916 Prof. H. A. Bumstead on Emission of Electrons 



the speed of the a-rays producing it in a manner entirely 

 analogous to the variation in the gaseous ionization under 

 like conditions. But the possibility of an error in this con- 

 clusion here suggests itself. When the a-particle is not near 

 the end of its range it passes through the thin aluminium 

 electrode and the latter receives its positive charge solely 

 on account of the electrons emitted by it. Near the end 

 of the range, however, some of the a-particles (those 

 entering most obliquely) will stop in the foil, and it is 

 possible that the observed increase in the positive charge 

 acquired by the electrode is due to the positive charges on 

 these a-particles and not to any true increase in the S-radi- 

 ation. It is easy to test this by charging the case negatively 

 and measuring the negative charge received by the electrode 

 from the electrons emitted by the case, for under these cir- 

 cumstances the stoppage of a-particles by the electrode would 

 decrease, instead of increasing, the effect. Unfortunately 

 when this was tried a decisive result was not obtained. The 

 current observed in the electroscope remained nearly constant 

 while several foils were interposed and then fell off more 

 gradually than with the positive charge on the case. As will 

 be shown in the following pages, however, this was not due 

 to the cause suggested above, but was in all probability the 

 result of the construction of the chamber. As has been 

 pointed out, many of the a-rays must have struck the sides of 

 the holes in the top at various angles up to grazing incidence. 

 Thus the electrons emitted from the case were not all pro- 

 duced by particles moving with approximately the same speed, 

 but by particles whose velocities varied considerably. Hence 

 the effect was somewhat analogous to ionization curves ob- 

 tained when the pencil of rays is not limited to a small angle 

 by a " Bragg screen." 



However, it was necessary to investigate the matter further. 

 As a more intense source of radiation was desirable, Professor 

 Boltwood was good enough to attempt the preparation of a 

 much stronger deposit of polonium. By a special method 

 he succeeded in depositing upon a 4 mm. plug as much 

 polonium as would be in equilibrium with about 0*(i mg. of 

 radium, and yet having so little foreign material with it that 

 it appeared as a mere discoloration upon the copper. With 

 this preparation, the difficulties of the experiment were 

 greatly lessened ; an electrometer could be used instead of 

 the electroscope and much larger and steadier readings could 

 be obtained. 



In order to avoid the difficulties mentioned above, which 

 prevented satisfactory readings when a negative potential 



