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. [Illustration: Fig. 12.] This type of instrument was first used by C. T. R. Wilson[100] 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[101]. 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.