# Collected Physical Papers/On the Continuity of Effect of Light and Electric Radiation on Matter

First published in the Proceedings of the Royal Society of London, Vol 70.
XIV
ON THE CONTINUITY OF EFFECT OF LIGHT AND ELECTRIC RADIATION ON MATTER
Though the theory of coherence gives a simple explanation of many cases of diminution of resistance in a mass of metallic particles subjected to electric radiation, yet there are other cases which cannot be explained by that theory. If coherence be due to electric welding, it would follow that all sensitive particles would exhibit a permanent diminution of resistance; in other words, the action should be non-discriminative and there should be no self-recovery. I have shown, however, in a previous paper, that the effect of radiation is by no means non-discriminative. On the contrary, while its effect on the positive class of substances, e.g., Mg, Fe, Ni, is a diminution of resistance, its reaction on the negative class, e.g., K, Ag, Br, I, is precisely the opposite, namely an increase of resistance. Further, the conductivity change is not always permanent, since several substances are known which quickly recover and attain their original conductivity on the cessation of radiation, as if a force of restitution were called forth to restore them to their original condition.
I was thus led to suppose that the effect of radiation is to produce a state of molecular strain. Evidences will presently be adduced which will furnish proofs in support of this view.
If a substance is molecularly distorted by the action of an external agent, we may naturally expect that there would be produced changes in the physico-chemical ​property of the substance. As a familiar instance phosphorous is changed from the yellow to the red variety by the action of visible radiation. After the allotropic transformation into the red variety, phosphorus has become chemically less active, insoluble in CS$_2$, and of higher specific gravity. Similarly its other properties, such as its elasticity, its position in the voltaic series, its electric conductivity, etc., undergo a corresponding modification. An identical molecular phenomenon, seen from different aspects, may thus appear to be diverse. Looking from an electric point of view it is found that the conductivity of red phosphorus is greater than that of the yellow variety. It is thus possible to measure the induced molecular change by measuring the correlated variation of any of the properties described above. The choice of a particular method will be governed by special convenience under given conditions.
It would thus be possible to detect the effect of molecular strain induced by visible or invisible radiation by the following more or less sensitive methods. It is to be borne in mind that the effect of radiation is almost confined to the skin or outer layer of the substance.
(1) Method depending on the variation of the adhesive or cohesive power of a substance, e.g., in a daguerreotype plate, the mercury vapour adheres by preference to the light-impressed portions only. Images may in a similar manner be developed by water vapour.
(2) Method depending on the variation of chemical activity undergone by the strained substance, or the method of photographic development. The acted and unacted portions are ​differently attacked by the developer. The action is not altogether independent of the effect described below.
(3) Method depending on the variation of electric potential, by which an E. M. F. is produced between the acted and unacted portions of a substance originally iso-electric.
(4) Method depending on the conductivity variation produced by the strain.
In the following investigations I shall employ specially the two last methods, and hope to demonstrate the fundamental unity of effects of visible and invisible radiation on matter. The subject is very extensive, and I propose to deal with it, as briefly as is compatible with clearness, in the three accompanying papers:—
I. "On the Continuity of Effect of Light and Electric Radiation on Matter." In this paper various experimental methods and results will be given, which can only be explained on the supposition that the observed effects are due to strain.
II. "On the Similarities between Mechanical and Radiation Strains." If the effects as described above are really due to strain, then similar results might be brought about by artificially producing strain in regard to which there can be no possibility of doubt, such as strain produced by mechanical means.
III. "On the Strain Theory of Photographic Action." Having shown the strain effect due to light, I will show how some of the obscure phenomena in photography receive a simple explanation on the above theory.
Effect of Electric Radiation
1. Method of Conductivity Variation
This method is specially suited for studying the effect of electric radiation on discontinuous particles. Since the action of radiation is one of surface, the larger is the superficial area the greater is the result; it is evident that loose particles expose a large surface to the incident radiation. Moreover, as the effective total resistance of the mass of particles is due to resistances of surface contacts, any change induced in the surface layers will cause great variation of the total resistance. In a continuous solid, on the other hand, only a comparatively thin molecular layer on the surface is acted upon; but this has little effect on the conductivity of the mass in the interior, protected by the outer conducting sheet. A slight conductivity variation can, however, be detected if the continuous solid takes the form of an extremely thin layer. I shall presently show that for the detection of molecular strain in a continuous solid the electromotive variation is the more suitable.
