Fig. 8.— Zinc Rod in Arc Light, protected by Glass Screen. The lenses are of quartz, but there is no need for any lenses in this experiment; leakage of electricity begins directly the glass plate is withdrawn.
It is easy to fail in reproducing this experimental result if the right conditions are not satisfied; but if they are it is absurdly easy, and the thing might have been observed nearly a century ago.
Zinc discharging Negative Electricity in Light;
Gold Leaf Electroscope;
Glass and Quartz Panes;
Quartz Prism.
Take a piece of zinc, clean it with emery paper, connect it to a gold leaf electroscope, and expose it to an arc lamp. ([Fig. 8]). If charged positively nothing appears to happen, the action is very slow; but a negative charge leaks away in a few seconds if the light is bright. Any source of light rich in ultra-violet rays will do; the light from a spark is perhaps most powerful of all. A pane of glass cuts off all the action; so does atmospheric air in sufficient thickness (at any rate, town air), hence sunlight is not powerful. A pane of quartz transmits the action almost undiminished, but fluorspar may be more transparent still. Condensing the arc rays with a quartz lens and analysing them with a quartz prism or reflection grating, we find that the most effective part of the light is high up in the ultra-violet, surprisingly far beyond the limits of the visible spectrum[4] ([Fig. 9, next page]).
This is rather a digression, but I have taken some pains to show it properly because of the interest betrayed by Lord Kelvin on this matter, and the caution which he felt about accepting the results of the Continental experimenters too hastily.
It is probably a chemical phenomenon, and I am disposed to express it as a modification of the Volta contact effect[5] with illumination.
Fig. 9.— Zinc Rod discharging Negative Electricity in the very Ultra-violet Light of a Spectrum formed by a Quartz Train. The best discharging light is found far beyond the limits of the visible spectrum.
Return now to the Hertz vibrator, or Leyden jar with its coatings well separated, so that we can get into its electric as well as its magnetic field. Here is a great one giving waves 30 metres long, radiating while it lasts with an activity of 100 h.p., and making ten million complete electric vibrations per second. It is made of four large copper sheets soldered together two and two, strung up by thin rope to a gallery, and each pair connected with the other by several yards of No. 0 pure copper rod, interrupted by a pair of sparking knobs ([Fig. 10]).