16 Prof. Rutherford and Miss Brooks : Comparison of 



radiation is thus greater than that due to the emanation 

 which causes it. 



An experiment was tried to see whether the application of 

 an electric field had any effect on the rate of production of 

 excited radioactivity. The emanation w T as introduced into a 

 long closed cylinder with an insulated central electrode and 

 allowed to stand undisturbed for several hours with no elec- 

 tric field acting. The excited radioactivity in such a case 

 was distributed by the processes of diffusion to all parts of 

 the interior surface of the cylinder. On account of the small 

 area of the central rod very little excited radiation was pro- 

 duced on it. After several hours a steady state is reached, 

 such that the rate of conveyance of fresh radioactive material 

 to the surface is balanced by the decay of radiating power of 

 the matter already deposited. A P.D. of 300 volts was then 

 applied so as to make the central rod the cathode, and the 

 ionization current, observed with the electrometer, was 

 immediately determined. 



The excited radioactivity now decays on the inside of the 

 cylinder and is concentrated on the brass rod. No appreciable 

 change of the current was observed with time, although the rate 

 of leak due to the excited radioactivity was quite comparable 

 with that due to the emanation. This experiment shows that 

 the rate of production of the excited radiation for a fixed 

 quantity of emanation is the same whether an electric field 

 is acting or not. 



Excited Radioactivity due to Radium and 

 Thorium Compounds, 



The excited radiation from thorium compounds was ob- 

 tained by placing a platinum plate, charged negatively, in 

 a closed vessel containing thoria in the manner explained in 

 a previous paper (loc. cit.). After about a day's exposure, 

 the platinum plate was removed and placed inside the testing 

 vessel in place of the radioactive material, and the diminution 

 of the rate of leak observed for successive layers of thin metal 

 foil. The same amount of absorption of the radiation was 

 observed, if the platinum plate was made radioactive by being 

 placed near some thoria in a closed vessel with no electric 

 field acting. In such a case the excited radiation is not 

 confined to the platinum plate, but is spread over the whole 

 interior of the vessel in which the thoria is confined. 



Two methods were adopted to obtain excited radiation due 

 to radium. In one case, as with thorium, the platinum plate 

 was made the cathode in a closed vessel containing radium. 



