542 Mr. H. P. Walmsley on the Distribution of the 



2*76 per cent, of the cathode activity. Thus at least 97*24 

 per cent. of the active deposit on the cathode was positively 

 charged. These are the results obtained with electrodes of 

 2*00 mm. diameter. If rods of other sizes are used, the same 

 general results are obtained but the percentages are slightly 

 modified. The case of the earthed chamber also collects 

 uncharged active deposit. To determine its amount an inner 

 lining of thin aluminium foil was placed in this chamber and 

 the activities collected by the foil, the cathode, and the 

 anode were measured by introducing them successively in a 

 cylindrical ionization chamber of the same dimensions as 

 those of the exposing apparatus and taking the leaks on an 

 electrometer. The active deposit for these comparisons was 

 collected, using a potential difference of 960 volts on the 

 chambers, exposing the surfaces in every experiment at least 

 three hours to the emanation. The results are tabulated : 



Diameter of 

 Electrode. 



Cathode 

 activity. 



Anode activity. 



Foil activity. 



2-00 mm. 

 3'23 mm. 



90-3 

 90-1 



2-49 

 2-79 



7-23 

 7-12 



In his most recent paper on the subject, Wellisch *• found 

 a similar effect. His apparatus consisted of a cylinder into 

 which passed a central rod which served as cathode. Measur- 

 ing the activities collected by the rod and the cylinder, he 

 found that the percentage cathode activity reached a maxi- 

 mum, and was then uninfluenced by any increase in voltaoe. 



Influence of Metal on the Anode Activity. 



To examine whether the anode activity arises in any way 

 from an absorption of emanation by the metallic electrode, a 

 comparison of the anode activities collected by rods of 

 different metals was undertaken. The cases of both chambers 

 were at —960 volts, and both electrodes were earthed. 

 The amounts of active matter collected in long exposures 

 under these conditions by rods of lead, tin, and aluminium 

 were compared in turn with that collected by the same brass 

 electrode. To obtain consistent results, it was found neces- 

 sary to use clean surfaces. Before an exposure, the rods 

 were well polished with rouge, and moisture films on the 



* Wellisch, Phil. Mag. xxvi. p. 623 (1913). 



