﻿tlw Active Deposit of Radium in an Electric Field. 717 



desired to increase the ionization current in the test-vessel, 

 is not shown in the diagram ; the rays were admitted through 

 an aperture in the lead directly beneath the aluminium 

 bottom of the vessel. 



For potentials up to ] 200 volts a battery of small accumu- 

 lators was employed. For the larger potentials a Wimshurst 

 machine was used which was driven by an electric motor ; 

 it was found that the voltage obtained by this means was 

 surprisingly steady, although the slight variations were 

 sufficient to prevent any accurate measurements of the ioni- 

 zation current through the gas. The voltage was regulated 

 by means of an adjustable spark-gap, and was measured 

 with a Bran n electrometer. 



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, &c, 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 lh hours; readings of the ionization current were 

 taken at intervals so as to note the growth of activity and the 

 establishment of equilibrium. The emanation was blown 

 out by means of a strong current of air from a force-pump ; 

 the ebonite plug containing the central electrode was then 

 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 subsequently shown, an accurate determination of the 

 activity on the case was the most important factor in the ex- 

 periments. 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 electrometer 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 sus- 

 pended 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. 



