﻿excited by the Alpha Rays from Polonium. 819 



keep the pressure low. The readings were not taken until 

 the pressure was less than 1/1000 of: a mm. of mercury, but 

 yet, the pressure may have decreased still further while the 

 readings were being taken, and on account of the decrease 

 in the pressure the ionization current may have also decreased. 

 It is evident that this would cause a decrease in the positive 

 rate of charging of the electrode. 



It is seen, therefore, that, according to the above expla- 

 nations of the charging of the electrode by the different 

 currents, there are only two things which might cause a 

 decrease in the rate of charging of the electrode positively, 

 as the potential on the polonium was raised positively. 

 Either there might be a more profuse reflexion of a rays 

 from the electrode, and so a reduction in its rate of charging, 

 or there might be a gradual decrease in the ionization current 

 due to the lowering of the pressure of the air in the 

 apparatus. 



Now it is evident from the curve given in fig. 2 that there 

 was a gradual decrease in the rate of charging of the elec- 

 trode after the potential of the polonium was increased 

 beyond 20 volts. In order to find out how much of this 

 decrease or whether any at all was due to the withdrawal of 

 more air from the apparatus after it was exhausted to a very 

 low pressure, the experiments described in the following- 

 section were performed. 



4. Ionization Experiments. 



In the first experiment the polonium was charged to a 

 positive potential of 77 volts. It was found, § 3, that with 

 the polonium at this potential the £ radiation from A was 

 practically all stopped. The Gaede pump was started and 

 the air which had stood in the apparatus for a week, at 

 atmospheric pressure, was pumped out until the pressure as 

 measured by the McLeod gauge was less than 1/1000 of a 

 mm. of mercury. The time taken by the pump to do this 

 was about 15 minutes. Then, while the pump was kept 

 continuously going, readings were taken of the rate of 

 charging of the electrode at different intervals of time. It 

 was hoped in this way to get some idea of the effect of the 

 withdrawal of the air on the ionization current between the 

 polonium and the electrode. In Table IT. the results obtained 

 are given. 



3 II 2 