It has been stated that in the positive class there is produced a diminution, and in the negative an increase of resistance. These opposite reactions seem at first difficult to understand, but about their reality there can be no doubt. In another paper to be shortly communicated, I shall give an account of an independent inquiry in which the positive, the neutral, and the negative classes of substances are differentiated by their characteristic curves. From the strongly positive substance like Fe, Mg, to the pronounced negative like K, there are numerous gradations. In silver we have a ​material which is almost on the line of demarcation; it passes easily from one condition to another under the influence of external variations.
2. External Change
If the response, positive or negative, is really an expression of some changed molecular condition, we may expect it to be modified not only by the chemical nature of the substance, but also (1) by previous history of the substance, (2) by temperature, and (3) by pressure.
Influence of previous history.—As regards the first, I have already shown in a previous paper that a substance strained by radiation often exhibits opposite or reversal effects. Freshly powdered particles often show erratic results, but the effect becomes consistent after annealing, which also increases its sensibility evidently by increasing the molecular mobility. The improvement in sensitiveness often obtained by shaking of the particles is no doubt due to the same cause. In the various types of molecular receivers, whether responding to electric radiation, light, or mechanical vibration, similar effect is observed. The receiver generally speaking, improves gradually with working; but after long-continued action it gets overstrained and exhibits fatigue.
Influence of temperature.—As regards temperature, I have often found that on excessively cold days the receivers exhibit a diminution of sensibility, which becomes restored by warming. Various reactions which were very strongly exhibited in the warm climate of India, I found to be much diminished in Europe. Cautious application of heat often increases, not only the sensibility, but also the power of self-recovery. ​High temperature, however, produces erratic behaviour by causing violent molecular disturbance.
Influence of pressure.—Pressure also has pronounced effect on molecular response. Moderate increase of pressure increases the sensibility, but too great an increase may cause a loss of sensibility. In substances which are nearly neutral, pressure variation may even cause reversal of response.
The same receiver may, owing to some obscure molecular modification, exhibit a response opposite in sign to the normal. But under continued stimulation of radiation, the abnormal response becomes converted into the normal. Parallel instances will also be noticed in the case of response to mechanical stimulus and to light. It is thus seen how the response is dependent on the molecular condition, and how a change of this condition may culminate in a reversal of response, say, from a diminution to an increase of resistance. The nature of the chemical substance, the molecular condition, the intensity and duration of radiation, the pressure, the temperature, and even the electromotive force acting on the circuit, are the factors instrumental in the determination of the resultant response. I have already shown how the cumulative action of continuous radiation may produce molecular reversal. There may thus be one or more reversals.
3. Recording Apparatus
In the following investigations, it is necessary to observe the conductivity or the electromotive variations, induced by external stimulus of various durations. It is also necessary to note the time-relations of direct and after-effects. The conductivity and electromotive ​variations can be deduced from the observed deflections of the galvanometer. When these are rapid, the observation requiring great alertness becomes very fatiguing. This difficulty is still greatly accentuated when simultaneous time-observations have to be taken. It thus becomes necessary to have at least two observers; the process of observation is very tedious, and the accumulation of results extremely slow. But in the apparatus now to be described, the mode of procedure has been very much simplified, affording facilities for quick and accurate record of responsive reactions.
The moving platform of the apparatus carries a squared paper (divided into 1/10 inch) on which the record is made. The platform moves uniformly by clock-work, and the rate of travel of the paper may be
roughly adjusted by means of different-sized pulleys, or more finely by the clock-work governor. The usual ​rate is 1 inch in 30 seconds, and one small division of the paper measured horizontally is thus equal to 3 seconds. But very much quicker or slower rates may easily be obtained by means of the different-sized pulleys. The spot of light from the galvanometer is thrown down on the paper by an inclined mirror. The movement of the galvanometer spot takes place at right angles to the direction of motion of the paper. There is a guide rod at right angles to the motion of paper, along which the recording pencil is moved. The excursion of the galvanometer spot can thus be easily followed with a pencil, and it is quite easy to do this, when the fluctuation period is about 2 seconds. In the experiments to be described, this period varied from 2 seconds to several minutes. A curve is thus directly obtained, with conductivity or electromotive variation as ordinate, and the time as abscissa. The curves given in the accompanying papers are exact copies of the direct records.
