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On the Influence of a Magnetic Field on Photoelectric Phenomena
H. Dember. Über die Beeinflussung der Lichtelektrizität durch ein Magnetfeld. Phys. Zeitschr. 21, 508, (1920).
The author discovered a very strong influence of an external magnetic field on the photoelectric activity of diamagnetic metals (bismuth, antimony, palladium). The experiment was arranged as follows: the photoelement was a brass box (for protection from external electrostatic influences), inside which were placed a thin bismuth plate and, opposite it, a brass grid, connected to the corresponding poles of a battery. The positive charge of the bismuth plate was measured with a quadrant electrometer. Illumination was produced by a quartz mercury lamp through a quartz window in the box. The air was pumped out of the box by a rotating Gaede pump and a diffusion pump. To obtain a uniform electric field, the bismuth plate was surrounded by a guard ring. The box was placed between the poles of a Du Bois electromagnet in such a way that the magnetic lines of force were parallel to the electric lines and perpendicular to the plane of the bismuth plate. With an electric field of 0.67 V per 1 cm, the following change in the strength of the photocurrent was obtained (Table 1):
Table 1.
| Strength of the magnetic field | Percentage change of photocurrent |
|---|---|
| 0 Gauss | 0% |
| 600 " | 15 |
| 1000 " | 15.5 |
| 3380 " | 21.0 |
| 4550 " | 23.8 |
| 6800 " | 26.7 |
Reversal of the direction of the magnetic field has no noticeable influence. Increasing the accelerating electric field diminishes the influence of the magnetic field: (Table 2).
Table 2.
| Electric field per 1 cm | Decrease of photocurrent in a magnetic field of 4900 Gauss |
|---|---|
| 0.22 V. | 38.6% |
| 1.98 | 24.3 |
| 6.93 | 17.1 |
| 25.3 | 3.2 |
| 263.0 | 2.1 |
| 710.0 | 1.8 |
A freshly prepared surface and a surface that had been in contact with air for a long time at atmospheric pressure behave, with respect to the influence of the magnetic field, in exactly the same way. Temperature has a very strong effect on the phenomenon. Thus, at the same field of 7460 gauss, the decrease in the photocurrent reached 22% at 25°C and 50% at −79°C (solid carbon dioxide).
The author compares the effect he found with the change in the conductivity and diamagnetic susceptibility of bismuth in a magnetic field, both of these phenomena depending very strongly on temperature. Apparently, in this case we are dealing with the influence of the magnetic field on the free electrons inside the metal, which in bismuth have a very considerable mean free path. Dember indicates, as a possible cause of the effect he observed, a change in the trajectories of the photoelectrons emitted from comparatively deep layers of the metal in the magnetic field.
S. Vavilov.