1912] Secondary Rays Excited by the Alpha Rays. 157 



ing of the electrode to increase since the charges on the polonium were 

 positive. There is, however, one factor which may have influenced the 

 experiment and which must be considered here, and that is that the Gaede 

 pump was kept going continuously in order to keep the pressure low. 

 The readings were not taken until the pressure was less than i-iooo 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 ionisation 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 explanations 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 reflection of « rays from the 

 electrode, and so reduce its rate of charging or there might be a gradual 

 decrease in the ionisation 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 electrode after the poten- 

 tial 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 withdraw- 

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

 pressure the experiments described in the following section were per- 

 formed. 



IV. — Ionisation Experiments. 



In the first experiment, the polonium was charged to a positive po- 

 tential of 77 volts. It was found, that with the polonium at this poten- 

 tial the 8 radiation was practically all stopped. The Gaede pump was 

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

 mospheric pressure, was pumped out until the pressure as measured by 

 the McLeod gauge was less than i/iooo 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 diff'erent intervals of time. It was hoped in 

 this way to get some idea of the effect of the withdrawal of the air on the 

 ionisation current between the polonium and the electrode. In Table II 

 the results obtained are given. 



