﻿824 Mr. V, E. Pound on the Secondary Rays 



ionization current. These are the a ray current from the 

 polonium, the 8 ray current from the polonium, and the 

 secondary ray current from the electrode. It has been 

 shown by an experiment in § 3 that a positive potential 

 of 20 volts is sufficient to stop practically all the & ray 

 current. Therefore, in this experiment, the only currents 

 which charged up the electrode besides the ionization current 

 were the a ray current and the secondary radiation current. 

 Either both or one of these currents, then, must have had a 

 decrease in intensity as well as the ionization current in 

 order to produce the total decrease in the rate of charging 

 of the electrode as found by experiment. The decrease in 

 intensity of the a rays from polonium with the time has 

 been studied by various experimenters, and they have found 

 the intensity fails to half value in 140 days. The time taken 

 by the experiment was about three hours, so that in this 

 short interval of time the decrease in intensity of the a rays 

 was practically nothing. Hence there must have been a 

 considerable decrease in the intensity of the secondary radia- 

 tion emitted by the carbon from the time when the first 

 reading of the rate of charging of the electrode was 

 taken. 



The causes, then, of the decrease in the rate of charging 

 of the electrode with the time as found in the above experi- 

 ment was a small decrease in the ionization current through the 

 air as the pressure was reduced, and a large decrease in the 

 secondary rays sent off by the carbon. The same causes will 

 account for the decrease in the rate of charging of the brass 

 electrode used in the first two experiments of this section, 

 and must also be considered when the results of the ex- 

 periment described in § 3 are explained. Before continuing 

 experiments with the object of a further elucidation of the 

 results described in § 3, it was thought well first to make an 

 extended study of the phenomenon of the decrease in the 

 intensity of the secondary radiation as made evident by the 

 foregoing experiments. 



5. Experiments on the "Fatigue " of Secondary Rays. 



The experiments described in §4 show that there was a 

 decrease in the secondary radiation sent off by carbon 

 bombarded by a rays as the air was pumped out of the 

 apparatus in which the carbon was placed. This decrease 

 may be called a fatigue of the secondary rays, for the effect 

 is similar to that observed in the case of the photoelectric 

 fatigue. The phenomenon of the photoelectric fatigue has 



