Metals under the Influence of Alpha Iiays. 323 



Returning to fig. 4, it will be observed that Curve I, which 

 was supposed to be due to the a-rays alone, shows a diminution 

 of the current as the potential on the case is increased, which is 

 similar to that of Curve III but much less in amount. This 

 was at first difficult to explain ; it is quite evident that the 

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

 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 the 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 disc* 



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 radia- 

 tion 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 II of fig. 4. The sensitive- 

 ness 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 with 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 elec- 

 trode 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 

 (1. 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 con- 

 sists of electrons, we may show by a simple calculation that 

 the experimental results are accounted for in a very satisfac- 

 tory manner. 



* At this stage of our experiments the very interesting paper of Werten- 

 stein (Le Radium, ix, p. 6, 1912) came to hand. By measuring the ionization 

 due to Ea C 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. 