4. Transition of a Molecular Receiver from Non-recovering to Self-recovering Condition
When a substance is strained by radiation there is produced a sudden variation of conductivity. The substance automatically recovers from the strained condition (1) if it has not been overstrained by an excessive stimulus, or (2) if its electric elasticity is very great. I have found, in general, that after careful adjustment of the receiver it exhibits tendency towards self-recovery, if the intensity of incident radiation is not too strong. In the case of substances which are, electrically speaking, highly elastic, such as K, there is an automatic recovery even after strong stimulation. The difference ​exhibited by various substances in regard to self-recovery is one merely of degree. I give below typical cases which exhibit the gradual transition from so-called non-recovery to complete self-recovery.
The galvanometer used is a dead-beat D'Arsonval of moderate sensitiveness. The receiver was appropriately fixed on a heavy base. This was supported on one or two sets of pneumatic tyres so as to protect the sensitive receiver from mechanical vibration.
Positive Type.—In fig. 33 (a) is shown the effect of radiation on Fe$_3$O$_4$ when cold. Only the upper portion of the curve is given; the flash of radiation produced a deflection of the galvanometer of sixty-four divisions.
It will be observed that it had recovered to the extent of three divisions in the course of 60 seconds; if the rate ​of recovery had been uniform, there would have been complete recovery in about 21 minutes, but in the later stages, the recovery is very slow. In (b) is shown the effect of increased molecular mobility due to cautious warming. There was now semi-recovery in 60 seconds; the quickness of recovery went on improving, and after a while the recovery was completed in 30 seconds.
Negative Type.—Fig. 34 exhibits the effect on the negative type as exemplified by Sn and Pb coated with Br. It appears that warmth is favourable to quick recovery; for in India with Pb coated with Br the self-recovery was obtained with the greatest ease. In the colder climate of England it takes a longer time for
the receiver to become adjusted to the condition of self-recovery. At first the flash of radiation produces a sudden increase of resistance, from which there is no immediate recovery, but an inspection of the ​galvanometer spot at once makes it evident that some internal struggle has been going on; the spot trembles, as if to overcome some internal friction; the substance then exhibits a sudden recovery. After these preliminary molecular adjustments the recovery becomes perfectly automatic and instantaneous. Each flash of radiation then produces a responsive galvanometer twitch, immediately followed by recovery.
Some of the stages are well seen in the curves given. In (a) the recovery takes place in 50 seconds; in (d) in only 6 seconds; (e) and (f) show recovery in lead coated with bromine. The recovery gradually became very quick, from 4 seconds to 3 seconds; after that it was too quick for record.
In all the above cases it will be noted that the curve of recovery is convex to the abscissa; that is to say, it is at first very rapid, but in the later stage it becomes relatively slow.
5. A Self-recovering and Metrical Receiver
The most perfect type of self-recovering receiver that I succeeded in constructing was made of the strained variety of silver described in a previous paper. It was there shown that this variety of silver exhibits, under the action of radiation, an increase of resistance. I had with me a portion of this variety prepared more than a year ago, and it is probable that time had improved its quality. I made with it a receiver consisting of 3 mm. thickness of the powder between two electrodes; the pressure was adjusted by means of a micrometer screw which pressed one of the electrodes. The applied electromotive force was 0.4 volt, and the resistance of the receiver was equal to 20 ohms. The ​receiver showed the usual increase of resistance with a tendency to self-recovery. In about half an hour it began to exhibit the most perfect self-recovery, and for the next 3 hours of continuous work it went on giving an extraordinary consistency of response.
The intensity of incident radiation was varied by changing the distance of the radiator. In fig. 35 are given responses to individual flashes at distances of 40 and 15 cm. It will be seen that the effects are very
consistent, the occasional variation being probably due to some of the oscillatory sparks not having been as efficient as the others.
