250 WhcelocJc — J^ature of Ionization Produced hy a Rays. 



takes place wlieii the source is 2"G'^'" from the chamber tlian in 

 either of tlie other cases. 



This is as would he expected ou the hypothesis of iouizatiou 

 in columns. At 2"()"" the density of ions in the columns is 

 greatest, and therefore greater recombination would take place. 

 With the source at 2'3"" from the chamber the ionization is a 

 little less than at 2*6'^" ; that is, the density of ions in the columns 

 is slightly less and tlie percentage lack of saturation is corre- 

 spondingly smaller. With the source at 3"2'^"' the current is 

 about two-thirds that obtained with the source at 2"6'^'" or 2'3'='". 



Fig. 8. 



10 20 40 



Volts per cm. > 



Fig. 8.-- -A, source at 3-3=™ ; B, source at S-G"" ; C, source at 3-3'^ 



100 



This is due to the fact that some of the a particles just barely 

 enter the chamber because of being shot ofi at relatively large 

 angles Avith the vertical. 



On the other hand, the initial recombination hypothesis of 

 -Bragg fails to account for the differences in the experimen- 

 tal curves obtained at different parts of the range, when the 

 electric field is applied at right angles to the path of the a 

 particle. 



The velocity of the a particle is greatest with the source at 

 2-3™', least with the source at 3-2'^"', and has an intermediate 

 value at 2-6™. The per cent of the ions recombining should 

 therefoi-e be least at 2-3"", greatest at 3-2™, and have an inter- 

 mediate value at 2*6™', which is not the case. 



