156 Transactions of the Canadian Institute. [vol. ix 



charged particles would have the effect of charging the electrode posi- 

 tively. Finally, there would be a current through the air in the chamber, 

 due to the ionisation of the air by the a rays. This last current would 

 charge the electrode positively, since the polonium was positively charged. 

 This current which we will call the ionisation current, would be, in all 

 probability, very small, on account of the small quantity of gas in the 

 chamber. 



Let us now consider what would be the effect on these different 

 currents of increasing the positive potential on the polonium from zero 

 upwards. The number of « particles emitted per second by the polon- 

 ium could not be changed by increasing the potential for it is found im- 

 possible to change the rate of emission of the «, ft and y rays from the 

 radioactive substances by any known agency. The increase of the po- 

 tential on the polonium might, however, increase the speed with which 

 the ot particles passed from the polonium to the electrode. If this 

 were the case, since it has been shewn by different experimenters, in- 

 cluding Geiger and Marsden*^ that « rays are reflected to some extent 

 from the substances they strike this increase in velocity might cause a 

 more profuse reflection of the a. rays from the electrode. Hence as the 

 potential was increased there would be fewer and fewer « 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 shewn 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 elec- 

 trode. The primary cause of this discharge would be, of course, the 

 bombardment of the electrode by the « 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 ionisation current from the polonium to the 

 electrode through the gas, and this would cause the positive rate of charg- 



* * Geiger and Marsden, Proc. Roy. Soc. Ser. A. 82, July 31, 1909. 



