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

1. ↑ Wilson, Proc. Roy. Soc. Vol. 68, p. 152, 1901.
2. ↑ If the apparatus is required to be air-tight, the gold-leaf system can be charged by means of a piece of magnetized steel wire, which is made to touch the rod R by the approach of a magnet.