A TEST FOR ELECTRON POLARIZATION 467 



fore, to vary the dihedral angle between the plane of incidence of the 

 second reflection and that of the first. There are two positions of the 

 movable system for which these planes coincide. For these "parallel " 

 positions the current entering the collector should be at a maximum 

 provided the electron beam is unpolarized initially and becomes asym- 

 metric at reflection; for the intermediate "transverse" positions the 

 current should be at a minimum. In the analogous experiment in 

 optics the intensity / of the twice reflected beam satisfies the formula 



/= /o(l + pcosld), 



where 9 represents the azimuth angle of the movable system measured 

 from either of its parallel positions, and p an amplitude coefficient 

 which serves as a convenient measure of the polarization effect. 



In the experiment with electrons our procedure has been to measure 

 the intensity of the twice reflected beam for various values of 6 — 

 though chiefly for the values corresponding to the cardinal positions — 

 to assume the same form of relation between intensity and angle as in 

 optics, and to evaluate the coefficient p. 



The reflection of electrons from a crystal surface is, like that of 

 X-rays, "selective in wave-length"; the intensity of the reflected beam 

 attains maximum values at various critical wave-lengths or speeds of 

 bombardment. This effect is, of course, accentuated in a beam which 

 has suffered two reflections. In the test for polarization we have made 

 observations in the range of bombarding potentials from 10 to 200 

 volts, chiefly at five different electron speeds at which there are 

 intensity maxima of the beam twice reflected. 



Preliminary observations indicated that at each of these critical 

 speeds the intensity of the reflected beam is. to a first approximation, 

 independent of angle. The acutal values found for p were some of 

 them positive and some negative, and none greater absolutely than 

 0.02, which was about the order of uncertainty involved in the deter- 

 minations of the collector currents. These results were described in a 

 letter to the Editor of "Nature." ^ 



In the present article the experiment is described more fully, and 

 additional data are adduced from which it is concluded that the value 

 of p, if different at all from zero, cannot be greater than 0.005. 



The principal parts of the apparatus are the gun for supplying a 

 homogeneous beam of electrons, the two crystal reflectors, and the 

 collector. These are contained in two metal boxes or enclosures shown 

 in longitudinal sections in Fig. 1. The right hand or gun enclosure 

 contains the electron gun and the first reflector, and is attached rigidly 



2 C. J. Davisson and L. H. Germer, Nature, 122, 809 (1928). 

 31 



