material, it is necessary to employ methods for measuring much smaller currents than can be detected conveniently by an ordinary galvanometer. The most convenient apparatus to employ for this purpose is one of the numerous types of quadrant electrometer or an electroscope of special design. For many observations, especially where the activity of the two substances is to be compared under constant conditions, an electroscope offers a very certain and easy method of measurement. As an example of a simple apparatus of this kind, a brief description will be given of the electroscope used by M. and Mme Curie in many of their earlier observations.
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Fig. 11.
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 eyepiece 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
