ut] METHODS OF MEASUREMENT 71 
radiation emitted by the active substance. Unless very active 
material is being examined, the difference of potential between P 
and P’ can easily be made sufficient to produce saturation. 
When necessary, a correction can readily be made for the rate 
of leak when no active material is present. In order to avoid 
external disturbances, the plates PP’ and the rod C, are surrounded 
by metal cylinders, # and F’, connected with earth. 
51. A modified form of the gold-leaf electroscope can be used 
to determine extraordinarily minute cur- 
rents with accuracy, and can be employed 
in cases where a sensitive electrometer is 
unable to detect the current. A special 
type of electroscope has been used by 
Elster and Geitel, in their experiments on 
the natural ionization of the atmosphere. 
A very convenient type of electroscope to 
measure the current due to minute 1oniza- 
tion of the gas is shown in Fig. 11. 
This type of electroscope was first used 
by C. T. R. Wilson? in his experiments of 
the natural ionization of air in closed 
vessels. A brass cylindrical vessel is taken 
of about 1 litre capacity. The gold-leaf i hs 
system consisting of a narrow strip of gold-leaf Z attached to a flat 
rod R is insulated inside the vessel by the small sulphur bead S, 
supported from the rod P. In a dry atmosphere a clean sulphur 
bead is almost a perfect insulator. The system is charged by a 
light bent rod CC’ passing through the ebonite cork D. The rod 
C is connected to one terminal of a battery of small accumulators 
of 200 to 300 volts. If these are absent the system can be charged 
by means of a rod of sealing-wax. The charging rod CC” is then 
removed from contact with the gold-leaf system. The rods P and 
C and the cylinder are then connected with earth. 
The rate of movement of the gold-leaf is observed by a reading 
microscope through two holes in the cylinder, covered with thm 
mica. In cases where the natural ionization due to the enclosed 





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1 Proc. Roy. Soc. Vol. 68, p. 152, 1901. 
