HALLWACHS

Electric Discharge by Light

In a recent publication Hertz has described investigations on the dependence of the maximum length of an induction spark on the radiation received by it from another induction spark. He proved that the phenomenon observed is an action of the ultraviolet rays of light. No further light on the nature of the phenomenon could be obtained, because of the complicated conditions of the research in which it appeared. I have endeavored to obtain related phenomena which would occur under simple conditions, in order to make the explanation of the phenomenon easier. Success was obtained by investigating the action of the electric light on electrically charged bodies. Already before Hertz, Schuster and after him Arrhenius have described phenomena which seem to be closely connected with that of Hertz. Hertz’ observations go further than theirs in that in them there is given a hint of the first cause of the phenomenon.

The investigations which I have undertaken on the influence of the electric light on electrically charged bodies, were mostly carried out in the following way. A thoroughly cleaned circular plate of zinc about 8 cm. in diameter was set on an insulating stand and connected by a wire with a gold leaf electroscope. In front of the zinc plate and parallel with it stood a large screen of sheet zinc about 70 cm. broad and 60 cm. high. In the middle of this screen was a selenite window through which the rays of a Siemens arc lamp on the other side of it could fall on the plate. The system of plate and gold leaves was well insulated. In a day the divergence diminished by about ¼ while a research was in progress there was no noticeable change. Also if the arc lamp was in action while the window was covered with some suitable substance the insulation remained perfect.

If the plate and the electroscope, upon which latter the rays cannot fall, are negatively charged, then as soon as the light rays fall on the plate the gold leaves begin to come together rapidly; if they are positively charged the leaves at first sight show no motion at all and only after a long time will a more exact examination show a noticeable motion.

In order to obtain a quantitative measure of the rate at which the leaves come together, a scale was marked out on the glass plate of the Beetz electroscope and the displacements of the ends of the gold leaves were observed on this scale through a lens from a distant fixed point. In this way it was found, for example, that if the distance of the freshly cleaned zinc plate from the arc light was 70 cm. in the case of a negative charge, the divergence diminished in 5 seconds by 70 per cent, in 10 seconds the leaves had come together. When the charge was positive there was obtained a diminution of 10 per cent only in 60 seconds. The action can be observed up to very great distances, for example, 3 m. Its intensity depends very much on the size of the arc.

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From the experiments here described it appears that the phenomenon observed is particularly excited by the ultraviolet rays of light. Since magnesium also is effective the cause of the phenomenon cannot be electrostatic forces. An impairment of the insulation cannot explain the effect, because then both positively and negatively charged bodies would be affected in similar ways. The experiments on the absorption of the action by the interposition of bodies forbids us to look for the cause in material particles which are shot out from the arc; also these would be kept from the plate by the screen placed before it. Finally, that a rise of temperature caused by the rays cannot be used to explain the effect appears from this, that the red and infrared rays are inactive.

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