﻿834 



Mr. V. E. Pound on the Secondary Bays 



We have seen, therefore, that it is possihle to get rid of 

 the fatigue effect altogether by withdrawing the gas from 

 the surface of the carbon before beginning the bombardment 

 by « rays. The " fatigue/' then, which has been described 

 in the previous sections must be due to a decrease in the 

 secondary radiation from the gas occluded at the surface of 

 the brass or carbon electrode used. 



In view of the effect which the presence of gas has on the 

 secondary radiation it was thought well to repeat the experi- 

 ment described in §3, and first performed by Logeman. 

 Instead of using a brass electrode a carbon electrode was 

 used. Also the carbon was left in the vessel at low pressure 

 for two days in order that the gas occluded in the carbon 

 should disappear or reach a value which would be constant 

 for the low pressure used. Then readings were taken of the 

 rate at which the carbon electrode charged up as the positive 

 potential on the polonium was varied from volts to about 

 1700. The results are given in Table XIII. below, and a 

 curve drawn from these results is shown in fig. 8. 





Table XIII. 





Voltage on 



Pressure of air 



Time from initial 



Current (o 



polonium. 



in vessel. 



reading. 



electrode. 



volts 



-<O01 



minutes 



-447 



1-6 „ 



j» 



12 „ 



-191 



7 ,, 



n ■ 



16 



-4-7 



18 „ 



•i 



28 



54 



39 ,, 



>» 



31 „ 



89 



1(38 „ 



u 



38 



131 



251 „ 



fJ 



44 



141 



334 „ 



>> 



48 „ 



144 



401 „ 



>» 



62 



150 



538 „ 



>j 



72 



160 



788 „ 



>» 



95 



165 



1095 „ 



>» 



105 



172 



1337 , 



■„ 



121 „ 



175 



1708 



>5 



137 



181 



The curve shown in fig. 8 rises very rapidly from —447 

 to about 90 as the potential on the polonium is increased 

 from to 40 volts, and then more slowly as the potential is 

 increased beyond 40 volts. This is somewhat different from 

 the results obtained in § 3 with the brass electrode and 

 shown graphically in fig. 2, for the curve in fig. 2, it will 

 be seen, rises rapidly while the potential on the polonium is 

 raised from to 40, and then falls again as the potential is 

 further increased. The cause of this fall in the rate of 

 charging of the brass electrode as the potential on the 



