# 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.
Introduction.
Though the theory of coherence gives a simple explanation of many cases of diminution of resistance in a mass of metallic particles under electric radiation, yet there are other cases which are not explicable 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. In my previous paper, however, I have shown that the effect of radiation is by no means non-discriminative. To the contrary, while its effect on the positive class of substances, e.g., Mg.Fe.Ni, is a diminution of resistance, it acts on the negative class, e.g., K.Ag′.Br.I, in a precisely opposite way, that is to say, it produces in these 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 it is hoped will furnish proofs as to the correctness 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 example, take the case of phosphorus changed from the yellow to the red variety by the action of visible radiation. We find that in the allotropic condition of red, the phosphorus has become less active chemically, 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, &c., are likely to undergo a corresponding modification. The same molecular phenomenon, seen from different aspects, may thus appear to be diverse. Looking from an electric point of view we do find that the conductivity of red phosphorus is greater than that of the yellow variety. We thus see the possibility of measuring the 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.
If the above view is correct then it would be possible to detect the effect of molecular strain due to visible or invisible radiation by the following more or less delicate 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 in preference to the light-impressed portions only. Images may in a similar manner be developed by water vapour. Under the action of electric radiation, particles of certain metals are known to stick together. But this is by no means universal.
(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 fundamendal 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 experiments will be described and results 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 in (1.) are really due to strain, then similar results might be brought about by artificially producing strain regarding which there can be no possibility of doubt, for instance, strain by mechanical means. In this paper I shall show the remarkable parallelism between those two classes of phenomena throughout an extensive range.
III. "On the Strain Theory of Photographic Action." Having shown the strain effect due to light, I will show how some of the most obscure phenomena in photography receive a simple explanation on the above theory.
Effect of Electric Radiation.
1. Method of Conudctivity Variation.
This method is specially well suited for studying the effect of electric radiation on discontinuous particles. For the action of radiation being one of surface, the larger the area of this the greater is the result, and in loose particles the effective surface is very much enlarged. In this case, again, the effective total resistance of the mass of particles being due to resistances of surface contacts, any change in the property of surface layers will greatly modify 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.
I have before said that in the positive class there is produced a diminution, and in the negative an increase of resistance. These opposite properties at first seem 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. In the paramagnetic ​and diamagnetic classes we also come across characteristic differences, though these divisions do not coincide with the classes giving the positive and negative electric touch. There is, however, one similarity; for just as the paramagnetic effect is more intense, so is the conductivity variation associated with the positive, generally speaking, much stronger than the effect on the negative class. 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 circumstances.
2. External Influences.
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 the previous history of the substance, by the temperature, (3) by pressure.
Influence of previous History.—As regards the first, I have already shown in my previous paper that a substance strained by radiation often exhibits opposite or reversal effects. Freshly powdered particles often show erratic results, but the effects become consistent after annealing; this often increases the sensibility also, by increasing the molecular mobility. Fresh particles are sometimes found to exhibit very little sensitiveness. At first I thought that this might be due to some kind of fatigue; the following, however, showed that such could not be the case. I found that in these cases the sensibility was increased by subjecting the substance to strong radiation or even by passing a few induction shocks. The increase of sensitiveness thus produced appears to be due to the removal of molecular sluggishness. 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, the same effect is noticed. Generally speaking, the receiver in the first place improves gradually with working. But as it gets overstrained it exhibits fatigue.
Influence of Temperature.—As regards temperature, I have in many instances found that on excessively cold days some receivers exhibit a diminution of sensibility, removed by warming. Several effects which were very strongly exhibited in the warm climate of India, I found to be much diminished here. Cautious application of heat often increases, not only the sensibility, but also the power of self-recovery. But excess of temperature 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 loss of sensibility. In substances which are nearly neutral, pressure variation may even cause reversal of response.
Again, the same receiver may, owing to some molecular modification, exhibit a response opposite in sign to the normal. But subjection to this continued stimulation of radiation, generally speaking, converts the abnormal response into normal. Exactly parallel instances will be noticed in the case of response to mechanical stimulus and to light.
