# Collected Physical Papers/On the Similarities between Radiation and Mechanical Strains

First published in the Proceedings of the Royal Society of London, Vol 70.
XV
ON THE SIMILARITIES BETWEEN RADIATION AND MECHANICAL STRAINS
In the previous paper various effects have been described caused by visible and electric radiation. Considerations were adduced which tended to show that these effects were due to molecular strain induced in the substance by the action of radiation. The whole history of the change produced by radiation, both the direct and after-effects, were graphically recorded in the various curves. The strain effect produced in a substance was shown to be attended with conductivity or electromotive variation. We shall next inquire whether strain, which is undoubtedly produced by mechanical means, gives rise to response by conductivity and electromotive variations.
As regards the conductivity variation due to mechanical strain, it is well known that in the construction of standard resistance coils, winding the wire on a spool causes a distinct variation of resistance, and that this strain effect can only be removed by annealing. The difference between the resistance of a substance when strained and after it is annealed is sometimes very considerable.
The effect of electric radiation in changing the conductivity of a mass of discontinuous particles is very great. It is to be borne in mind that the effect of electric radiation is only skin-deep. As the action is one of surface, the larger the surface the greater is the effect produced. As already stated, the effective surface ​accessible to radiation in loose particles is very much enlarged. Moreover, the resistance offered to the particles is not due to the individual solid lumps, but to the resistance of surface layer. It is precisely the surface layers that are affected by radiation, and hence the marked variation of resistance.
When the particles become continuous, the radiation can only affect the extremely thin layer of molecules on the surface, the mass in the interior being shielded by the outer conducting sheet; the molecular changes produced on the surface layer do not affect to any appreciable extent the conductivity of the mass.
For detection of strain effect in continuous solids the method of electromotive variation is the more suitable. It has been shown that light causes a P. D. between the acted and unacted plates. I shall next deal with the question whether mechanical stimulation gives rise to an electromotive variation between the acted and unacted plates.
1. The Strain Cell
Fig. 42. The Strain Cell.
If a cell be made of two clean wires cut from the same piece, with water as the electrolyte there should theoretically be no P. D. between the two. But in practice a small P. D. exists between the wires, owing to slight difference in their molecular condition.
This initial difference can, however, be annulled by appropriate means, for example by subjecting them to mechanical vibration for a short time. After these precautions are taken, results are obtained which are extraordinarily consistent.
Now if one of the two wires be continuously twisted, an increasing P. D. is induced during torsion between the acted and the unacted wires. This may be measured by the deflection of the sensitive galvanometer. A curve of response could thus be obtained with electromotive force, measured by the galvanometer deflection, as ordinate, and the time during which disturbance is kept up as abscissa. Such curves were directly obtained by the recording apparatus described in my previous paper. The wire was twisted at a uniform rate. The successive dots represent the completion of 360°. To ​keep the deflection within the scale, a megohm was interposed in the circuit. The resistance of the cell was about 5000 ohms. The absolute values of electromotive force corresponding to the galvanometer deflections were subsequently obtained by noting the effect of a known electromotive force.
2. Effect of Torsional Disturbance
Most of the metals—exceptions presently to be described—become negative during molecular disturbance caused by torsion, i.e., the current through the liquid is from the acted to the unacted wire. As there is a considerable vagueness in the terms positive and negative, which has led to much confusion, I would name the acted wire as becoming zincoid or Z, when under an external disturbance the current flows through the electrolyte from the acted to the unacted wire. Again, in certain cases the reverse is true; the current flows from the unacted to the acted wire; the acted wire will in that case be designated as cuproid or C.
The induced electromotive variation is not due to twist as such, but to molecular disturbance induced during increasing twist. For if the wire be held stationary in the twisted position, the molecular disturbance with the attendant electromotive variation will gradually disappear (fig. 43). Other facts will be brought forward to show that the effect is due to the molecular disturbance.