Certain analogies with the phenomena of Phosphorescence and Thermo-luminescence.—A remarkable phenomenon will be noticed in the recovery curve. It will be seen that the complete recovery is effected after a series of minor oscillations. In other words, there seems to be an after-vibration which persists for a time in substances subjected to radiation. This is very ​suggestive in regard to a not altogether different after-effect of light in the fluorescent and phosphorescent bodies. In the case of Ag′ receiver, owing to its molecular mobility, the recovery is automatic. But in the case of so-called non-recovering substances, the strain persists for a considerable time; the recovery can, however, be hastened by removing molecular friction through gentle heating. In connection with this, I will quote an interesting observation previously made. In an iron receiver strained by radiation there was quick recovery after heating, and careful inspection showed a slight oscillatory movement of the galvanometer spot during the process. Here the strain produced by radiation remained latent to be released by heat. In the phenomenon of thermo-luminescence, the strain effect of light also remains latent till set free by the application of heat.
Effect of Continuous Radiation.—Still more interesting are the superposed effects of a series of flashes of radiation. The first flash produces a certain molecular distortion, attended with conductivity variation, from which it tends to recover. This force of restitution will be shown to increase with increasing distortion. Now if, before the substance has recovered from the first shock, a second flash be superposed, it will produce further distortion; but the effect will not be quite so strong, inasmuch as the force of restitution is increasing. Thus a series of superposed flashes will produce a limiting effect, which is kept balanced by the force of restitution. Under increased intensity of radiation, the balancing position is at a higher level. The after-effect, moreover, is more persistent under stronger intensity of stimulation.
​In fig. 36 are shown the effects of rapidly succeeding flashes of radiation caused by the spring vibrator of a
​Ruhmkorff's coil. In (a) the radiator was kept at a distance of 40 cm.; the radiation was continued for 15 seconds, after which 15 seconds was allowed for recovery. A longer time would have allowed a more complete recovery, but this would have entailed a great loss of time in the long series of experiments contemplated. The recovery is therefore partial, the return curves not exactly reaching the original starting position. The record of response to the source of radiation at a distance of 40 cm. shows that there is a partial recovery between quickly recurring flashes, observed in the fluctuating response about the balanced position. Now when the intensity of radiation is increased by decreasing the distance of the radiator to 25 cm. (see b), the strain effect persists for a longer time, and the flashes arrive before the substance can have recovered to any extent; there is thus less fluctuation in the balanced position. The intensity of radiation was further increased by placing the radiator at a distance of 15 cm. (see c) the fluctuations now disappeared; and the galvanometer deflection was held rigid as long as the radiation was kept on; in fact, we have here an effect which physiologists describe as "tetanic." On the cessation of radiation there was an immediate recovery. It will be noticed how extraordinarily consistent are the succeeding values of response. The resultant effect being due to the additive effects of numerous flashes, an occasional failure of an individual flash is of little or no importance. The series of responses in (d) was taken after half an hour; it will be noticed how very consistent they are among themselves and how similar to those in (c), showing that even after half an hour's ​continuous work there had been but little fatigue with the attendant change of sensibility.
Relation between the Intensity of Radiation and the Conductivity-variation.—The resistance of the receiver being not very large, the external resistance of the shunted galvanometer and of the cell are not negligible in comparison; the change of deflections is, therefore, not proportional to the variation of resistance. To interpret the absolute values of the deflections, a resistance box was substituted for the receiver, keeping the rest of the circuit just as before. In this way the absolute values of the resistances corresponding to particular deflections were found. Some of these are given on the left-hand side of fig. 36.
The galvanometer deflections, when the radiator was at distances of 40, 25 and 15 cm., were 23, 33 and 42 divisions respectively. Owing to the comparative steadiness of the last two deflections there is no uncertainty about them; but on account of the fluctuation in the deflection when the radiator is at a distance of 40 cm., it is difficult to find the exact value of the deflection; the mean of the various deflections gives twenty-three divisions. The absolute values of resistances corresponding to these deflections are 180, 380, and 1020 ohms. The original resistance being 20 ohms, the variations due to the different radiation intensities are 160, 360, and 1000 ohms.
