486 Wellisch and Bronson — Distribution of the Active 



adjustable spark-gap, and was measured with a Braun electro- 

 meter. 



The method of procedure was in general as follows : after 

 the introduction of the emanation the vessel was allowed to 

 remain in position under the desired conditions of potential, 

 pressure, etc., for a period sufficiently long to enable the 

 emanation and the resulting activity to get into equilibrium. 

 This period was usually about three hours, and was never less 

 than 1-J hours ; readings of the ionization current were taken 

 at intervals so as to note the growth of activity and the estab- 

 lishment of equilibrium. The emanation was blown out by 

 means of a strong current of air from a force-pump ; the ebon- 

 ite plug containing the central electrode was removed, care 

 being taken that this electrode did not touch the case. 



The activity on the case was measured by observing the 

 ionization current to which it gave rise ; for this purpose a 

 fresh electrode was suspended in the vessel by means of the 

 brass holder shown in the diagram ; by thus avoiding the use 

 of ebonite the activity could be measured with great precision. 

 This procedure was rendered necessary because, as is subse- 

 quently shown, an accurate determination of the activity on 

 the case was the most important factor in the experiments. 

 It was found that handling ebonite insulation, even with great 

 precaution, resulted in the production of disturbing electrical 

 effects which were sufficient to render uncertain the subsequent 

 measurements. The activity at any time after the removal of 

 the emanation was then measured by observing the electro- 

 meter rate. 



In order to determine the activity on the central electrode 

 this electrode was removed from its holder by means of a pin, 

 which passed through a small hole, and was then suspended 

 in the second vessel in the same manner as described above ; 

 the electrometer rate could then be taken at any definite time 

 dating from the removal of the emanation. 



In practice it was found convenient to measure the case 

 activity at 10, 15, and 20 minutes, and the central electrode 

 activity at 25, 30, and 35 minutes after the emanation had been 

 removed ; the maximum activity, i. e. the activity when in 

 equilibrium with the emanation, was then calculated by means 

 of the figures given by one of us* for the rate of decay of the 

 activity resulting from a long exposure. The consistency of 

 this calculated maximum determined the number of readings 

 necessary for the measurement of the activity, but usually those 

 just mentioned amply sufficed. 



* Bronson, Phil. Mag. (6), xii, p. 73, 1906. In order to calculate the max- 

 imum activity from the rates determined at the times given above, the 

 observed rates have to be multiplied by the factors 2-02, 2 25, 2*41, 2'58, 

 2 75, and 2 96 respectively. 



