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CONTEMPORARY ADVANCES IN PHYSICS 329 



equation excepting fx, — v is the hardest to estimate accurately — -and 

 so we can solve the equation for /x. 



When the experiment is done there appears, however, a very 

 remarkable thing. Instead of there being a long band upon the plate, 

 there are just two spots. Instead of the beam having been broadened 

 out into a continuous fan, it has evidently been split into two separate 

 pencils. It looks as though the field had acted first of all upon the 

 magnets, by setting them all vertical, — half of them with north pole 

 up, and half with north pole down. Not this phenomenon alone, but 

 many others in Nature show us that this is just what happens. You 

 may perhaps feel for the moment that it is intelligible, after all; the 

 compass-needle turns to the north — why should not the little atomic 

 magnets, as soon as they enter the field, turn their south poles toward 

 the north pole of the magnet which attracts them? Well, this would 

 not account for the magnets which constitute the beam which bends 

 away from the w^edge-shaped north pole, instead of toward it; and 

 indeed it does not even account for the beam which bends toward 

 the wedge-shaped pole. Classically the field should have no orienting 

 effect whatsoever upon the atoms, and yet it evidently does.^ This is 

 one of the phenomena of the atomic world which we cannot properly 

 visualize in terms of the behavior of objects large enough to be tangible 

 and visible. All that I can do is to assert it, and to say that it justifies 

 us in using this formula to calculate /z. When we use it, the value 

 which we find for the magnetic moment of the sodium atom in its 

 normal state is 



eh/4:Tmc, 



which happens to be one of the values in the sequence which I just 

 wrote down. 



I repeat that according tp the theory in its present stage, the elec- 

 tron-orbits in the normal sodium atom have a net magnetic moment of 

 zero. This value eh/4:Trmc is, therefore, the magnetic moment due to 

 the spin of the valence-electron — it is the magnetic moment of the 

 spinning electron. I write it in the appropriate place, and then with 

 the aid of the ^- value derived from the gyromagnetic effect I write 

 down the value of angular momentum which we assign to the spinning 

 electron : 



p = \{hl2T). 



To this roster of three statements about the spinning electron I now 

 make a final addition. The Gerlach-Stern experiment on sodium 

 shows that a beam of sodium atoms — which for this purpose is the 

 equivalent of a beam of spinning electrons — -is divided into two by a 



