[STEWART] ON EXCITED RADIOACTIVITY 99 
of a battery of small storage-cells, the other terminal of the battery 
being joined to the case of the electroscope. 
A fine steel wire which was attached at its upper end to the 
rod, D, extended below the sulphur bead, and was there bent into a 
loop surrounding the brass strip. This steel wire could be readily 
drawn into contact with the strip, A, by means of a small magnet, 
and the measuring system by this operation was raised from time to 
time to any desired potential. 
The apparatus was also provided 
with a movable base, G, and bodies to 
be tested for radioactivity were placed 
in the electroscope resting on this base. 
Besides having a small capacity, 
this measuring system possessed the 
advantage of being entirely free from 
leakage along its supports. In practice 
the conducting rod D, was maintained 
throughout any measurement, at the 
initial potential of the gold leaf, and, 
consequently, when the latter indicated 
any loss of charge, it was evident that 
such loss arose from a leakage through 
the gas surrounding the measuring sys- 
tem. Any conduction across the sul- 
phur bead could only be in such a di- 
rection as to partially counteract the 
loss arising from leakage through the 
gas. 

In operating the electroscope, it 
was found that a potential of 150 
volts applied to the system produced a convenient deflection of the 
goid leaf, and, as this voltage was ample to produce the saturation 
current under spontaneous ionization, it was adopted generally 
throughout the investigation as the standard. 
With the scale provided in the micrometer, readings could be 
made to ‘01 mm., and, as the microscope in a measurement was always 
adjusted so that a movement of one-tenth of a millimetre corres- 
ponded approximately to a fall in potential of one volt, it was possible 
to measure without difficulty a change of a fraction of a volt in the 
potential of the gold leaf. 
