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 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.

Fig. 17.

An apparatus of this kind is very convenient for testing the absorption of the radiations by solid screens, as well as for making comparative studies of the activity of different bodies. Unless very active preparations of radium are employed, a battery of 300 volts is sufficient to ensure saturation when the plates are not more than 5 centimetres apart. If substances which give off a radio-active emanation are being tested, the effect of the emanation can be eliminated by passing a steady current of air from a gas bag between the plates. This removes the emanation as fast as it is produced.

If a clean plate is put in the place of A, a small movement of the electrometer needle is always observed. If there is no radio-active substance in the neighbourhood, this effect is due to the small natural ionization of the air. We can correct for this natural leak when necessary.

65. We have often to measure the activity due to the emanations of thorium or radium, or the excited activity produced by those emanations on rods or wires. A convenient apparatus for this purpose is shown in Fig. 18. The cylinder B is connected with