1912] Secondary Rays Excited by the Alpha Rays. 161 



If the results that were obtained while the pressure of the air was 

 decreasing are compared with the results obtained while the pressure 

 was increasing as given in Table IV. and shewn graphically in Fig. 3, 

 it will be seen that at any given pressure, the rate of charging of the elec- 

 trode was greater as the pressure was decreasing than as it was increasing. 

 It will also be seen that the rate of charging at a pressure of .035 mm. of 

 mercury as the pressure was increasing v/as very much less than the rate 

 of charging at much smaller pressures as the pressure was decreasing. 

 These results shew clearly then, that the decrease in the rate of charging 

 of the electrode as the air was pumped out of the apparatus was not all 

 due to a decrease in the ionisation current through the gas. For, since 

 the ionisation current is only dependent on the pressure of the air in the 

 chamber, it should have the same value at like pressures whether the 

 pressure was decreasing or increasing. 



If the decrease in the rate of charging of the electrode with the time 

 was not all due to a decrease in the ionisation current through the gas 

 there must be some other reason for this decrease. As shewn in Section 

 III there are three other currents which cause the electrode to charge up 

 besides the ionisation current. These are, the « ray current from the 

 polonium, the 8 rays current from the polonium and the secondary 

 ray current from the electrode. It has been shewn by an experiment in 

 Section III that a positive potential of 20 volts is sufficient to stop prac- 

 tically all the 8 ray current. Therefore, in this experiment, the only 

 currents which charged up the electrode besides the ionisation current 

 were the « 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 ionisation current in order to produce the total decrease in 

 the rate of charging of the electrode as found by experiment. The de- 

 crease in intensity of the « rays from polonium with the time, has been 

 studied by various experimenters and they have found the intensity falls 

 to half value in i4odays. 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 mus-t have been a 

 considerable decrease in the intensity of the secondary radiation 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 experiment was a small 

 decrease in the ionisation current through the air as the pressure was 

 reduced and a large decrease in the secondary rays sent off by the car- 

 bon. The same causes will account for the decrease in the rate of charg- 

 ing of the brass electrode used in the first two experiments of this sec- 

 tion, and must also be considered when the results of the experiment de- 

 scribed in Section III are explained. Before continuing experiments with 



