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are obliged to make some more drastic assumption. If I had unlimited 

 time before me, I might sketch the history of our assumption; but as 

 I don't, I will come straight to the present situation. We assume 

 first, that in the iron atoms in the rod the electron-orbits are so oriented 

 with respect to each other that their magnetic moments kill one another 

 off completely. We then assume that every electron has a magnetic 

 moment and an angular momentum of its own, intrinsic to it and 

 inherent in it, and altogether independent of whether or not the 

 electron is revolving in an orbit. Just as the earth has a rotation of 

 its own in addition to its elliptical course around the sun, so we imagine 

 that the electron has a rotation of its own; this rotation has an angular 

 momentum, and with it there is connected a magnetic moment. 

 (You will remember doubtless that the earth also has a magnetic 

 moment, but this is one of the analogies which it is better not to force 

 too far.) \A'hen we magnetize the iron rod, it is the electrons which 

 we are turning; the vectors which we cause to point all in the same 

 direction are the magnetic moments and the angular momenta which 

 are inherent in the electrons, and the value of their ratio is the value 

 which is characteristic of the "spinning electron," as we call it. 

 Therefore, amplifying the notation a little, I write: 



fJ^lP = gie/lmc) 



U = 1 for electron-orbits, 

 g = 2 for spinning electron, 



and now I leave the spinning electron for a few minutes, in order to 

 turn again to the theory of electrons revolving in their orbits. 



You all realized, of course, that when I converted the Amperian 

 whirl of current into an electron running around an orbit, I was 

 adopting the atom-model known by the names of Rutherford and 

 Bohr; for these were the original thinkers who impelled all the rest 

 of us, following in their footsteps, to think of the atom as a positively- 

 charged nucleus around which electrons are revolving like planets 

 around the sun. This is an atom-model in which magnetism is 

 inherent — a Rutherford-Bohr atom is intrinsically a magnet. Anyone 

 who did not know the history of the model might well assume that it 

 was designed expressly to account for magnetism, and any such person 

 might also quite reasonably assume that all the physicists of the early 

 nineteenth century thought of it simultaneously as soon as the electron 

 was discovered. Well, it was not designed expressly to account for 

 magnetism, and most of the physicists of the early nineteenth did 

 not think of it — or if they did, they thought of it only to reject it. 

 At that time, the atom-model with the orbital electrons seemed to be 

 disqualified by a very potent reason; for according to the classical 



