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 chapter ). The experiments were made on the active plate when its activity was a minimum, in order to avoid complications due to the presence of [Greek: beta] rays. The film of radium was so thin that only a very small fraction of the [Greek: alpha] rays was absorbed.

Fig. 33.

The active plate A was insulated in a metal vessel D, and was connected to one pole of the battery, the other pole being earthed. The upper electrode, which was insulated and connected with a Dolezalek electrometer, consisted of a rectangular copper vessel BC, the lower part of which was covered with a thin sheet of aluminium foil. The [Greek: alpha] rays passed through the foil, but were stopped by the copper sides of the vessel. This arrangement was found to reduce the secondary ionization produced at the surface of the upper plate. The outside vessel D could be connected with either A or B or with earth. By means of a mercury pump, the vessel was exhausted to a very low pressure. If the rays carry a positive charge, the current between the two plates measured by the electrometer should be greater when A is charged positively. No certain difference, however, between the currents in the two directions was observed, even when a very good vacuum was obtained. In some arrangements, it was found that the current was even greater when the lower plate was negative than when it was positive. An unexpected experimental result was also noticed. The current between the parallel plates at first diminished with the pressure, but soon reached a limiting value which was not altered however good a vacuum was produced. For example, in one experiment, the current between