The intensities of radiation at the above distances may approximately be taken as proportional to 1/40$^2$, 1/25$^2$, 1/15$^2$, or as 14 : 36 : 100. The corresponding molecular effects as measured by the increase of resistance are found to be as 16 : 36 : 100.
​It will thus be seen how accurately the indications of the Ag′ receiver measure the intensity of radiation. Further progress in the study of different phenomena connected with electric radiation has been seriously hampered owing to the want of means for measurement of intensity of electric radiation. But this difficulty, as will be seen from the above, is not insuperable.
The strict proportionality of molecular effect can be taken as true only through a limited range. From the results of various experiments, into the detail of which I can not at present enter, it appears that, generally speaking, the curve of response (with molecular effects as ordinates, and the intensities of stimulus as abscissa) is not a straight line. It is at first slightly convex, then straight, and in the last part concave. It is only in the second part that the curve is approximately straight.
In considering the effect of electric radiation in changing the conductivity of the particles, no explanation can be regarded as complete, unless it explains not only the diminution, but also the increase of resistance; the phenomenon of automatic recovery and the opposite effects exhibited by the same receiver under different molecular conditions, have also to be explained. The increase of resistance of the Ag′ receiver and its instantaneous recovery are directly opposed to the theory of coherence.
The state of balance between the effect induced by radiation and the force of restitution on the one hand, and the different equilibrium positions with different radiation intensities on the other, point to the phenomenon being due to molecular strain produced by radiation.
​Fatigue of the Receiver.—I wished to trace the gradual appearance of fatigue in the Ag′ receiver, and for this purpose kept it acted on with slight intermissions for nearly 3 hours. At the end of that time it began to show unmistakable signs of fatigue. Fig. 37 shows
the effect when the radiator was at a distance of 20 cm.; the deflections were now only twenty-one divisions, whereas before this the deflection was thirty-three divisions with the radiator at the increased distance of 25 cm. Formerly the recovery commenced immediately on the cessation of radiation, now there was a short period of hesitation and then it began to recover somewhat slowly. The extent of recovery also grew less and less, add at last the receiver suddenly exhibited the reversal effect, by showing a diminution of resistance.
A parallel instance under the continued action of light will be demonstrated later.
6. Phenomenon of Reversal
(a) Effect of Sub-minimal Intensity of Stimulus.—Another very curious phenomenon met with is the opposite effects of radiation below and above the critical intensity. I have in a previous paper shown that, whereas the effect of radiation of moderate intensity on As is to induce an increase of resistance, the effect of feeble intensity of stimulus is to produce a diminution. Positive classes of substances exhibit parallel results. The opposite effects of feeble and strong stimulation are exhibited not only under electric radiation, but also under mechanical stimulus (cf. p. 204). This result is certainly very curious, but I will show later on that exactly similar effects are produced under mechanical stimulation.
Possibly connected with the above is the following: A receiver subjected to radiation of moderate intensity, often exhibits a short-lived negative twitch immediately followed by the normal response. This is probably due to the fact that it takes some time for the sensitive substance to absorb the whole amount of incident radiation. The first moiety absorbed may thus fall below the critical intensity, and thus gives the preliminary twitch of opposite sign to the normal which occurs later by the absorption of a larger amount of radiation. The response-curve thus exhibits a negative twitch which precedes the normal.
(b) Reversal due to Overstrain.—I have also shown that overstrain due to the continued action of radiation gives rise to a reversal of response, the reversal being partial or complete. I give below a curve (fig. 38) for Fe$_3$O$_4$ under continuous radiation, where, after the ​maximum effect was reached, there was a distinct trend towards reversal.
The positive class of substances, the normal response of which is positive or diminution of resistance under moderate stimulation, gives negative response (increase of resistance) under sub-minimal stimulus. This is specially noticeable in a receiver rendered inert by prolonged rest. Annealing makes the response normal, as also previous stimulation. Strong and prolonged stimulation tends to reverse the normal negative response into positive. All these peculiarities of response under electric radiation will also be found characteristically exhibited by other receivers responding to the stimulus of light or of mechanical vibration.