We thus see how the response is dependent on the molecular condition, and how a change of this condition may even give rise to 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 used for detection, are the factors which are instrumental in the modification of the final response. I have already shown how the increasing effect of continuous radiation may produce molecular reversal. There may thus be one or more reversals. It is probable that the other variables may also produce similar reversals.
3. Recording Appatatus.
In the following investigations, the electrical effects, either the conductivity or the electromotive variations, due to external disturbances of various durations, have to be observed. It is also necessary to note the time-relations of the after-recovery from these effects. The conductivity and electromotive variations can be deduced from the observed varying deflections of the galvanometer. When the variations are rapid, the observation requires great alertness and is very fatiguing. This difficulty is still greatly enhanced when simultaneous time-observations have to be taken. It thus becomes necessary to have at least two observers; the process of observation is made slow and tedious, and the accumulation of results by this method is very tardy. But in the apparatus now to be described, the mode of procedure has been very much simplified, affording facilities for quick and highly accurate observations.
The apparatus is a modified railway myograph. (See fig. 1.) The moving platform carries squared paper (divided into 1/10 inch) on which the record is made. The platform moves uniformly by clockwork, and the rate of travel of the paper may be roughly adjusted by means of different-sized pulleys, or more finely by the clockwork 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 easily be
followed with a pencil, and it is quite easy to do this, when the fluctuation period is more than 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 from 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 will recover from the strained condition (1) if it has not been overstrained by an excessive stimulus, or (2) if its electric elasticity be very great. I have found in general that on careful adjustment of a receiver there is a tendency to 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 when ​the stimulus is strong. The difference exhibited by various substances as regards self-recovery is one merely of degree. I give below typical cases which exhibit the gradual transition from so-called non-recovery to complete recovery.
The galvanometer used is a dead-beat D'Arsonval, in which the period taken for the maximum excursion and the return to zero from that position is less than half a second.
In these experiments the receiver was appropriately fixed on a heavy base. This rested in turn on a steady pedestal with one or two sets of pneumatic tyres interposed.
Positive Type.—In fig. 2 (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, as we shall see, recovery is rather slow. In (b) is shown the effect of increased molecular mobility due to cautious warming. Now there was 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. 3 exhibits the effect on the negative type as exemplified by Sn and Pb coated with Br. It appears that in these, warmth is favourable to quick recovery. For in India with Pb coated with Br the self-recovery was obtained with the greatest ease. Here a more careful adjustment was necessary. 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, and then after awhile the internal friction is overcome, and the substance suddenly recovers. But soon after this preliminary stage the recovery becomes perfectly automatic and instantaneous. Each flash of radiation 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 slower.
5. A Self-recovering and Metrical Receiver.
But the most perfect type of self-recovering receiver that I have succeeded in constructing was made of the strained variety of silver described in my previous paper. I there showed 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 by having about 3 mm. thickness of the powder between two electrodes, one of which, by means of a micrometer screw, could be made to produce a gradual compression. 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 at first, 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.
A short rod was the source of radiation throughout these experiments. The intensity of incident radiation was varied by changing the distance of the radiator. In fig. 4 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 certain oscillatory sparks not having been as efficient as the others.
Certain Analogies with the Phenomenon of Phosphorescence and ​Thermo-luminiscence.—Another 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 with reference 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, but the recovery may be hastened by removing molecular friction through gentle heating. In connection with this, I will quote an interesting observation described in my paper previously mentioned. In an iron receiver strained by radiation there was quick recovery after heating, but "on careful inspection a slight oscillatory movement of the galvanometer spot was noticed during the process." Here the strain produced by radiation which remained latent was released by heat. In connection with this, one cannot help being reminded of the phenomena of thermo-luminescence, where the strain effect of light 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 will tend to recover. I may anticipate certain results which will be described later, in saying that the force of restitution increases 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. If the intensity of radiation is increased, the balancing position will be different.
It also appears from the result of other experiments that the after-effect persists for a little longer time when the stimulus is stronger. I shall show that this is the case when interpreting the curves of effect.