The wires used in the following experiments were from commercial specimens. The length was in every case about 9 cm., but the diameters were not the same.
The responses under electric radiation, and under the stimulus of mechanical vibration will presently be ​shown to exhibit remarkable similarities. Still more extraordinary are the similarities that exist even in abnormalities, several instances of which will be given later; of these I shall mention here only one. It
was shown in experiments with electric radiation that substances sometimes fall into a sluggish molecular condition, when the responses almost disappear. Strong stimulation (induction shocks, etc.) or annealing is then found to restore the sensitiveness. The same peculiarity is observed in the strain-cell. Lead, for example, specially on cold days, is apt to fall into a sluggish condition, when it becomes almost irresponsive, But it regains its sensitiveness after intense vibration or after annealing.
All metals (including the noble metal Pt) when molecularly disturbed exhibit electromotive effect. The intensity of electromotive variation depends on the nature and physical condition of the substance. The intensity oi effect does not, however, depend on the ​chemical activity of the substance, for the electromotive variation in the relatively inactive tin is greater than that of zinc. The electrolyte used in the following experiments is common tap-water, but similar effects are also obtained with distilled water.
3. Self-recovery
It was said that the acted wire, usually speaking, becomes zincoid. This is not universally the case, for there are substances which become cuproid under mechanical stimulation. I have previously said that electric radiation produces opposite effects on different substances; silver is often found to show an effect (increase of resistance) opposite to that of generality of metals. It is very curious that silver is also often found to exhibit an opposite electromotive effect under mechanical stimulation, that is to say, the acted wire becomes C.
As long as the wire is not overstrained there is always a recovery. Observe the character of recovery in the curve for Zn when the twisting was stopped. It will be noticed that the recovery is very rapid at first, but slow in the later part, and that the recovery is complete.
4. Irreversible Molecular Effect of Twist
In the case of electric radiation or light, the impulses are of a vibrational nature, unlike the uni-directioned mechanical twists used in the above experiments. To make the two sets of phenomena comparable, we should have the mechanical disturbance of a vibrational nature also. I therefore next tried to see what the effect would be by reversing the direction of the twist, and found that the induced electromotive force is independent of the direction of twist, that is to say the electromotive ​variation is the same, whether the torsion is right-handed or left-handed.
I next tried the effect of a complete torsional vibration. I twisted the wire suddenly through +90°, then back to zero, then to −90°, and again back to zero, the complete vibration being executed in half a second. It will be seen that under these conditions we have a mere vibration and no resultant twist. This gives rise to an electromotive variation, the magnitude of which simply depended, as will be shown later, on the amplitude of vibration. It did not matter in the least whether the vibration commenced with a right- or left-handed twist.
It may be stated here that similar electromotive variation is obtained by molecular disturbance produced by a mechanical tap.
I shall now describe the effect of mechanical stimulus of varying intensities and durations. The intensity may be varied by varying the amplitude of vibration. We shall also study the effect of a single stimulus, or the summated effect of rapidly succeeding stimuli.
A set of experiments on the effect of mechanical stimulus may thus be carried out parallel to those on the effect of radiation stimulus. It would then be instructive to compare the response-curves of mechanical with those of radiation stimulus.
5. Effect of a Single Stimulus
For studying the effect of mechanical stimulus, a voltaic element made of "tin" wire is very suitable. Normal responses are easily obtained after annealing.
​As has been said before, any other metal may be used; I have, in fact, obtained as good results with platinum. But the advantage of tin is that the electromotive variation is comparatively strong; under favourable conditions this may be as high as 0.4 volt; another advantage is that it shows very little fatigue. On freshly making the cell, signs may be exhibited of abnormal irritability; this is due to the fact that a stable molecular condition has not yet been reached; but a more settled state soon supervenes, and after that successive responses are obtained which are extraordinarily regular and consistent amongst themselves.
