104 Prof. Rutherford on Excited Radioactivity and 



(1) Platinum wire, charged —110 volts, exposed 15 

 minutes in a cylinder containing thoria. First observation 

 five minutes after removal of wire from emanation 

 cylinder. 



Time Movement of Electrometer in 



in Minutes. scale-divs. per sec. 



P9 



7-5 2-8 



24 4-0 



43 4'6 



58 5-2 



78 5-9 



99 6-5 



In this case the activity had increased over three times in 

 99 minutes and had not reached its maximum value. 



(2) Aluminium foil as cathode in parallel plate apparatus of 

 fig. 1. Time of exposure 41 minutes. First observation 

 6 minutes after removal. 



Time. 



Radioactivity. 







1 



21 minutes. 



1-6 



31 „ 



1-8 



57 „ 



2-0 



70 „ 



2-2 



91 „ 



2-5 



120 „ 



2-9 



160 „ 



2-9 



180 „ 



2-9 



22 hours. 



1-0 



49 „ 



0-21 



In this case, for the purpose of comparison, the initial value 

 of electrometer current is taken as unity. The activity 

 increases to nearly three times its initial value after an 

 interval of two hours, and then slowly decreases at the normal 

 rate, i. e., it falls to half value in about 11 hours. 



Similar results were obtained if the plate was made active 

 without the action of the electric field. The increase of 

 activity with time is independent of the nature of the elec- 

 trode, or of the concentration of the radioactive matter upon 

 it. It was not found possible to influence the rate of increase 

 of activity with time or the final maximum by heating the 

 wire to about a red heat. 



With increase of time of exposure of the electrode in the 

 thorium emanation, the ratio of increase of activity after 



