nor to affirm it; but if such have ever been invented, they 

 have remained quite outside the currents of modern thought. For us, 

 weight is a property which we attribute to the atom. Since this is, 

 after all, a professional audience, I will now change over to that other 

 word which many people have such difficulty in distinguishing from 

 "weight": I will say that mass is a property which we attribute to the 

 atom. In a way, that is a negative statement. It means that we do 

 not hope to explain mass in terms of something more fundamental; 

 it means that we accept mass as being itself so fundamental that even 

 the elementary particles have it. When I say "elementary particles," 

 I am still referring in part to the atoms, though it is a somewhat 

 careless usage to do so; but I am referring also to electrons both 

 positive and negative, to protons, to alpha-particles, to nuclei — to all 

 the particles, in effect, of which the atoms are built up. Also I ought 

 to include the corpuscles of light, but this lecture will be quite long 

 enough if I leave them almost unmentioned. All of these particles, 

 then, are endowed with mass; each of them has a characteristic mass 

 of its own, which we do not attempt to explain, but which we do try 

 to measure as closely as we can. 



There is another property, familiar to you though not to everyone, 

 which we accept as equally fundamental and equally unexplainable 

 with mass : it is electric charge. We attribute it also to the elementary 

 particles, though not, it is true, to all of them. We assign it to the 

 electrons, of course, and to protons and alpha-particles and all of the 

 hundreds of nuclei which distinguish the elements and the isotopes 



' A lecture delivered before the American Physical Society at Chicago on Novem- 

 ber 27, 1936, and before the American Institute of Electrical Engineers at New York 

 on May 6, 1937. 



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