APPENDIX IV.
ON THE DISELECTRIFICATION OF METALSAND OTHER BODIES BY LIGHT.
Referring to a footnote to my Royal Institution lecture, onpage 11, Messrs. Elster and Geitel have been good enough to call my attention to a great deal of work done by them in the same direction. To make amends for my ignorance of this work at the time of my Royal Institution lecture, and to make it better known in this country, I make abstract of their Papers as follows:—
With a view to Arrhenius’ theory concerning atmospheric electricity, we arranged experiments on the photo-electric power of sunlight and diffuse daylight at Wolfenbüttel from the middle of May to the middle of June, 1889. Hoor alone had observed the effect of sunlight; other experimenters had failed to find it, but we find a discharging effect even in diffuse daylight.
We take an insulated zinc dish, 20 cm. diameter, connect it to a quadrant electrometer or an Exner’s electroscope, and expose it in the open so that it can be darkened or illuminated at pleasure. Sunlight makes it lose a negative charge of 300 volts in about 60 seconds. A positive charge of 300 volts is retained. The dissipation of negative electricity ceases in the dark, and is much weakened by the interposition of glass. But light from the blue sky has a distinct effect. Fill the dish with water, or stretch a damp cloth over it, and the action stops. A freshly-scrubbed plate acquires a positive charge of 2½ volts, which can be increased by blowing.
With freshly-cleansed wires of zinc, aluminium, or magnesium attached to the knob of the electroscope, a permanent negative charge is impossible in open sunlight. Indeed, magnesium shows a dissipating action in diffuse evening light. Such wires act like glowing bodies. Exposing an electroscope so provided in an open space it acquires a positive charge from the atmosphere. No abnormal dissipation of positive electricity has been observed.
Our success last time was largely due to the great clearness of the sky in June, and we wished to see if we could get the same effect at the beginning of the winter.
The following is our summary of results:
Bright fresh surfaces of the metals zinc, aluminium, magnesium were discharged by both sun- and daylight when they were negatively charged; and they spontaneously acquired a positive charge, whose amount could be increased by blowing.[35]A still more notable sensitiveness to light is shown by the amalgams of certain metals, viz., in the order of their sensitiveness, K, Na, Zn, Sn. Since pure mercury shows no effect, the hypothesis is permissible that the active agent is the metal dissolved in the mercury. If so, the following are the most active metals:—
K, Na (Mg, Al), Zn, Sn.
All other metals tried, such as Sn, Cd, Pb, Cu, Fe, Hg, Pt, and gas carbon, show no action. The same is true of nearly all non-metallic bodies; but one of them—namely, the powder ofBalmain’s luminous paint—acted remarkably well in sunlight. Of liquids, hot and cold water, and hot and cold salt solution were completely inactive; consequently wetting the surface of metals destroys their sensibility to light.
The illumination experiments can be arranged in either of two ways. For experiments in free space we use zinc, aluminium, or magnesium wires, or small amalgamated spheres of zinc provided with an iron rod. With these it can be easily shown that the illuminated surface of certain metals act in the same way as a flame collector.
For demonstration experiments the apparatus described[36]is better, and with this we show the following:—
Amalgamated zinc, negatively charged, discharges almost instantly in sunlight; and if near a positively-electrified body charges itself positively.
The same thing happens, though more slowly, in diffuse daylight. Red glass stops the action, but the following let some through:—Selenite, mica, window glass, blue (cobalt) glass.
Fig. 59.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′.
Fig. 59.
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′.
If sparks are just able to occur between a brass knob and a cleanamalgamated 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.)
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 authors point out analogies between the above effects and those they had observed in the action of glowing bodies in air, and theymention 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.Explanation of Fig. 60.—The sodium and mercury are introduced through the tube S into the globe K. The tube S is then closed, a pump applied to X, and exhaustion carried on for some days. T is an open funnel sealed into the tube (as is done in some vacuum tubes made by Holtz) to show a curious unilateral conductivity of rarefied gas. The object of this funnel is to permit metal from the interior, free from scum, to be introduced from K to D when the whole is tilted. Thus a bright surface is exposed to the earth ring R. It can be charged negatively, and its leak under illumination be measured, through the terminal D. Sometimes the tube is inverted, so that the active surface may be at D′, further from the earth wire.
Fig. 60.
