# Collected Physical Papers/On Electromotive Wave accompanying Mechanical Disturbance in Metals in Contact with Electrolyte

First published in the Proceedings of the Royal Society of London, vol. 70.
XX
ON ELECTROMOTIVE WAVE ACCOMPANYING MECHANICAL DISTURBANCE IN METALS IN CONTACT WITH ELECTROLYTE
Take a rod of metal, and connect the two points A and B with a galvanometer by means of non-polarisable electrodes. (Fig. 71a.) If the point O is struck, a wave of molecular disturbance will reach A and B. It will be shown that this is attended by a wave of electric variation. The mechanical and the attendant electrical disturbance will reach a maximum and then gradually subside. The resultant effect on the galvanometer will be due to E$_A$ − E$_B$ where E$_A$ and E$_B$ are
​the electric variations induced at A and B. The electric changes at A and B will continuously balance each other, and the resultant effect on the galvanometer will be zero, when the mechanical disturbance reaches A and B at the same time and with the same intensity, when the molecular condition is similar at the two points, and when the rate of rise and subsidence of disturbance is the same at the two points. In order that a resultant response may be exhibited, matters have to be so arranged that (1) the disturbance reaches one point, say A and not B, and vice versâ. This may be accomplished by the method of block. Again, a resultant differential action may be obtained even when the disturbance reaches both A and B, if the electrical excitability of one point is relatively exalted or depressed by physical or chemical means. Besides the method of the block there are thus two other means of obtaining a resultant response, (2) by the method of relative exaltation, (3) by the method of relative depression.
Method of Block
The electromotive effect described below can be obtained with all metals. A piece of "tin" wire (an alloy of tin and lead used as electric fuse) will be found to give very good results. A specimen of wire 1 mm. in diameter, 10 cm. in length, is mounted in the apparatus. Two strips of cloth moistened with water or dilute salt solution are securely tied round two points A and B. Electric connections are made with the indicating galvanometer by means of non-polarisable electrodes (Zn in ZnSO$_4$ solution). Special precautions are taken so that there is no variation of contact. If a sharp tap be ​now given near A, a transitory electrical current in response to the disturbance will be found to flow round the circuit, in a definite direction. Disturbance of B gives rise to a current in the reverse direction. For quantitative measurement it is necessary to have the intensity of stimulus maintained uniform, or increased or decreased in a definite manner. Instead of a tap, the stimulus of torsional vibration is more satisfactory. By maintaining the amplitude of vibration constant, or increasing or decreasing the amplitude, we can either keep the stimulus constant, or increase or decrease it in a quantitative manner. I shall first describe some of the typical results which may be obtained with the simple "straight wire form" of the apparatus. Worked with care it gives consistent and good results. For quantitative measurements requiring the greatest exactitude the "cell form," to be presently described, will be found more satisfactory.
Recording Apparatus.—The galvanometer used is a sensitive dead-beat D'Arsonval. The records are taken by means of a cylindrical modification of the response recorder described in a previous paper, or by means of photography. In the latter method, a clockwork moves the photographic plate at a uniform rate and a curve is traced on the plate by the moving galvanometer spot of light. The disturbance of molecular equilibrium caused by the stimulus is attended by an electromotive variation, which gradually disappears on the restoration of molecular equilibrium. The rising portion of the response curve shows the electromotive effect, due to stimulus, and the falling portion the, recovery. The ordinate represents the electromotive variation, and the abscissa the time.
Experiments for Exhibition of Balancing Effect
If the wire had been carefully annealed, the molecular conditions of its different points are approximately the same. The wire will therefore be practically iso-electric throughout its length. If the wire be now held near the middle by the clamp, and a vibration through an amplitude of, say, 90° be given to the end A, an upward deflection will be produced; an equal and opposite deflection will be produced by similar vibration of B (fig. 71, c). If both the ends are simultaneously vibrated, the electromotive variation at the two ends will continuously balance each other, and the galvanometer spot will remain quiescent (fig. 71, d). The clamp is next removed, and the wire vibrated as a whole; the stimulation of A and B being the same, there is no resultant deflection. Having found the balancing point for the clamp (which is at or near the middle), if the clamp be now shifted to the left, on simultaneous vibration of A and B, the A effect will relatively be the stronger (inasmuch as the torsional vibration of A is increased and that of B decreased), and there is produced a resultant upward deflection. Thus keeping the rest of the circuit untouched, by merely moving the clamp from the left, past the balancing position to the right we get either a positive or zero or a negative resultant effect. This can be repeated any number of times. The experiment shows further that when the amplitude of vibration is kept constant, the intensity of electromotive effect is increased by shortening the wire. The normal direction of the current of response in the wire is in the majority of metals, from the relatively less to the relatively more excited point.
