64. Testing apparatus. The arrangement shown in Fig. 17 is very convenient for many measurements in radio-activity. Two parallel insulated metal plates A and B are placed inside a metal vessel V, provided with a side door. The plate A is connected with one terminal of a battery of small storage cells, the other pole of which is earthed; the plate B with the electrometer, and the vessel V with earth. The shaded areas in the figure indicate the position of ebonite insulators. The active material to be tested is spread uniformly in a shallow groove (about 5 cms. square and 2 mms. deep) in the brass plate A. In order to avoid breaking the battery connection every time the plate A is removed, the wire from the battery is permanently connected with the metal block N resting on the ebonite support. In this arrangement there is no possibility of a conduction leak from the plate A to B, since the earth-connected vessel V intervenes.