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 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, E and F, connected with earth.
56. A modified form of the gold-leaf electroscope can be used
to determine extraordinarily minute currents
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 ionization
of the gas is shown in Fig. 12.
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Fig. 12.
This type of instrument was first used by C. T. R. Wilson[1] 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 system, consisting of a narrow strip of gold-leaf L attached to a flat rod R, is insulated inside the vessel by the small sulphur bead or piece of amber S, supported from the rod P. In a dry atmosphere a clean sulphur bead or piece of amber is almost a perfect insulator. The system is charged by a light bent rod CC´ passing through an ebonite cork[2]. 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