​The form of the response curve, stimulus remaining constant, is modified by the molecular condition of the wire. A wire in a sluggish condition shows feeble response, the recovery being also slow. The same wire after it has been vibrated for a time exhibits a stronger response. Longer time is required for recovery from the action of a stronger stimulus.
Comparison of Electric Excitability of Two Points by the Method of Balance
As already stated, when the clamp is put at the balancing position, alternate equal stimulations of A and B produce equal and opposite electromotive responses, and when the two ends are stimulated simultaneously there is no resultant effect.
Increased Excitability produced by Preliminary Vibration.—If now one-half of the wire, say the A half, be vibrated for a time, the electric excitability of that half will be found to be more or less permanently augmented, presumably by increased molecular mobility conferred by vibration. The response of A would now be found to be greatly enhanced, as compared with its previous response, the response of B remaining the same as before. If now both the ends are simultaneously vibrated, the previous balance will be found to be upset, the resultant showing that A in consequence of previous stimulation, has been rendered the more excitable.
If B be now vibrated for a time, the former approximate balance will be re-established by the enhanced responsiveness of B. Thus the following results ​are obtained with the clamp at the approximate balancing point:
| | Response of A. | Response of B. | Resultant response. |
| | Divisions. | Divisions. | Divisions. |
| Approximate balance | +5 | −4.5 | +0.5 |
| After the end A has been vibrated. | +10.5 | −4.5 | +6.0 |
| After the end B has been vibrated for an equal length of time. | +10.5 | −9.5 | +1.0 |
Effect of Chemical Reagents.—It will be shown that keeping the electrolyte by which contact is made constant, the electric excitability of the wire depends on the molecular condition of the wire. Certain electrolytes, such as dilute solution of NaCl, dilute solution of bichromate of potash and others, are normal in their action, that is to say, with such contacts the response to stimulation is practically the same as with distilled water contact.
Contact made with dilute NaCl solution may therefore be regarded as the normal contact. There are again certain chemical reagents which enhance the electrical excitability; others on the contrary produce great depression, or abolition of excitability.
Electric Comparator
We may compare the relative electric excitability conferred by chemical reagents by the method of balance. Having previously obtained a balance (with water or ​dilute NaCl solution at A and B), one contact, say A, is touched with a few drops of very dilute Na$_2$CO$_3$, which is an exciting agent. The electric excitability of A will now be found to be greater than that of B as demonstrated by the upsetting of the previous balance, the resultant current being now towards the more excitable A.
| | Response of A. | Response of B. | Resultant Response. |
| Both contacts of normal saline. | +12 | −12 | 0 |
| Contact A touched with Na$_2$CO$_3$ solution. | +32 | −12 | +20 |
Similarly, when A is depressed by a trace of oxalic acid, the electric excitability of A is less than that of B, the resultant deflection being now downwards (current of response towards the relatively more responsive B). It is to be remembered that in all cases the resultant current of response in the wire is towards the more excitable point.
An interesting line of investigation rendered possible by a modification of method of balance described above is to compare the relative excitability induced by various chemical reagents, the influence of different strengths of the same reagent, and the modification of the effect by the duration of application. We may thus compare the effect of the reagent in relation to the normal effect of water or dilute NaCl solution. There is again an extremely delicate method of comparison of the ​relative effects of a series of compounds like Na$_2$CO$_3$, K$_2$CO$_3$, etc. Balance having been previously obtained between the normal sensitiveness of A and B, the two different solutions are now applied at the two points; the slightest difference in their relative action is at once exhibited by the upsetting of the balance during stimulation, the direction of the resultant deflection indicating the more stimulating reagent.
