Electrons by Metals under Influence of Alpha Rays. 477 



the fields used were too small to accelerate the a-rays suf- 

 ficiently to cause an appreciable decrease in the S-ray current. 

 The fact that the soft radiation accompanied the a-rays in the 

 thorium deposit as well as in polonium suggested that it 

 might be a secondary effect ; if this were so, then the 

 currents plotted in Curve I. (fig. 4) would be due not to the 

 a-rays alone, but there would be a small admixture of the 

 assumed secondary rays from the lower side of the aluminium 

 foil on the disk *. 



A small circle of the thicker aluminium foil, of the same 

 diameter as the opening in the brass cylinder C (fig. 1) was 

 pushed up against the polonium. in this way any soft 

 radiation coming directly from the polonium would be 

 stopped ; but a secondary radiation due to the impact of the 

 a-particles on the inner walls of the brass cylinder C would 

 not be stopped unless a foil were interposed below the 

 cylinder by means of the wheel. The experiment was made 

 as before, by applying various positive potentials to the case 

 and taking- alternate readings with and without the foil 

 below the cylinder. 



The results are shown in Curve IT. of fig. 4. The sensi- 

 tiveness of the electrometer had changed by about 5 per cent, 

 since the experiments with the uncovered polonium ; the 

 values of all the currents were reduced in the same ratio so 

 that the measurements ivitk the interposed foil should agree. 

 In Curve I. the crosses represent the measurements when 

 the polonium was covered, the circles those obtained when it 

 was not covered, a thin foil being between the cylinder and 

 electrode in both cases. A comparison of Curves III. and 

 II. shows that the direct radiation contains a component 

 which is much less affected by the retarding field than the 

 secondary radiation alone. This agrees with the results of 

 Wertenstein (I. c.) who, however, worked with a magnetic 

 instead of an electric field. If we assume that the soft 

 radiation is made up of two portions, one coming directly 

 from the polonium, and not retarded by the field, while the 

 other is secondary and consists of electrons, we may show 

 by a simple calculation that the experimental results are 

 accounted for in a very satisfactory manner. 



* At this stage of our experiments, the very interesting paper of 

 Wertenstein, Le Radium, ix. p. 6 (1912), came to hand. By measuring 

 the ionization due to RaC in gases at low pressures, he has demonstrated 

 the existence of two soft radiations ; one is secondary, and deviable in 

 a magnetic field, and doubtless consists of electrons, while the other is 

 not appreciably deflected in a magnetic field of 1100 units. The 

 remainder of our work was done with a knowledge of Wertenstein's 

 results and the next experiment was directly suggested by his paper. 



