I9I2] 



Secondary Rays Excited by the Alpha Rays. 



169 



In view of the effect which the presence of gas has on the secondary 

 radiation it was thought well to repeat the experiment described in Sec- 

 tion III and first performed by W. H. Logeman. Instead of using a 

 brass electrode a carbon electrode was used, also the carbon was left in 

 the vessel at low pressure for two days in order that the gas occluded in 

 the carbon should disappear or reach a value which would be constant 

 for the low pressure used. Then readings were taken of the rate at which 

 the carbon electrode charged up as the positive potential on the polonium 

 was varied from o volts to about 1700. The results are given in Table 

 XIII below, and a curve drawn from these results is shewn in Fig. 8. 



The curve shewn in Fig. 8 rises very rapidly from -447 to about 90, 

 as the potential on the polonium was increased from o to 40 volts and 

 then more slowly as the potential is increased beyond 40 volts. This is 

 somewhat different from the results obtained in Section III with the brass 

 electrode and shewn graphically in Fig. 2, for the curve in Fig. 2, it will 

 be seen, rises rapidly while the potential on the polonium is raised from 

 o to 40 and then falls again as the potential is further increased. The 

 cause of this fall in the rate of charging of the brass electrode as the po- 

 tential on the polonium was raised above 40 volts in a measure may be 

 attributed, as has been suggested, either to a more profuse reflection of 

 a rays from the electrode as the potential of the polonium was raised or 

 to a gradual decrease in the ionisation current due to a lowering of the 



