excited by the a. Bays of Polonium. 



413 



another reading was taken of the rate of charging of the 

 polonium without the magnetic field. After this the brass 

 plate was cooled by pouring liquid air in the tube E, and 

 the readings on the rate of charging of the polonium were 

 continued once more. 



Table XV. 



Air in Apparatus. 

 Brass Electrode. Voltage on Brass = — 81 voits. 



Pressure of 



Time from initial 



Current through 



Current to 



air in 

 apparatus. 



reading of 

 Table XIV. 



magnet. 



polonium. 



<-001 mm. 



28 min. 



amp. 



-238-5 





Cooled Brass v\ 



ith Liquid Air 





"003 rum. 



35 min. 



amp. 



-235-5 



•005 „ 



40 „ 



„ 



—227 



, -005 „ 



50 „ 



„ 



-217 



•005 „ 



65 „ 



„ 



-214 



•004 „ 



80 „ 



„ 



—210-5 



•004 „ 



95 „ 



„ 



-210-5 



•004 „ 



110 „ 



„ 



—210-5 



•003 „ 



140 „ 



o „ 



-209 



•003 „ 



170 „ 



o „ 



-210-5 



•003 „ 





1 „ 



- 42-5 



•003 „ 





9'5 „ 



- 35-6 



•003 „ 





16 „ 



- 356 



•003 „ 





21-5,, 



- 36-5 



From the numbers given in the table it will be seen that 

 immediately after the brass began to cool down to the tem- 

 perature of the liquid air the pressure in the apparatus rose 

 slightly and then fell again as the pumping was continued. 

 The rate at which the polonium charged up, however, steadily 

 decreased as the brass plate cooled down. As the effect of 

 a rise in pressure would be to increase the ionization current 

 in the chamber, it follows from the occurrence of this decrease 

 in the rate of charging of the polonium that the secondary 

 radiation from the brass plate must have dropped off as its 

 temperature lowered. 



The values of the a ray excited secondary radiation from 

 the brass plate at temperatures 20° C. and —192° C. as 

 deduced from Tables XIV. and XV. are given in Table XVI. 



