Explanation of Fig. 59.—B′ is a brightly polished amalgamated zinc plate attached to the negative pole of a Holtz machine, with the positive knob from 6 to 10 centimetres distant. The source of a light is a strip of burning magnesium ribbon 30 to 50 centimetres away. Whenever the spark is able just to choose the path B B′, light shining on the zinc plate checks it and transfers the spark to A A′.
Wied. Ann., 39, p. 332.—On a Checking Action
of Illumination on Electric Spark and Brush Discharge.
If sparks are just able to occur between a brass knob and a clean amalgamated zinc cathode, illumination of the latter by ultra-violet light tends to check them. (This is a curious inversion of Hertz’s fundamental experiment on the subject. It is an effect I have not yet observed; but Elster and Geitel’s arrangement differs from mine[37] in that the surfaces are at a steady high potential before the spark, so that light can exert its discharging influence, whereas in mine the surfaces were at zero potential until the spark-rush occurred. Hertz’s arrangement was more like mine, inasmuch as he illuminated the knobs of an induction coil on the verge of sparking. It appears, then, that whereas the action of light in discharging negative electricity from clean oxidisable metallic surfaces is definite enough, its influence on a spark discharge differs according to the conditions of that discharge—in cases of “steady strain” it tends to hinder the spark; in cases of “sudden rush” it tends to assist it.—O. J. L.)
Wied. Ann., 41, p. 161.—On the use of
Sodium-Amalgam in Photo-electric Experiments.
Elster and Geitel have repeated some of Righi’s experiments on the discharge of negative electricity from metals in rarefied air, and find, in agreement with him, that a reduction of pressure to about one millimetre increases the discharge velocity about six or seven times. They proceed to try sodium-amalgam exposed to daylight in exhausted tubes, and describe apparatus for the purpose. Such an arrangement simply cannot hold a negative charge in bright daylight, even although it be unprovided with quartz windows. Even paraffin lamps and sodium flames exert some action.
They observe that under the action of light the boundary surface of the metal and glass changes, and the metal begins to cling to the glass. They suppose that Warburg’s vacuum tubes of pure sodium may behave similarly, and show photo-electric sensibility.
The Same, p. 166.—On a Checking Action of Magnetism
on Photo-Electric Discharge in Rarefied Gases.
The authors point out analogies between the above effects and those they had observed in the action of glowing bodies in air, and they mention Lenard and Wolf’s experiments (Wied. Ann. XXXVII., p. 443), tending to show that the effect is due to a disintegrating or evaporative effect of light on surfaces. Elster and Geitel had observed that the discharging power of glowing bodies was diminished by application of a magnetic field, the effect being the same as if the temperature was lowered; and they proceed to try if the discharge of negative electricity from illuminated surfaces in highly-rarefied gas could also be checked or hindered by a magnetic field. They find that it can.
Fig. 60.