Explanation of Fig. 60.—The sodium and mercury are introduced through the tube S into the globe K. The tube S is then closed, a pump applied to X, and exhaustion carried on for some days. T is an open funnel sealed into the tube (as is done in some vacuum tubes made by Holtz) to show a curious unilateral conductivity of rarefied gas. The object of this funnel is to permit metal from the interior, free from scum, to be introduced from K to D when the whole is tilted. Thus a bright surface is exposed to the earth ring R. It can be charged negatively, and its leak under illumination be measured, through the terminal D. Sometimes the tube is inverted, so that the active surface may be at D′, further from the earth wire.
Using the light from sparks admitted through a quartz window into the vacuum tube when a negatively charged amalgamated zinc surface was exposed near an earth-connected platinum ring, and between the poles of a small electromagnet, they found that when the tube was full of air at10 mm. pressure the magnet had but little effect, but that at 0·15 mm., whereas without the magnet the charge of -270 volts disappeared completely in five seconds, when the magnet was excited it only fell about half that amount in the same time. With hydrogen at 0·24 mm. the result was much the same, and at either greater or less pressure in both cases the magnet had less effect. In oxygen the loss of charge was not quite so rapid; and, again, at a pressure of O·1mm., the magnet more than halved the rate. But in CO₂ the rapidity of loss was extreme.[38]Either at 1·1 mm. or at 0·005 mm. the charge of 270 volts leaked away completely in two seconds when the magnet was not excited; but in the latter case (low pressure) exciting the magnet reduced the speed by about one-half. At the pressure of 1·1 mm. the magnet did not seem to produce an effect. With daylight the results are similar.
Fig. 61.Explanation of Fig. 61.—P is the plate of amalgamated zinc, and R is the earth ring, as before. Ultra-violet light is introduced through a quartz window Q from a spark gapr. The vessel has a joint at the middle, so that the sensitive plate can be got at and changed. Magnet poles are applied outside this vessel in various positions.
Fig. 61.
Explanation of Fig. 61.—P is the plate of amalgamated zinc, and R is the earth ring, as before. Ultra-violet light is introduced through a quartz window Q from a spark gapr. The vessel has a joint at the middle, so that the sensitive plate can be got at and changed. Magnet poles are applied outside this vessel in various positions.
The authors then discuss the meaning of the result, and its bearing on the opposition hypotheses of Lenard and Wolf and of Righi. Lenard and Wolf’s view is that the loss of negative electricity is due to dust disintegrated from the surface by the action of light, but whose existence they consider is established by an observed effect on steam jets. Righi, on the other hand, believes that gas molecules themselves act the part of electric carriers. Elster and Geitel consider that the magnetic effect observed by them supports this latter view, it being known that a magnet acts on currents through gases; and they surmise that the impact of light vibrations may directly assist electric interchange between a gas molecule and the surface, by setting up in them syntonic stationary vibrations, something like resonant Leyden jars. It is to be remembered that phosphorescent substances, such as Balmain’s paint powder, exhibit marked photo-electric effect in daylight.
Fig. 62.Explanation of Fig. 62.—A simpler arrangement, like the one above (Fig. 61), whereby clean liquid alkali metals can be introduced into the experimental chamber B, from the preliminary chamber A, through a cleansing funnel, F, which dips its beak into the interior.
Fig. 62.
Explanation of Fig. 62.—A simpler arrangement, like the one above (Fig. 61), whereby clean liquid alkali metals can be introduced into the experimental chamber B, from the preliminary chamber A, through a cleansing funnel, F, which dips its beak into the interior.
The unilateral character of the electric motion, and the charging of neutral surfaces by light, require special hypotheses, concerning an E.M.F. at the boundary of gases and conductors, such as Schuster and Lehmann have made.
A vacuum tube suitable for experiments with sodium-amalgam or pure sodium, or the liquid sodium-potassium alloy, is described, with the aid of which a current (shown by the charge of an electroscope) can be maintained by a dry pile through the rarefied gas above the metal when it is illuminated from ordinary windows.
Experiments also on differently-coloured lights. Summary of results. The photo-electrically active metals arrange themselves in the following order—Pure K, alloys of K and Na, pure Na. Amalgams of Rb, K, Na, Li, Mg (Tl, Zn); the same as their voltaic order. With the most sensitive term of the series a candle six metres off can be detected, and the region of spectral red is not inactive. The later terms of the series demand smaller waves, and even for potassium blue light gives a much greater effect than red. No discharge of positive electricity is observable with these substances.
Hitherto only Balmain’s paint powder has been observed to be active among non-metallic substances. Now they try other phosphorescent bodies, and arrive at the following results:—
Fluorspar is conspicuously photo-electric, both in sunlight and daylight, especially the variety of fluorite called stinkfluss.