Resultant Response by Method of Relative Depression or Exaltation
From what has been said, it will be seen that by rendering A and B unequally excitable, a resultant response may be obtained. The block may be abolished, and the wire may be vibrated as a whole; the response will now be due to the differential effect at A and B. For producing difference in excitability we may subject one point, say A, to a preliminary vibration, or apply at the point a suitable chemical reagent. By the application of the latter there will be a small P. D. between A and B, which will simply produce a displacement of the zero. (By means of a potentiometer the galvanometer spot may be brought back to the original position). The displacement of the zero does not affect the general result. The direction of this more or less permanent current, due to the small P. D., gives no indication of the direction of current of response; the direction of the latter is determined by the rule that the responsive current flows towards the more excitable point. The electromotive response induced by mechanical stimulation is algebraically superposed on the existing P. D. ​The deflection takes place from the modified zero, to which the spot returns during recovery. I give four records (fig. 72): in (a) A is touched with Na$_2$CO$_3$ (which is an excitant): a permanent current flows from B to A: response to stimulus is in the same direction as the permanent current (positive variation); in (b) A is touched with a trace of oxalic acid (which depresses the excitability), the permanent current is in the same direction as before, but the current of response is in the opposite direction,(negative variation); in (c) A is touched with dilute KHO (3 parts in 1000), the response is exhibited by a positive variation; in (d) A is touched with stronger KHO (3 parts in 100), the response is now exhibited by a negative variation. The last two apparently anomalous results are due to the fact (which will be demonstrated later) that KHO in minute quantities is an excitant, while in larger quantities it is a depressant.
Fig. 72. Response by Method of Relative Depression or Exaltation.
(a) Response when A is treated with sodium carbonate—an apparent positive variation. (b) ,,,,,,,,,, oxalic acid—an apparent negative variation. (c) ,,,,,,,,,, very dilute potash—positive variation. (d) ,,,,,,,,,, strong potash—negative variation.
       This response is up when A is more excitable and down when B is more excitable.
       Lines thus — — — indicate direction of permanent current.
| | Permanent current. | Current of response. |
| A treated with sodium carbonate• | $\leftarrow$ | $\leftarrow$ |
| ,,,,oxalic acid•• | $\leftarrow$ | $\to$ |
| ,,,,very dilute potash• | $\leftarrow$ | $\leftarrow$ |
| ,,,,strong potash•• | $\leftarrow$ | $\to$ |
Current of response is always towards the more excitable point.
Detection of Minute Physico-chemical Change
I will now describe an experiment which will show in a striking manner how exceedingly delicate is the method of electric response to stimulation, and how by its means we can detect and measure traces of physico-chemical changes in different parts of the same solid. Take a wire and touch two points, one with Na$_2$CO$_3$ solution the other with oxalic acid. Wash the wire. There is no trace left of the previous treatment. Let one contact be permanently made at a normal or previously unacted point N. Let the other exploring contact be moved along from the other end towards N, the wire being mechanically stimulated during the test. The galvanometer spot remains quiescent as long as the exploring contact is over normal areas. But as soon as it touches the zone on which is impressed the invisible image of physico-chemical change, the differential effect of stimulus at once reveals it by producing a vigorous ​movement of the galvanometer spot. At N$_1$ there was no movement, but there was an upward movement of
Fig. 73. Electro-molecular Explorer.
response when the explorer came over the stimulated area "Carbonate." As the explorer passed on to N$_2$ there was a cessation of movement, but when it reached the depressed area marked "Oxalic" there was a vigorous movement downwards (fig. 73).
Interference Effects
I have already described a case of interference in the galvanometric effect when the two points A and B in similar molecular conditions are simultaneously acted on by the same mechanical stimulus. Under these ​conditions the electric variation at the two points continuously balance each other, and there is no resultant effect.