7. Physical Nature of the Change
From the fact that the conductivity variation above described takes place In platinum and other noble metals, and the further fact that the action goes on even ​when the substance is kept immersed in a protecting medium such as naphtha, it would appear that the observed effect is primarily due to physical strain. By chemical action it is generally understood that the change is irreversible and in one direction. But in the cases of complete self-recovery exhibited by various substances, there is an automatic return to the original condition. It should, however, be borne in mind that, as a result of the strain, the chemical activity of the substance may undergo a change; a chemical action would occur if the strained substance be immersed in a medium for which it has a relatively stronger attraction.
8. Electromotive Variation produced by Electric Radiation
In consequence of molecular strain produced by electric radiation a difference of potential would likely be induced between the acted and unacted portions of a substance. A voltaic cell could be made with two plates of similar substance; there would then be no P. D. between the two. But on exposing one of the two plates to the effect of radiation, a difference of potential might be established between the acted and unacted plates. The differential effect, if it exists, could then be detected by a galvanometer or an electrometer. There are, however, many difficulties in rendering this method practical. First of all, in making a voltaic combination, some kind of electrolyte is necessary, but unfortunately all electrolytes are opaque to electric radiation. This difficulty could, however, be obviated to some extent by taking an electrolyte which is almost a non-conductor (e.g., amylic alcohol) so as to be partially transparent to ​electric waves. The second difficulty is, however, far more serious for owing to the diffuse action of the comparatively long electric waves it is impossible to shield one plate while exposing the other. If both the plates are equally acted upon, there would then be no electromotive variation. It was only after the conclusion of another line of investigation on the electromotive variation produced by mechanical stimulus that a clue was obtained to overcome the difficulty. I then learnt that the effects of the same stimulus on two pieces of the same metal, forming a voltaic element, are not the same if the molecular conditions of the two are different. Under such a condition a P. D. exists between the two, and stimulation of both causes a variation of the existing electromotive force.
I therefore expected to detect the effect of electric radiation by an induced variation of the original electromotive force. And if the effects are at all parallel to those found in the conductivity variation method (as diminution or increase of resistance) the corresponding effects might be observed by a diminution or an increase of the existing electromotive force.
In carrying out experiments to verify the above suppositions, I found my anticipations to be justified. I at first made a cell by taking two varieties of silver. A piece of cotton wool moistened with amylic alcohol was placed in a glass tube. Ag and Ag′ were placed on opposite sides of the moistened cotton, thus forming a voltaic element, the amylic alcohol acting as the electrolyte. Two electrodes compressed the powder, till a current was observed to flow. As in ​the case of receivers for exhibition of conductivity variation, a careful adjustment of pressure is necessary for obtaining the best result. In order that the effect observed might be purely due to electromotive variation and not to the variation of conductivity, the cell was connected with a capillary electrometer. Owing to the opacity of the electrolyte the intensity of radiation has to be strong; the radiator was therefore placed at a distance of 6 inches from the cell. The incident radiation induced a responsive variation of the electromotive force. Long continued radiation induced a reversal as observed in the case of conductivity-variation previously described.
I give below the results of three experiments with different combinations:—
| | Original E. M. F. | New value after the action of radiation | Percentage reduction. |
| I……… | 1.26 V. | 1.15 V. | 9 |
| II……… | 0.39 V. | 0.312 V. | 20 |
| III……… | 0.065 V. | 0.039 V. | 40 |
I was next desirous of obtaining a continuous record of electromotive changes induced by the continued action of electric radiation. For this purpose I used a galvanometer.
A cell was made, in the way previously described, with two specimens of magnesium powder. Owing to some differences in the two portions there was an initial P. D. of 0.042 V. between the two electrodes. The E. M. F. of the cell was balanced by the potentiometer method, a sensitive galvanometer (with an interposed high resistance) being used as the detector of electromotive variation. Fig. 39 (a) shows the deviation from ​the balanced position by radiation which nearly reduced the potential difference to half its original value. A second experiment [see (b)] gave almost identical results.
Fig. 39. E. M. variation in a Mg receiver. The original E. M. F. was 0.042 V. This was reduced to 0.021 V. by electric radiation.
It is thus seen that the response-curve by electromotive variation under electric radiation is similar to that obtained by the method of conductivity variation.
