562 BELL SYSTEM TECHNICAL JOURNAL 



magnetic moment. The spin-axes of the electrons of the primary 

 beam we imagine to be oriented at random. But among the electrons 

 scattered by the target this is no longer true; those scattered in any 

 small solid angle have a non-uniform distribution in orientation; the 

 resultant of all their spins is a vector normal to the plane containing 

 the primary beam and the direction of scattering. Thus, the result- 

 ant spin vector is everywhere tangent to circles about the primary 

 beam, in the same general way as the magnetic vector in the field about 

 a wire carrying an electric current. If the polarization process is one 

 of selection — if, that is, the scattering material selects from the incident 

 electrons those of a particular orientation to scatter in a particular 

 direction, then it is to be expected that double scattering will exhibit 

 just the type of asymmetry which Rupp actually observes. 



The effect of a magnetic field upon a spinning electron is to cause its 

 spin vector to precess about an axis parallel to the direction of the 

 field. The same statement may be made in regard to the resultant 

 spin vector of an assemblage of electrons such as that which consti- 

 tutes the beam incident upon the gold foil in Rupp's experiments. 

 This action is competent to explain the various changes which Rupp 

 observes in the state of polarization of the primary beam when it has 

 traversed the differently directed fields. The magnitude of the effect 

 yields a value of the ratio of the magnetic moment of the electron to 

 its angular momentum and this agrees well with the theoretical value 

 of this ratio, e/mc. 



It will have been remarked perhaps that in picturing these polariza- 

 tion effects, we have reverted to the conception that electrons are 

 particles. It is difficult to see how they are to be explained in terms 

 of the characteristics of waves. There is, however, no particular 

 reason for making the effort. Here, as in other circumstances, the 

 choice of conception is entirely a matter of convenience. 



