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 plates41. 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