CONTEMPORARY ADVANCES IN PHYSICS 325 



electromagnetic theory, an electron revolving in an orbit ought to 

 radiate all of its energy in a very short time and fall into the nucleus. 

 Bohr was the man who overrode this objection. He overrode it, not 

 in order to construct a theory of magnetism in defiance of it, but in 

 order to construct a theory of spectra in defiance of it. This theory 

 has been extraordinarily successful. Our theory of magnetism is 

 hardly more than a by-product of that theory of spectra; and this, in 

 an odd sort of way, enhances its credit. A theory devised expressly 

 for a certain purpose is always less impressive than one which follows 

 incidentally from a successful theory devised for quite another purpose ; 

 and the contemporary theory of magnetism is a wonderful example of 

 this latter and more impressive type. 



The main element of Bohr's theory of spectra — if one can speak of 

 one element as the main one, which is really not quite proper — is an 

 assumption about the angular momentum of the electron in its orbit 

 or, let me say, the angular momentum p of the electron-orbit. It was 

 assumed that the electron may revolve, without radiating its energy, 

 in any orbit of which the angular momentum is an integer multiple of 

 hjliv, — h now standing, of course, for the famous quantum-constant 

 of Planck which is the emblem of modern physics. I write this down 

 as follows: 



p = (1,2,3,4, ■■■)(h/2ir). 



Bohr was thinking at first about the hydrogen atom; but hydrogen is 

 an inconvenient example to use in talking about magnetism, and iron 

 is a very complicated case indeed, so I will talk entirely about the 

 sodium atom. 



The sodium atom has a nucleus with a charge of -f lie, and eleven 

 electrons circulating in orbits around it. This certainly sounds 

 formidably complex, but it happens — and I shall later remind you of 

 this fact — that the orbits and also the spins of ten of the electrons are 

 so oriented with respect to one another that their angular momenta 

 and their magnetic moments completely neutralize each other. I 

 shall therefore ask you to imagine these ten inner electrons as a sort 

 of cloud. The eleventh electron of the sodium atom — known techni- 

 cally as the "valence" electron — cruises around this system; some- 

 times it is traveling in an orbit completely outside the cloud, sometimes 

 in an orbit which cuts across the cloud, but never in an orbit which 

 is entirely or even mainly inside the cloud. The ten inner electrons 

 which constitute the cloud neutralize a part of the force with which 

 the nucleus acts upon the valence-electron; but they make — I repeat 

 — not the slightest contribution to the angular momentum or to the 

 magnetic moment of the atom. 