That the response is due to molecular disturbance in the acted wire may be shown by the following experiment. The wire is clamped below; when the wire is subjected to torsional vibration, there is produced a strong molecular disturbance with the attendant electromotive variation. The wire is next released from the clamp and vibration imparted as before; there is now no electromotive response.
In fig. 44 is given a series of curves for different "intensities" of vibration. For want of space I have given a few only of each series. As a matter of fact, the succeeding series would have been mere repetitions of those which preceded. I have taken as many as 500 successive records, and each record is a mere duplicate of the rest. The substance does not exhibit any appreciable fatigue, especially if a period of rest be allowed for complete recovery. It will be seen that the rise is quick, whereas the fall is comparatively slow, specially in the later part.
With strain cells, there is no permanent change; the stimulated wire returns exactly to its original ​condition on the cessation of stimulus. In the border region between Physics and chemistry no sharp line of demarcation can be drawn. In the case of tin cell the two wires are originally alike; under mechanical vibration a difference of potential is induced between the strained and unstrained wires. The induced electromotive variation disappears when the acted wire recovers
from stimulation. We may describe the same fact in chemical language by saying that owing to molecular strain the stimulated wire becomes chemically more ​active (zinc-like), and that the wire recovers its original condition on the cessation of stimulus.
6. Increased Effect with Increasing Intensity of Vibration
In fig. 44 are given the curves of response for single vibration, of amplitudes of 90°, 180° and 360°, the period lasting for 0.5 second. It will be noticed that the intensity of response increases with the energy of vibration.
7. Effect of Summation of Stimuli
In the case of effect of rapidly succeeding flashes of electric radiation on Ag′, it was shown (see fig. 36d) that the partial effects became fused together and that there was a limiting effect, kept balanced by the force of restitution. With rapidly succeeding mechanical stimuli, we again obtain precisely similar results. Fig. 45 (a, b) shows the effect of continuous vibration on tin cell, with different intensities of vibration, the vibration-frequency being twice in a second. The curve gradually rises and attains a maximum, at which position it is held almost rigid as long as the vibration is kept up. But on the stoppage of stimulation there is an immediate recovery, and if sufficient time be allowed the recovery is complete, as seen in the last curve of the series. The disturbance was kept up for 1 minute, and the period of recovery allowed was also 1 minute. In this way I obtained a long-continued series of similar responses, there being little fatigue; this is the case when a period of repose intervenes. But if the vibration is kept up without intermission, signs of fatigue begin to appear and the curve tends to fall. in some metals there may even be a reversal. Observe the flat top of ​the curve similar to that of Ag′ under electric stimulus mentioned above. Also the effects of different intensities of vibration, as shown in figure 45, (a) and (b).
In (d) is shown the effect of vibration on Ni. After reaching the maximum there is a tendency towards reversal. Ni also shows greater signs of fatigue.
In (c), fig. 45, is shown the interesting curve for a given piece of Ag. The effect is very much feebler, and curiously enough it gave response of an opposite sign, the vibrated wire becoming cuproid. It was said that silver occupied a peculiar position as regards response to electric radiation, sometimes responding in one, and again in an opposite manner, probably owing to its readiness to pass from one molecular condition to another, under slightly different external conditions. With mechanical vibration, too, I find silver, exhibiting ​positive or negative responses, the acted wire becoming on different occasions either Z or C.
Fig. 46.
(a) Effect of electric radiation on Ag′ (conductivity variation). (b)„light on HgBr (E.M. variation).
(d) Effect of continued action of electric radiation on Ag′ (conductivity variation). (e)„„„mechanical vibration on Tin (E.M. variation). (f)„„„light on AgBr E.M. variation).
