1912] Secondary Rays Excited by the Alpha Rays. 155 



the polonium, as measured by the quadrant electrometer. A curve rep- 

 resenting these results is shewn in Fig. 2. 



It is evident from the results given in the Table, and from the curve, 

 that when the polonium was at zero voltage the charge gained by the 

 brass electrode was negative. It is also evident that as the voltage on 

 the polonium was increased positively the charge gained by the electrode 

 increased from a negative charge to a positive one, and that at a poten- 

 tial of about 20 volts the rate of charging became a maximum. This 

 result agrees with the result published by Logeman. This can be seen 

 from the numbers recorded in his paper, a few of which are given in the 

 following table. The results can be readily compared because the dis- 

 tance between the polonium and the opposite electrode was about the 

 same as that in Logeman's apparatus. 



Logeman's Results. 



Distance between electrodes 5 mm. 



It will be seen however, from the curve, that as the potential on the 

 polonium was increased beyond 20 volts, the rate of charging of the elec- 

 trode gradually decreased. This effect does not appear to have been 

 observed by Logeman. In searching for the explanation of it, further 

 interesting properties of the secondary rays were found by the writer 

 which have not as yet been noted by other experimenters. 



Before seeking for an explanation of the results given in Table I, 

 and shewn graphically in Fig. 2, it is necessary to enumerate the differ- 

 ent currents which would give a charge to the brass electrode B. In the 

 first place there would be a current due to the passage of the a rays from 

 the polonium across the space separating the polonium from the elec- 

 trode. Since the a rays are positively charged particles this current 

 would charge the electrode positively. Then there would be a current 

 of negatively charged particles from the polonium which would reach 

 the electrode. This current is known as the 8 radiation and always 

 accompanies a discharge of a particles. The passage of this current 

 would give a negative charge to the electrode. Again there would be a 

 stream of negatively charged particles emitted by the electrode. 

 The researches of Aschkinass and Logeman have proved this stream to 

 exist whenever a substance is bombarded by « rays, and it has been 

 called the secondary radiation. The emission of this stream of negatively 