In fig. 5 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., and 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 thus seen to be partial, the return curves not exactly reaching the original starting position. It will be seen from the effect at 40 cm. that even ​there we have partial recovery between quickly recurring flashes, and there is thus observed a fluctuation 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 little longer time, and the flashes arrive before the substance can recover to any extent; thus there is less fluctuation in the balanced position. ​But when the radiator was placed at a distance of 15 cm. (see (c) ), the fluctuations almost 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 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 has little or no importance. The series of responses in (d) was taken after half an hour, and 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 no 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, and the variation 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. 5.
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 uncertainity 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 at least 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 only be taken as ​true 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 disturbance as abscissæ) 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 varying the conductivity of the particles, we have to bear in mind that no explanation can be regarded as complete, unless it explains not only the diminution, but also the increase of resistance; also the phenomenon of automatic recovery and of the opposite effects which are exhibited by the same receiver under different molecular conditions. 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 distortion produced 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 effect being due to some 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. 6 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, and 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 noticed later on.
6. Phenomena of Reversal.
(a.) Reversal due to Sub-Normal Intensity of Stimulus.—Another very curious phenomenon met with is the opposite effects of radiation below and above the critical intensity. Thus I have shown that, whereas under certain conditions the effect of radiation of moderate intensity on As is to produce an increase of resistance, the effect of feeble intensity of stimulus is to produce a diminution. Exactly parallel, though opposite, effects are sometimes seen produced in the positive class of substances. This result is certainly very curious, but I will show later on that exactly similar effects are produced under mechanical stimulus.
Possibly connected with the above is the following: When a receiver is subjected to radiation of moderately strong intensity, I have often noticed 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, hence the preliminary negative twitch, while a little later, on the absorption of the whole amount, we get the normal response. Thus under the continued action of radiation, the response curve exhibits a negative twitch at the beginning followed by the normal positive effect (see also fig. 17).
(b.) Reversal due to Overstrain—In addition to the above, I have also shown that reversal effects are produced by overstrain due to the continued effects of radiation; and these reversals may be partial or complete. This depends on the nature of the substance, and also on the adjustments. I give below a curve (see fig. 7) for Fe$_3$O$_4$ under continuous radiation, where, after the maximum effect was reached, there was a distinct trend towards reversal.
Under certain conditions, we may thus have in a positive substance an increase of resistance or negative effect under feeble radiation; this is specially seen when the receiver had been undisturbed for a long time, and the substance had undergone certain unknown molecular modification. Annealing makes the responses normal; or under moderately strong intensity, the abnormal negative response becomes changed into positive, to be again reversed (or tend to be reversed) under strong and continued action of radiation. All these peculiarities ​will also be found characteristic of 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 we generally understand an irreversible operation. But in the cases of complete self-recovery exhibited by various substances, we have 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 be changed, and if the strained substance happen to be immersed in a medium for which under strain it has a relatively stronger attraction, there may then be a chemical action. But this would be merely a secondary effect.
8. Electromotive Variation produced by Electric Radiation.
I said that if radiation causes molecular strain, there might be produced a difference of potential 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 to the effect of radiation, a difference of potential may be established between the acted and unacted plates. The differential effect, if it exists, could then be detected by a galvanometer ​or an electrometer. In the carrying out of this method there are, however, many practical difficulties. 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. But the second difficulty is far more serious. 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 on, there would then be no electromotive variation. From these considerations any attempt to detect the effect of electric radiation by electromotive variation appeared to be hopeless. It was only after the conclusion of another line of investigation on the electromotive variation produced by mechanical stimulus that I learnt that the effects of the same stimulus on two pieces of the same metal, forming a voltaic element, are different if the molecular conditions of the two are not originally the same. Under such a condition a P.D. exists between the two, and stimulation of both causes a variation of the existing electromotive force.
From the similarities of the effects of radiation and mechanical strain (see the following paper) I was convinced that with radiation, too, I would get unequal effects on the two plates having a slight initial electromotive difference. The effect of radiation would then be to produce a variation of the original electromotive force. And if the effects are at all parallel to those observed in the conductivity variation method (as diminution or increase of resistance) we may likewise expect to find a diminution or increase of electromotive force.
