In order to see whether the positive charge due to the α 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 electromagnet, so that the magnetic field was parallel to the plane of the plates[[151]]. 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.
| Potential of lower plate | Current in | arbitrary units | |
|---|---|---|---|
| Without magnetic field | With magnetic 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 α particles, carrying a charge e, which are absorbed in the upper plate. Let ι₀ 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 the potential is reversed. Let ι1 be the charge per sec. communicated to the upper electrode when the lower plate is charged positively and ι2 the value when charged negatively. Then
ι1 = ι₀ + ne,
ι2 = ι₀ + ne;
adding we get
ι1 + ι2
ne = ------ .