When one point is acted on by a chemical reagent, not only is its electric excitability changed, but its time relations—its latent period, the time-rate of its acquiring the maximum electric variation, and the recovery from the effect of stimulus—become also modified. Using the block method, we may place a drop of excitant Na$_2$CO$_3$ on A and depressant KBr on B. On simultaneous vibration of A and B, the A effect being relatively much stronger than B effect, the resultant is an upward deflection. But on moving the balancing clamp away from A (thus decreasing the stimulation intensity of A and increasing that of B) we can find a point where the A effect is equal and opposite to the B effect. But owing to change of time relations, simultaneous vibration of A and B no longer gives a continuous balance; we obtain instead a diphasic variation. The diphasic curve thus obtained is exactly the same as the resultant curve deduced from the algebraic summation of the A and B curves obtained separately.
Continuous Transformation from Positive to Negative through an Intermediate Diphasic Response.—In the following record, fig. 74, I succeeded in obtaining a continuous transformation from positive to negative due to induced changes in the relative sensitiveness of the two contacts. I found that traces of after-effect due to application of Na$_2$CO$_3$, even after it is washed off, remain for a time, the increased sensitiveness conferred disappearing gradually. Again, if we apply Na$_2$CO$_3$ solution to a fresh point, the sensitiveness gradually increases. There is another interesting point, viz., that the ​beginning of response is earlier when the applicaticin of Na$_2$CO$_3$ is fresh. In the experiment whose record is given, the wire is held at one end, and successive uniform vibrations imparted to the wire as a whole at intervals of one minute by means of a torsion head at the other end. Owing to after-effect of previous applications of Na$_2$CO$_3$, the sensitiveness of A is at the beginning great, hence the resultant response at the commencement is positive or upward. Dilute solution of Na$_2$CO$_3$ is next applied to B. The response of B (down) begins earlier, and continues to grow
Fig. 74. Transformation from positive to negative through intermediate diphasic variation. Thick dots represent times of application of stimulus.
stronger and stronger. The response therefore shows a preliminary negative twitch of B followed by positive variation of A. The negative grows continuously. At the fifth response, the two components, negative and positive, of the double response become equal; after that, the negative becomes very prominent, the positive dwindling into a feeble and inconspicuous response.
Modification of the Apparatus into "Cell Form"
The series in fig. 75 explains the transformation from the "straight wire" to "cell" form. The wires A and B, cut from the same piece, are clamped separately below; vibration of A (the amplitude of which is measured by a graduated circle) gives rise to a responsive current in one direction; vibration of B produces a current in the opposite direction. Every experiment can thus be verified by corroborative and reversal effects. The intensity of electromotive response varies with the substance, and is sometimes considerable, for example, with "tin," a single vibration may give rise to as high a value as 0.4 volt. The intensity of response does not depend on the chemical activity of the substance, for the electromotive variation in the relatively inactive tin is greater than that in zinc. Again, the sign of response, positive or negative, is sometimes modified by the molecular condition of the wire (see below). In the cell form of apparatus the wires are immersed to a definite depth in the electrolyte; there is thus a perfect and invariable contact between the wire and the electrolyte. The wire is clamped below, and torsional vibration gives rise to a strong electrical response. If the wire be carefully unclamped, vibration is found to cause no electrical response. As all the rest of the circuit is kept absolutely the same in the two different sets of experiments, the results offer conclusive proof that the responsive electromotive variation is solely dire to the mechanical stimulation of the acted wire. The excitatory effect due to stimulation persists for a time. This is demonstrated by keeping the galvanometer circuit open during the application of stimulus, and completing it at various short intervals ​
​after its cessation, when a persisting electrical effect diminishing rapidly with time will be observed. When the wire is brought to the normal condition, successive responses to uniform stimuli are exactly similar in the case of metals which, like tin, show no fatigue. I usually interpose a high external resistance, varying from 1 to 5 megohms, so that the galvanometer deflections are proportional to the electromotive variations; the internal resistance of the cell and the variation of that resistance by the addition of chemical reagents are thus rendered quite negligible. Ordinarily tap-water is used as the electrolyte. The responses obtained with tap-water are practically the same as those obtained with distilled water. Zinc wires in ZnSO$_4$ solution give responses similar in character to those given by, for example, Pt or Sn in water.
