﻿the Active Deposit of Radium in an Electric Field. 725 



The figures given in Table II., although sufficiently con- 

 sistent to justify the conclusion just given, nevertheless 

 exhibit slight irregularities which are being made the subject 

 of further investigation. In the first instance the values for 

 the percentage cathode activity for potentials in the neigh- 

 bourhood of 1000 volts show a slight falling off as coni| ared 

 with those corresponding to the smaller potentials. The 

 explanation of this effect appears to lie in a distortion of the 

 field in the neighbourhood of the ebonite plug, arising from 

 some action of the a radiation on the ebonite; this distortion 

 would result in some of the cathode activity being deposited 

 on the plug instead of on the central electrode. Corre- 

 sponding difficulties arose for the ionization measurements 

 at the reduced pressures. 



The increase of the percentage cathode activity in the 

 neighbourhood of 2000 volts is probably real, although the 

 number of experiments in this connexion is not yet sufficient 

 to justify any definite statement. It is, however, of interest 

 to record that throughout the exposure with these high 

 potentials a large current due to ionization by collision was 

 passing through the gas. 



Finally, it might here be mentioned that a complete set of 

 observations for the distribution of the activity with various 

 applied potentials, both for air at 1 atmosphere and for air 

 at a pressure of 250 mm., had previously been made, using 

 the same test-vessel, but with the guard-tube projecting a 

 short distance into the volume of the gas. By reason of 

 this some of the activity which belonged properly to the 

 cathode was measured as u case activity," so that the resulting 

 values for the percentage cathode activity were too small. 

 The curves obtained both for activity and ionization exhibited 

 the same general features as those previously described. 

 Some of the results obtained are given below ; these figures 

 are not comparable with those given above, which were all 

 obtained after the guard-tube had been cut down. 



Emanation in Air at 1 atmosphere. 



Potential in 



volts. 



Percentage cathode 

 activity. 



160 



880 



2750 



76-2 



841 

 862 