In carrying out experiments to verify the above suppositions, I soon found my anticipations to be fully 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; this formed a voltaic element. Two electrodes compressed the powder, till a current was observed to flow. The amylic alcohol acted as the electrolyte. Very careful adjustment of pressure is necessary, as in the case of receivers for exhibition of conductivity variation. 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. On allowing electric radiation to act on the cell, there was produced a variation of electromotive force; continued radiation even produced a reversal. The electric radiator—a rod used in the previous experiments—was also used in this case: owing to the opacity of the electrolyte the intensity of radiation has to be strong. The radiator was placed at a distance of 6 inches from the cell.
​I give below the results of three other 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 now desirous of obtaining a continuous record of electromotive changes produced by the continued action of electric radiation. For this purpose I used a galvanometer.
A cell was made, in the way previously described, with magnesium powder. Owing to some differences in the two portions of the powder there was an initial P.D. of 0·042 V. between the two electrodes. I now had the cell balanced by the potentiometer method, a sensitive galvanometer (with an interposed high resistance) being used as the detector of electric variation. Fig. 8 (a) shows the deviation from the balanced position by radiation which nearly reduced the potential difference to half its original value. By tapping, the original P.D. was restored, and a second experiment (see (b) ) gave almost identical results.
It is thus seen that the curve of electromotive variation due to radiation is similar to that obtained by the conductivity variation method.
It has already been shown that when the range of electric elasticity of the substance is not narrow, or when the strain is not too great, there is a recovery. 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 also produce a reversed effect. We shall now study whether visible radiation produces similar effects.
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. One and the same receiver responded in the same way when alternately acted on by visible and invisible (electric) radiation. It is, however, difficult to discriminate the effect of light from that due to the rise of temperature. That the effects observed were not solely due to temperature was evident from the fact that there was a tendency towards reversal, and that the same receiver which normally exhibited a diminution of resistance exhibited an increase of resistance when it underwent a molecular modification. The peculiarities of this universal radiometer was 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 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 other cases the potential is lowered by the action of light.
I now proceed to show the remarkable similarity of the curves of effect produced by electric radiation and light. For the photo-electric cell I used two silver strips fastened at the back by paraffin on 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.
The two plates being opposed, there would be no resultant effect if both were illuminated.
But if the two plates are slightly different, then the effects on the two are not the same. There will then be an electromotive variation, even when both the plates are exposed.
9. Effects of Flashes of Light.
In fig. 9, (a), is shown the effect of flashes of light of 2 seconds duration on AgBr plate. The source of light—an incandescent gas-burner—was at a distance of 12 inches. If the plates are kept in the dark for several days, the sensitiveness is then very much enhanced, and ​effects described below may be obtained by much feebler light, such as that given by a paraffin lamp or a candle. The strong similarity to
the curve for single flashes of electric radiation (see fig. 4) will at once be noticed. 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. 9, (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 light source 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 transitory negative twitch immediately followed by the normal response. With light, too, it is frequently found, as has been observed by Minchin, that the immediate result is a transitory negative effect followed by the normal action. Under molecular modification there may sometimes be seen a reversed response.
If the action of light is continued, after the maximum effect is reached, there is produced a tendency towards, or actual reversal.
In fig. 10 is given an interesting series of results showing the growth of fatigue, the different phases culminating in actual reversal. (Compare with fig. 6.) It will be seen that in the second response was feebler, and
It will thus be seen that both electric radiation and light produce similar conductivity and electromotive variations. In both, two opposite effects are observed. The curves of effect in both are similar. 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.
In the next paper I shall adduce further evidence tending to show that these effects are due to molecular strain.
* ↑ "On Electric Touch and Molecular Changes produced in Matter by Electric Waves," 'Roy. Soc. Proc.' vol. 66.
* ↑ Loc. cit.
* ↑ Loc. cit.
* ↑ Loc. cit.
* ↑ Loc. cit.
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