Character and Intensity of Response dependent on Molecular Condition
The following experiments show how the phenomenon of response is intimately connected with the molecular condition of the acted wire:—
Effect of Annealing.—The photographic records, given in fig. 76, show the equal and opposite responses
Fig. 76. Series of responses, to uniform stimuli, of both A and B wires, before and after annealing.
​in A and B wires to a succession of uniform stimuli. Hot water was now substituted for the cold water (too high a temperature temporarily reduces the response); the cell was then allowed to cool to its old temperature. Records show how the process of annealing had greatly enhanced the amplitude of response.
Effect of Previous Vibration.—The increased sensitiveness conferred by previous vibration has already been referred to before. I give below a record (fig. 77) obtained with platinum; this and similar results obtained
with other metals clearly show the enhancement of response after preliminary stimulation.
Sometimes the wire gets into a very sluggish condition, when the response almost disappears; in other words, owing to some molecular modification, responsiveness is reduced from the normal positive value (by positive is meant that the acted wire becomes zinc-like or is zincoid) to zero. In these cases annealing or preliminary vibration are usually effective in transforming the sensitiveness from zero to the normal positive value.
Abnormal Response
But the modification of which I have spoken does not stop short of mere abolition of responsive power, but sometimes proceeds further, and actually reverses the sign of response—the excited wire becoming cuproid.
But even in such cases long-continued stimulation transfonms the abnormal negative to the normal positive. I give below photographic records which exhibit this. In fig. 78, α, the transformation took place during continued vibration. To detect the point of transformation, I experimented with a platinum cell which exhibited the abnormal effect, and took a long series of records of responses to uniform stimulation acting at
Fig. 78. (α) Abnormal negative (downward) response (a) of tin converged into normal positive (upward) response (c) after continued vibration (b).
(β) Shows points of transition from the abnormal negative to the normal positive (platinum).
intervals of a minute. In the record (fig. 78, β) I have been able to catch the point or rather points of transition.
Abnormal response of animal nerve.—It is interesting to note that similar abnormal response 18 also exhibited ​by animal nerve in condition of extreme sub-tonicity. This abnormal response of the nerve is also transformed into normal under continuous stimulation.
Returning to the response of metals we may distinguish the following typical cases. Beginning with the case of extreme molecular modification, we have (1) a condition which gives rise to the abnormal or negative response; after continued vibration the negative becomes less negative, and ultimately becomes converted into positive: (2) an irresponsive or neutral condition; vibration or annealing transforms it into positive: (3) a sluggish, feebly positive, becoming more and more positive after continued vibration: (4) a steady and permanent condition, when the responses are uniform: and lastly (5) when vibration is maintained for too long a time, the positive tends to become less positive, the responses decline—a state of things which is designated as fatigue.
Increased Electromotive Response under increased Intensity of Stimulus
When the intensity of stimulus is increased by increasing the amplitude of vibration, the electric
Fig. 79. Photographic records of responses (a) from 5° to 40°, (b) from 40° back to 5°. The vertical line = 0.1 volt.
​response undergoes an increase. The records given (fig. 79) are for amplitudes of vibration increasing from 5° to 40°, and decreasing from 40° to 5°. The successive stimuli are imparted at intervals of 1 minute. It will be noticed how the responses become enhanced under increasing stimulations.
Table I.—Showing the Increasing Electromotive Response under Increasing Amplitude of Vibration
| Vibration amplitude. | Deflection. 23 dns. = 0.1 volt. | Deflection. 23 dns. = 0.1 volt. | Deflection. 23 dns. = 0.1 volt. | Deflection. 23 dns. = 0.1 volt. |
| 5° 10 20 25 30 35 40 | $\downarrow$ | Ascending. 5.5 15 25.5 33 39 43 47 | Descending. 5 12 26 32 39 43 48 | $\uparrow$ |
| | $\to$ | $\to$ | $\to$ | $\to$ |
It will also be noticed that whereas recovery is complete in 1 minute when the vibration amplitude is small, it is not quite complete within that time, when the vibration amplitude is large. Greater strain prolongs the period of recovery. Owing to want of complete recovery, the base line is tilted slightly upwards. This slight displacement does not materially affect the results, provided the shifting is slight. From other records taken through a greater range of stimulation, it appears that in a curve obtained with responsive electromotive variations as ordinates and amplitudes of vibrations as abscissæ, the first part of the curve is, generally speaking, slightly convex to the abscissa. This convexity ​is more pronounced when feeble stimulation gives rise, as to be presently explained, to a response of opposite sign to that of the normal. The curve is straight in the middle and concave in the last part where the amplitude of response reaches a limit.
