Page:Radio-activity.djvu/174

 a type of secondary radiation, set up at the surfaces on which the [Greek: alpha] rays fall. The particles would be extremely readily absorbed in the gas, and their presence would be difficult to detect except in low vacua. J. J. Thomson at first obtained no evidence that the [Greek: alpha] particles of polonium were charged; but in later experiments, where the plates were closer together, the electro-*scope indicated that the [Greek: alpha] rays did carry a positive charge.

In order to see whether the positive charge due to the [Greek: alpha] rays from radium could be detected when the slow moving ions were prevented from escaping by a magnetic field, I placed the apparatus of Fig. 33 between the pole-pieces of a large electro-*magnet, so that the magnetic field was parallel to the plane of the plates. A very marked alteration was observed both on the magnitude of the positive and negative currents. In a good vacuum, the upper plate received a positive charge, independently of whether the lower plate was charged positively or negatively or was connected with earth. After the magnetic field had reached a certain value, a great increase in its strength had no appreciable effect on the magnitude of the current.

The following table illustrates the results obtained when the two plates were 3 mms. apart, and were both coated with thin aluminium foil.

Current in arbitrary units //\-\ Potential of   Without magnetic   With magnetic lower plate         field            field 0                   —               +·36 +2 volts              2·0              +·46} } ·39 -2  "                2·5              +·33}

+4  "                2·8              +·47}                                            } ·41 -4   "                3·5              +·35}

+8  "                3·1              +·56}                                            } ·43 -8   "                4·0              +·31}

+84 "                3·5              +·77}                                            } ·50 -84  "                5·2              +·24}

Let n be the number of [Greek: alpha] particles, carrying a charge e, which are absorbed in the upper plate. Let [Greek: iota]_{0}, see next page] be the current due to the slight ionization of the residual gas.

If only a small potential is applied to the lower plate, this current should be equal in magnitude but opposite in sign when