The connections are clearly seen from Fig. 11. The active material is placed on a plate laid on top of the fixed circular plate P, connected with the case of the instrument and with earth. The upper insulated plate P´ is connected with the insulated gold-leaf system LL´. S is an insulating support and L the gold-leaf.
The system is first charged to a suitable potential by means of the rod C. The rate of movement of the gold-leaf is observed by means of a microscope. In comparisons of the activity of two specimens, the time taken by the gold leaf to pass over a certain number of divisions of the micrometer scale in the eye-piece is observed. Since the capacity of the charged system is constant, the average rate of movement of the gold-leaf is directly proportional to the ionization current between P and P´, i.e. to the intensity of the 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 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.