880 Drs. Russ and Makower : Deflexion by Electrostatic 



voltmeter. After an exposure of ten minutes to the " recoil- 

 stream " from the wire, the distribution of activity over the 

 strip was tested in a manner similar to that described for 

 the preliminary experiments, except that the width of the 

 aluminium window in the base of the a-ray electroscope was 

 reduced from 3 millimetres to 1 millimetre. 



The distribution of activity on the strip is shown in fig. 4 



qioVoCb 



for three typical cases when no field was applied between 

 the plates and with 930 and 1920 volts respectively. To 

 show that there was no lack of symmetry in the disposition 

 of the apparatus, measurements were also made with the 

 field reversed. The results so obtained were in substantial 

 agreement with those shown in the diagram. 



It will be seen that with no field the activity of the strip 

 exhibited a well-marked maximum at its middle, while the 

 activity fell off rapidly and symmetrically on either side. 

 With 930 volts the point of maximum activity was displaced 

 2 millimetres, but the distribution was no longer symmetrical 

 about the maximum. With 1920 volts the curve of dis- 

 tribution showed two maxima, one near the centre of the 

 strip and the other about 4 millimetres from the centre. 



It is not clear what is the cause of these two maxima, but 

 one maximum always occurred at the middle of the plate and 

 was therefore due to " recoil-atoms " which probably for the 

 reason already suggested, reached the plate without being 