Freshly-broken surfaces discharge much more rapidly than old surfaces.
Blue waves, and not alone the ultra-violet, have a perceptible effect on fluorspar.
In a vacuum the mineral loses its photo-electric sensibility and its conductivity too. Contact with damp air restores its sensibility. Moistening with water weakens but does not destroy the sensitiveness. On the other hand, igniting the mineral destroys both its photo-electric power and its exceptional phosphorescent property.
Distinct traces of photo-electric power are shown by the following minerals also: Cryolite, heavy spar, celestine, arragonite, strontianite, calcspar, felspar and granite.
The hypothesis that the power of phosphorescing when illuminated is approximately a measure of the discharging power of light has been verified in many cases; the exceptions can probably be explained by the influence which the electrical conductivity of the illuminated substance exerts on the rate of discharge of electricity from its surface. This agreement confirms the view expressed by us on the occasion of experiments with Balmain’s paint, that, during electricaldischarge by light, actions take place which are analogous to those of resonance. Messrs. Wiedemann and Ebert had previously been led by other considerations to the same conclusion.
We are compelled by the results of the present experiments to conclude that a more rapid discharge of electricity into the atmosphere takes place in sunlight than in darkness from the surfaces of the earth, which is composed of mineral particles charged, as the positive sign of the slope of atmospheric potential indicates, with negative electricity.
Fig. 63.Explanation of Fig. 63.—Arrangement used by Elster and Geitel for exposing various phosphorescent minerals to daylight, while under inductive charge. They were put in powder in the tray P, and the transparent wire-gauze N above them was charged positively from a battery. The metal cover M M′ could be removed and replaced at pleasure, and the effect on a delicate quadrant electrometer connected to P observed. By this method considerable tension can be got up on the mineral surface, notwithstanding that it is close upon zero potential. The light effect depends on tension, not potential.
Fig. 63.
Explanation of Fig. 63.—Arrangement used by Elster and Geitel for exposing various phosphorescent minerals to daylight, while under inductive charge. They were put in powder in the tray P, and the transparent wire-gauze N above them was charged positively from a battery. The metal cover M M′ could be removed and replaced at pleasure, and the effect on a delicate quadrant electrometer connected to P observed. By this method considerable tension can be got up on the mineral surface, notwithstanding that it is close upon zero potential. The light effect depends on tension, not potential.
It seems to us evident that there exists a direct electric action of sunlight upon the earth, and that we have given experimental evidence in favour of the theory put forward by von Bezold and Arrhenius, according to which the sun acts on the earth, not by electrostatic or electro-dynamic action-at-a-distance, which would involve difficulties of a theoretical character, but through the medium of the electrical forces of light waves. We hope soon to establish the consequences of this theory in meteorology in another Paper, giving the results of two years’ observations on the intensity of the most refrangible rays of sunlight and of the slope of atmospheric potential.
Elster and Geitel describe the observations they have made for two years on solar radiation, at observing stations of low and high altitude, as tested by its electrical discharging power; and they plot curves of such effective radiation for days and months along with the curves of atmospheric potential observed at the same places. These curves are of much interest, and need study. Incidentally they find that, of the whole effective solar radiation, 60 per cent. was absorbed at altitudes above 3,100 metres; 23 per cent. of the remainder was absorbed in the layer between this and a station at 1,600 metres; and 47 per cent. was absorbed between this and 80 metres above sea level. Or, in other words, of 236 parts which enter the atmosphere 94 reach the highest observing station (Sonnblickgipfel), 72 the middle one (Kolm-Saigurn), and 38 the lowest (Wolfenbüttel). They discuss the question as to how far the daily variation of terrestrial magnetism is due to electrical currents in the atmosphere excited by sunshine and other meteorological matters.
(The Paper and plates are worthy of reproduction in full in thePhilosophical Magazine.)
Results:—The resistance of a Geissler tube provided with a cathode surface of pure alkali metal is diminished by the light from the sparks of an induction coil; especially when the pressure is ·1 to ·01 mm. of mercury. The resistance which rarefied gas opposes to an electric current in a magnetic field is greatest in the direction normal to the magnetic lines. The changes of resistance effected by any kind of light in a vacuum tube with alkali metal cathode can be measured galvanometrically. (A Daniell cell gives 100 divisions on a Rosenthal galvanometer when coupled up through such an illuminated tube, each division meaning about 10⁻¹⁰· ampere.)