8. Reversal Effects
Reversed Effect due to Sub-minimal Stimulus.—Just like the negative effect (i.e., opposite to the normal) often exhibited under electric radiation when the stimulus is below the critical intensity, so also feeble mechanical stimulus often produces an effect opposite to the normal. Thus with strain cell made of lead, ​I found that whereas the acted wire became cuproid with an amplitude of vibration of 4°, the same wire when vibrated through 45° became zincoid. Thus in a Pb cell (50,000 ohms in circuit):
| Amplitude of vibration. | Deflection. | Result. |
| 4° | 5 divisions to right | Acted wire C. |
| 45° | 70„left | „Z. |
The opposite effect under sub-minimal stimulus was too frequent to be accidental, but it did not occur invariably. On the occasions when it occurred, this negative effect disappeared after continued vibration. Thus on taking a record of effect of continued vibration, there is produced a negative twitch, which is converted later into a positive deflection, just as in the curves of effect of light (see below fig. 48).
Reversal produced by Continued Stimulation.—After the maximum effect the attainment of the further continuation of vibration tends to produce a reversal. This is specially the case with nickel in which the curve of response becomes completely reversed.
I have described the various molecular effects produced by mechanical stimulus under varying conditions, and shown how very similar they are even in details to the effects produced by electric radiation and light. How striking these similarities are will be seen from the following tabular statement and comparison of different curves.
9. Response common to Electric Radiation, Light, and Mechanical Vibration
1. The molecular effect produced may be detected either by conductivity or electromotive variation methods.
​2. Substances when not overstrained exhibit recovery; the recovery is, however, delayed when there is overstrain.
3. Response is modified by previous history, and the influence of the surrounding conditions. Slight rise of temperature and annealing are generally favourable to increased sensitiveness and quick recovery.
4. Under the action of electric radiation, light, and mechanical vibration, two opposite effects are exhibited; by the conductivity variation method this is seen in the diminution or increase of resistance; a positive or negative variation is obtained by the method of electromotive variation.
5. In the curve of response, in all the above cases, the ascending portion is abrupt, whereas the fall during recovery is at first rapid, then comparatively slow, the curve of recovery being thus convex to the abscissa.
6. Under rapidly succeeding stimuli, there is a fusion of individual effects; the curve rises to a maximum, when the force of restitution is kept balanced by the force of molecular distortion.
7. Sub-minimal stimulus often produces a response of opposite sign to that of normal. Too long-continued stimulation produces, or tends to produce, a reversal.
8. Under certain molecular modifications, the response is of opposite sign to that of the normal. Continued stimulation converts the abnormal into normal. The response curve may thus exhibit, at the beginning, a negative twitch followed by the normal positive.
A number of curves selected from experiments already described, are given (see fig. 46) to illustrate graphically the remarkable similarities of response under diverse modes of stimulation.
10. Effect of Stimulation by Light balanced by Mechanical Stimulation
I have hitherto spoken of the similarities of the radiation and mechanical strains, but have not yet said anything about their mutual relation.
It is known that in cases where electric radiation produces an increase of conductivity, mechanical vibration produces an opposite effect, i.e., an increase of resistance. It thus appeared that two opposite molecular effects were produced by the two different modes of stimulation.
In verification of this I investigated the effects of light and mechanical vibration in inducing electromotive variation in a strain cell. For this purpose I took a tin cell, and subjected one of the wires to the action of light and mechanical vibration alternately. The upper curve of fig. 47 shows the effect of light of
Fig. 47. Effect of light and torsional vibration on a Tin cell. Light makes the acted wire cuproid, mechanical vibration makes it zincoid.
a given intensity. It will be noticed that light makes the acted wire cuproid. But the action of mechanical vibration (see lower curve in same figure) makes the acted wire zincoid, and after several trials I found that a vibration with an amplitude of 3° produced a series of ​curves similar, but of opposite sign, to those produced by light. Mechanical vibration thus produced a molecular effect opposite to that of light.
I next allowed both the stimuli to act simultaneously on one of the wires; the action of light was then found to be exactly balanced by the action of mechanical vibration, an increase or diminution of either at once upsetting the balance.
The molecular effect of mechanical vibration thus appears, at least in the case of tin, to be opposite to that produced by light. This may be the case in general; the exception might be when one of the two stimuli is normal and the other sub-minimal.