412 



Mr. V. E. Pound on the Secondary Rays 



apparatus, and the only other explanation that could be 

 offered was that a gas came from the walls of the brass 

 vessel at liquid air temperature which did not come at room 

 temperature. If this were the case there would be less gas 

 in the brass at low temperature than at high, and the method 

 indicated in the previous section of studying the gaseous 

 layer at the surface of a substance by the quantity of 

 secondary rays coming from it could be used to test the 

 above explanation. Accordingly, a brass plate was placed 

 at K instead of a carbon one, and the amount of a. ray 

 excited secondary radiation from the brass plate was deter- 

 mined when it was at room temperature and when it was 

 at the temperature of liquid air. The series of readings 

 taken are given in Tables XIV. and XV. In order that the 

 brass when at room temperature might be as free from air 

 as possible the pump was made to exhaust the air from the 

 apparatus for three hours continuously before a reading was 

 taken of the rate of charging of the polonium. 



Table XIV. 



Air in Apparatus. 



Brass Electrode at 20° C. Voltage on Brass =—81 volts. 



Pressure of 



air in 

 apparatus. 



Time from initial 



reading 3 hours 



after starting pump. 



Current through 

 magnet. 



Current to 

 polonium. 



<-001 mm. 



min. 



amp. 



-238-5 



<-ooi „ 



5 „ 



„ 



-237 



<001 „ 



15 „ 



„ 



-238-5 



<-ooi „ 





1 „ 



- 40-5 



<001 „ 





to „ 



- 34-5 



<*001 „ 





37 „ 



- 34 



<-ooi .., 





22 ,. 



- 34-5 



It will be seen from the above table that at the end of 

 three hours' constant pumping by the Gaede pump the pres- 

 sure of the air in the apparatus had fallen very low, and that 

 the rate of charging of the polonium had become constant 

 before the mngnetic field was applied. 



Immediately after the readings on the rate of charging of 

 the polonium with increasing magnetic fields were taken. 