Effect of Sub-minimal Stimulus.—The response is of opposite sign to that of the normal, when the intensity of stimulus is sub-minimal. This characteristic effect I also find in the response of living tissues.
Maximum Effect
If instead of a single vibration of a given amplitude we superpose a rapidly succeeding series, the individual effects are added-up and a maximum deflection is produced which remains practically constant as long as the vibration is maintained. A single ineffective stimulus thus becomes effective by the additive effect of several. Too long-continued vibration may cause fatigue, but during half a minute or so, the maximum effect in tin is very definite. For example, a single vibration of 5° gave a deflection of 3.5 divisions; the same when continued at the rate of four times per second gave a maximum deflection of eighteen divisions. A single vibration of 10° of the same wire gave a deflection of 4.5 divisions, but continued vibration gave the definite maximum of 37.5 divisions. I give below a curve (fig. 80) exhibiting the maximum effects for different amplitudes of vibration.
Hysteresis.—Allusion has already been made as to the increased sensitiveness conferred by preliminary vibration. Being desirous of finding out in what manner this is brought about, I took a series of observations for an entire cycle, that is to say, a series of observations ​were taken for maximum effects, starting from 10° and ending in 100°, and backward from 100° to 10°. Effect of hysteresis is very clearly seen (fig. 80, A); there is a considerable divergence between the forward and return curves, the return curve being higher of the two. On repeating the cycle several times, the divergence becomes much reduced, the wire acquires a more constant sensitiveness.
Effect of Annealing.—I repeated the experiment with the same wire, after pouring hot water and allowing it to cool to the old temperature. It will be seen from the
​cyclic curve (fig. 80, B), (1) that the sensitiveness has become very much enhanced; (2) that there is relatively less divergence between the forward and return curves. Even this divergence practically disappeared at the third cycle, when the forward and backward curves coincided (fig. 80, C). The above results show in what manner the excitability of the wire is enhanced by purely physical means.
It is very curious to notice that the substitution of dilute Na$_2$CO$_3$ solution as electrolyte produces results very similar to that produced by annealing; that is to say, not only is there a great enhancement of sensitiveness, but there is also a reduction of hysteresis. Another curious point is that, whereas with ordinary fresh wire the addition of Na$_2$CO$_3$ greatly enhances the sensitiveness, after the wire has been annealed there is comparatively little further increase of sensitiveness due to the addition of the reagent.
Effect of Chemical Reagents
I reproduce photographic records of a few typical cases which will graphically illustrate the influence of chemical reagents. The mode of procedure is as follows. The cell is filled with water, and photographic records are taken of responses to single vibrations of constant amplitude, applied to one of the two wires at intervals of 1 minute. The responses are found to be uniform. Chemical reagents are now added, and responses obtained as before. These exhibit either an increase or a diminution, depending on the exciting or depressing nature of the reagent. It is also quite easy to obtain duplicated results by alternately vibrating the A and B wires. Uniform responses, alternately ​positive or negative, are first obtained; after the addition of reagent both are found to exhibit either an increase or a diminution. As has been said before a very high external resistance, varying from 1 to 5 megohms, is interposed in the external circuit, the slight variation of internal resistance of the cell due to the addition of the reagent being thus rendered quite negligible compared with the total resistance of the circuit. That there is no appreciable variation in the total resistance can be independently verified by applying a known electromotive force before and after the addition of the reagent, when the resulting deflection is found to be the same in the two cases. The deflections are thus simply proportional to the responsive electromotive variations.
Chemical Excitants.—The following record (fig. 81)
​exhibits the increased response due to the action of Na$_2$CO$_3$ on Pt. Another record shows an exactly similar effect on tin. The record of effect was taken two minutes after the application. Other records taken immediately after, show that the enhanced responsiveness takes place gradually with time.