Fig. 64.Explanation of Fig. 64.—A vacuum tube of rarefied hydrogen containing alkali metal as cathode, say the liquid K-Na alloy, or solid K or Na. A spark gap at S serves as alternative path, and a stream of sparks can occur to the plate P in the dark. But when light falls on the surface A this stream of sparks can cease, showing that the resistance of the vacuum tube is diminished.Fig. 65.Explanation of Fig. 65.—Showing position of magnetic poles with respect to the vacuum tube discharge. With the polesacrossthe line of discharge, as in Fig. on left, excitation of the magnet opposes the leak from the surface. With the poles as in Fig. on right, the discharge is not much affected, it is even sometimes slightly increased.Fig. 66.Explanation of Fig. 66.—Potassium vacuum bulbs containing ⅓ millimetre of hydrogen mounted and connected to battery and galvanometer, and arranged as a photo-electric photometer.
Fig. 64.
Explanation of Fig. 64.—A vacuum tube of rarefied hydrogen containing alkali metal as cathode, say the liquid K-Na alloy, or solid K or Na. A spark gap at S serves as alternative path, and a stream of sparks can occur to the plate P in the dark. But when light falls on the surface A this stream of sparks can cease, showing that the resistance of the vacuum tube is diminished.
Fig. 65.
Explanation of Fig. 65.—Showing position of magnetic poles with respect to the vacuum tube discharge. With the polesacrossthe line of discharge, as in Fig. on left, excitation of the magnet opposes the leak from the surface. With the poles as in Fig. on right, the discharge is not much affected, it is even sometimes slightly increased.
Fig. 66.
Explanation of Fig. 66.—Potassium vacuum bulbs containing ⅓ millimetre of hydrogen mounted and connected to battery and galvanometer, and arranged as a photo-electric photometer.
Attempts to make such a potassium cell into a photometer.
Plates of platinum, silver, copper need exceedingly ultra-violet light before they show any photo-electric power; zinc, aluminium, magnesium show it for visible violet and blue light; the alkali metals, in an atmosphere of rarefied hydrogen, advance their range of sensibility into the spectral red; while under the most favourable conditions they show a sensibility only inferior to that of the eye itself. The authors now use galvanometric methods of measuring the effect, instead of only electrometers, and they arrive at the following results:—
(1) The three alkali metals Na, K, Rb, have different sensibility for differently-coloured lights. For long waves their order of sensibility is Rb, Na, K; though rhubidium is far exceeded by the other two metals in white light.(2) Illumination of a plane alkali metal cathode surface with polarised light causes greatest discharge if the plane of polarisation is normal to plane of incidence; and least, if the two coincide.(This is a most remarkable observation. Its probable meaning is that the electric oscillations of light are photo-electrically effective in so far as they are normal to the surface on which they act; while electric oscillations tangential to the surface are scarcely operative. Different angles of incidence must be tried before the proof is complete.—O. J. L.)(3) Electric oscillations of very short period, such as are given by a Hertz oscillator, are commutated by illumination in the presence of alkali metals in rarefied gas, so as to be able to set up a constant electric tension in the gas.(A Zehnder tube[39]was used, and the momentary phases of the oscillation during which the metal is negatively charged are apparently taken advantage of by the illumination.)(4) The photo-electric dissipation showed by powdered fluorspar is dependent on the colour of the mineral, in such a way that the deepest blue, violet or green specimens are the most sensitive.
(1) The three alkali metals Na, K, Rb, have different sensibility for differently-coloured lights. For long waves their order of sensibility is Rb, Na, K; though rhubidium is far exceeded by the other two metals in white light.
(2) Illumination of a plane alkali metal cathode surface with polarised light causes greatest discharge if the plane of polarisation is normal to plane of incidence; and least, if the two coincide.
(This is a most remarkable observation. Its probable meaning is that the electric oscillations of light are photo-electrically effective in so far as they are normal to the surface on which they act; while electric oscillations tangential to the surface are scarcely operative. Different angles of incidence must be tried before the proof is complete.—O. J. L.)
(3) Electric oscillations of very short period, such as are given by a Hertz oscillator, are commutated by illumination in the presence of alkali metals in rarefied gas, so as to be able to set up a constant electric tension in the gas.
(A Zehnder tube[39]was used, and the momentary phases of the oscillation during which the metal is negatively charged are apparently taken advantage of by the illumination.)
(4) The photo-electric dissipation showed by powdered fluorspar is dependent on the colour of the mineral, in such a way that the deepest blue, violet or green specimens are the most sensitive.