Deposit of Radium in an Electric Field. 441 



reaching a maximum, and then tends to remain constant. 

 This effect becomes more manifest when large quantities of 

 emanation are used, in which case an exposure of 8 minutes 

 is sufficient to collect the maximum quantity of radium A, 

 while a long exposure of about 1 hour gives only 80 per cent, 

 of the maximum quantity of radium A. 



As this decrease with time in the amount of radium A is 

 comparatively small, various experiments have been under- 

 taken in order to make certain of its accuracyo Thus, no 

 anomaly could be found in the accumulation of radium A on 

 a plate in the absence of an electric field, or on a cathode 

 exposed to a small quantity of emanation, which shows the 

 efficiency of the method used. Further, in the case of short 

 exposures not exeeeding 1 minute the activities collected on 

 the electrode were found to be proportional to the duration 

 of the exposure, showing that the emanation was free from 

 gas activity in these experiments. Since the anomalous 

 results could be ascribed to the decrease of the efficiency of 

 recoil from the aluminium surface after a long exposure to 

 the emanation, the short exposures in these series of expe- 

 riments were usually made after the long exposures. 



It follows necessarily that the total amount of active matter 

 present on the electrode at any given time of the exposure 

 must correspondingly depart from the theoretical value. It 

 has been found that the activity acquired by an electrode 

 exposed to a large quantity of emanation practically reaches 

 its maximum after an exposure of 1 hour 30 minutes, instead 

 of the theoretical time of 3 or 4 hours. 



Experiments with JSeedle Electrodes. 



7. In the light of the results given above it seemed to be 

 of interest to investigate the activity acquired by a needle 

 electrode exposed to emanation and raised to a high potential 

 in order to produce a discharge from the needle to the walls 

 of the vessel which is accompanied by an electric wind of 

 the same direction. In these experiments a tin vessel K 

 (fig. 5) was used, which could be evacuated and filled with 

 emanation by means of a side tube t. The electrode is intro- 

 duced into the vessel through a rubber stopper S, and con- 

 sists of a brass rod R to which the needle N may be readily 

 attached by putting it into a narrow hole drilled in the lower 

 part of R. The brass rod is sealed into a glass tube g which 

 covers also the upper part of the needle, so that only the 

 lower part of the latter is exposed to the emanation. The 

 high potential was produced by a Wimshurst machine or by 



