﻿818 Mr. V. E. Pound on the Secondary Bays 



polonium from zero upwards. The number of a. particles 

 emitted per second by the polonium could not be changed by 

 increasing the potential, for it is found impossible to change 

 the rate of emission of the a, /3, and y rays from the radio- 

 active substances by any known agency. The increase of 

 the potential on the polonium might, however, increase the 

 speed with which the a particles passed from the polonium 

 to the electrode. If this were the case, since it has been 

 shown by different experimenters, including Geiger and 

 Marsden *, that a. rays are reflected to some extent from the 

 substances they strike, this increase in velocity might cause 

 a more profuse reflexion of the a rays from the electrode. 

 Hence, as the potential was increased there would be fewer 

 and fewer u particles which would remain attached to the 

 electrode, and this would cause the positive rate of charging 

 of the electrode to decrease. 



The effect of increasing the positive potential on the 

 polonium could only tend to retard more and more of the 

 8 rays which are negatively charged and of slow velocity. 

 Hence, on account of the stopping of these rays, the rate of 

 charging of the electrode positively must have increased. 

 In fact the sharp rise in the first part of the curve shown in 

 fig. 2 has been attributed by Logeman and others, and very 

 probably correctly so, to the stoppage of the 8 rays by the 

 positive charge on the polonium. 



Again, the increase of the positive charge on the polonium 

 must tend to produce a freer discharge of negative electricity 

 from the electrode, since a positive charge on the polonium 

 attracts negative from the electrode. The primary cause of 

 this discharge would be, of course, the bombardment of the 

 electrode by the a. rays, and this is what we have called the 

 secondary radiation. An increase in the amount of secondary 

 radiation discharged from the electrode would increase the 

 rate at which the electrode charged positively. 



Finally, the increase of potential on the polonium would 

 have the effect of increasing the ionization current from the 

 polonium to the electrode through the gas, and this would 

 cause the positive rate of charging of the electrode to increase, 

 since the charges on the polonium were positive. There is, 

 however, one factor which may have influenced the experi- 

 ment, and which must be considered here, and that is that 

 the Gaede pump was kept going continuously in order to 



* Geiger and Marsden, Proc Roy. Soc. Series A, vol. lxxxii. Jul? 31, 



]909. 