It has been shown that there is a recovery when the range of electric elasticity of the substance is not narrow, or when the strain is not too great; that on subjecting the substance to the continued action of radiation there is a limiting effect; that too long continued action tends to produce an electric reversal; that too feeble an intensity may give rise to a response of opposite sign to that of the normal. We shall next investigate whether visible radiation produces similar results.
Effect of Light
The molecular effect due to visible radiation may as in the previous case, be detected by the method of conductivity or electromotive variation. That light does produce conductivity variation is seen in selenium. I ​have also succeeded in detecting the effect of light in producing variation of contact resistance in a galena receiver. One and the same receiver responded in the same way when alternately acted on by visible and invisible (electric) radiation. The peculiarities of this universal radiometer were in every way similar to those of detectors for electric radiation.
It is, however, more satisfactory to study the effect of light in producing electromotive variation. Becquerel, Minchin, and others have shown that light produces an electromotive variation in a photo-electric cell. Like electric radiation, the effect of light is not confined to any particular metal or groups of metals, but all metals exhibit an electromotive variation under its action. Two opposite effects are likewise shown; in some cases the potential is raised, in others, the potential is lowered by the action of light.
I now proceed to show the remarkable similarity of the curves of response produced under electric radiation and under light. For the photo-electric cell I used two silver strips fastened by solid paraffin on two sides of a glass plate. The front surfaces were exposed to bromine vapour. The two strips formed the two plates of the photo-electric cell, the electrolyte being common tap-water.
If the two strips are exactly similar, then there is no P. D. between them, and the effect of light on either of the strips is the same. When both the plates are illuminated, there is then no resultant effect. But if the two plates are slightly different, then the effects on the two are not the same. An electromotive variation is induced even when both the plates are exposed to light.
9. Effect of Short Exposure to Light
In fig. 40 (a), is shown the effect of light of 2 seconds duration on one of the two AgBr plates. An incandescent gas-burner was placed at a distance of
​12 inches. Previous maintenance of the plates in the dark enhances the sensitiveness; the cell then responds to feeble light emitted by a candle. The similarity of responses to light and electric radiation is very remarkable (cf. fig. 35). The recovery curve is also convex to the abscissa. The time allowed for recovery was not sufficient to bring the substance back to its original condition. The successive starting-points are therefore slightly ascending. In the last curve in, the series (a) sufficiently long time was allowed, and the substance completely recovered in about 37 seconds. (b) shows the effect of light of about four times the intensity, the source being brought nearer to a distance of 6 inches. The effect is stronger, but not quite equal to four times the effect produced in the last case.
10.Effect of continuous Action of Light
Just as in the case of electric radiation, light produces a maximum effect, corresponding to a given intensity. Fig. 40 (c) shows the effect of continuous light of 30 seconds duration, the source of light being at a distance of 18 inches. Observe the tendency of the curve to become horizontal when reaching the maximum. (d) shows the effect of four times the intensity, the source of light being at a reduced distance of 9 inches. Here, too, the intensity of effect is not quite four times the effect of light at a distance of 18 inches.
11.Reversal Effects
With electric radiation it was found that the effect of a flash was sometimes a preliminary negative twitch immediately followed by the normal response. With light, too, it is frequently found, as has also been ​observed by Minchin, that the immediate result is a transitory negative followed by the normal action. A reversed response is also observed under molecular modification.
If the action of light is prolonged beyond the attainment of maximum effect, a tendency appears towards, or actual reversal.
In fig. 41 is given an interesting series of results showing the growth of fatigue, the different phases culminating in an actual reversal (compare with fig. 37). It will be seen that in the second, response was feebler,
Fig. 41. Fatigue and reversal in AgBr cell.
and a tendency towards reversal had already taken place after an exposure of about 9 seconds. In the third, the response is feebler still. In the fourth, the normal response lasting for 3 seconds, is extremely weak; there is then a reversal of response which is fairly strong. On the stoppage of light the reversed effect persists for some time. After the fourth, the responses become reversed.
It will thus be seen that both electric radiation and light produce similar conductivity and electromotive ​variations. Two opposite effects are observed in both. The response curves are similar in the two cases. Under the action of continued radiation both exhibit a limiting effect. Under too long continued action, both exhibit a tendency towards or an actual reversal.
(Proc. Roy. Soc. June 1901.)