Depressants.—Reagents, like KBr (10 per cent.), produce a depression in the response. There are again others which abolish the response almost completely, for example, 3 per cent. KHO solution (fig. 82, c). One of the most effective reagents which abolishes the response is oxalic acid. The depressing effect of this reagent is so great that a strength of 1 part in 10,000 is often sufficient to produce an abolition of response.
​Opposite Effects of strong and feeble dose.—The most curious effect is that exhibited by the same reagent when the strength of solution is varied. This is clearly seen in record (fig. 82), in which (a) gives the normal response in water; (b)) shows the enhancement of response by a highly dilute solution of KHO, 3 parts in a thousand. The response was completely abolished by a stronger solution, 3 parts in a hundred (fig. 82 c).
Effect of "poisons."—Certain agents like oxalic acid cause a total abolition of response, like the action of poisons on living tissues.
The facts described above appear to show that the enhancement or depression of response may, at least to a considerable extent, be due to the increase or diminution of molecular mobility. With a given stimulus, the height of response and the form of the response curve is determined by the element of molecular friction. In connection with this, it is instructive to observe the records of vibrations of a torsional pendulum, the friction being gradually increased by immersing the pendulum in a viscous fluid. The various types of response-curves in metals are found to be very similar to the above.
Of these I give an interesting example. With moderate friction the successive curves obtained with the pendulum are like those given in the left of fig. 83 (a). With increased friction the height of the curve is diminished, the maximum is reached later, and the recovery becomes much prolonged like the curve to the right. With still greater friction there is an arrest of recovery. It would appear as if the reagents which abolish response in metals produce a similar molecular arrest. ​The following photographic records seem to lend support to this view. If the oxalic acid be applied in large
quantities, the abolition of response is complete; but on carefully applying just the proper amount, I find that the stimulus evokes a responsive electric variation which is less than the normal, and the period of recovery is very much prolonged from the normal 1 minute before to 5 minutes after the application of the reagent. In the lower record (fig. 83) the normal response and recovery is seen in the left record. After the application of oxalic acid the record to the right shows a pronounced arrest, i.e., there is now no recovery. Note also that the maximum is attained much later. Stimuli applied after the arrest produce no effect, as if the molecular mechanism had become locked up.
Conclusion
1. Molecular disturbance produced by mechanical stimulation gives rise to an electromotive response. In the majority of cases, under normal conditions, the responsive electrical current in a wire is from the less to the more stimulated.
2. Response may be obtained by (1) method of block, (2) by methods of negative or positive variation.
3. The electromotive response disappears on the cessation of stimulus.
4. The intensity of the electrical response is modified by the molecular condition of the wire. Annealing, or previous stimulation enhances the electric response.
5. Abnormal response due to molecular modification is transformed into normal by previous stimulation.
6. The intensity of electric response is increased with increasing intensity of stimulation.
7. In a curve showing the relation between intensity of stimulus and resulting response, the first part is slightly convex to the abscissa, the second is approximately straight, and the third concave. The response tends to reach a limit.
8. Minimally effective stimulus becomes effective by summation of stimuli. A maximum effect is produced, determined by the intensity of individual stimulation.
9. Hysteresis is exhibited in cyclic curves. The forward and return curves tend to coincide after several cycles. Previous annealing reduces hysteresis, and after one or two cycles the wire assumes a constant sensitivity.
10. Chemical reagents may profoundly modify the electric excitability. Some of them increase the excitability, while others depress or abolish it.
​11. The effect of a feeble dose is often opposite to that of a stronger dose.
12. By touching different points of the same wire with different reagents, the excitability of these portions are rendered unequal. Hence a responsive electromotive variation is obtained by stimulating the wire as a whole. The current in the wire is from the less to the more excitable.
13. This method enables the detection of invisible traces of physico-chemical change in a wire.
14. Chemical reagents not only change the excitability but also the quickness of response. Two points having two different rates thus give diphasic and other interference effects.
(Proc. Roy. Soc. May 1902.)