CONTEMPORARY ADVANCES IN PHYSICS 323 



random, they add up to zero, and the rod as a whole possesses no 

 resultant angular momentum; it is just standing still. Now let the 

 rod be surrounded with a solenoid, and by means of a current in the 

 solenoid let it be magnetized to saturation. Now all the arrows 

 representing magnetic moments are pointing parallel to the axis of 

 the rod. But so are all the arrows representing atomic angular 

 momenta! their resultant is no longer zero — suddenly there has arisen 

 a resultant angular momentum, belonging to the totality of all the atomic 

 magnets, and quite large enough to be detected, instead of being tiny 

 like the angular momentum of an individual atom. Unless our theory 

 is fundamentally wrong somewhere, we should be able to observe this 

 resultant angular momentum. 



The experiment is done by hanging the rod vertically from a fine 

 suspension, and sending the magnetizing current through the solenoid. 

 At the instant of the magnetization, the rod turns sharply on its axis, 

 twisting the suspending fibre. Thus it manifests the angular mo- 

 mentum of which I have just been speaking — though I ought to say 

 that what we observe is of the nature of a recoil, or back-kick: when 

 the totality of the little atomic magnets suddenly acquires its resultant 

 angular momentum, the substance of the rod as a whole acquires an 

 equal and opposite amount (so as to keep constant the total amount 

 of angular momentum in the universe) and it is the latter which we 

 detect. The experiment is quite a delicate one, but its technique has 

 been developed to a remarkable degree since it was first attempted 

 twenty years ago by Einstein and de Haas. What we measure is the 

 ratio of the magnetization of the rod-as-a-whole to the angular mo- 

 mentum of the rod-as-a-whole; and this is just the same as the ratio 

 oi fjL to p for the elementary atomic magnets. There are not many 

 properties of matter of which we can say that the value measured on 

 a large piece of matter is the same as the value for the individual 

 atom; but there are a few, and this is one of them. 



Now in giving you the result, let me first emphasize the general 

 principle that here we have evidence of the spinning of elementary 

 particles, and of the interrelation between spinning and magnetism. 

 Next, I give you the numerical result itself. For iron and nearly all 

 of the other ferromagnetic materials, we find: 



/jl/P = e/mc 



or twice the theoretical value which I quoted a moment ago. 



This cannot be explained by assuming any peculiarity of size or 

 shape or frequency of the electron-orbits in the atoms, for as I just 

 said the theoretical formula is independent of all these things. We 



