Fig. 47.—Abolition of Response at both A and B Ends by the Action of NaOHFig. 47.—Abolition of Response at both A and B Ends by the Action of NaOHStimuli of 30° vibration were applied at intervals of one minute to A and B alternately. Response was completely abolished twenty-four minutes after application of NaOH.
Fig. 47.—Abolition of Response at both A and B Ends by the Action of NaOH
Stimuli of 30° vibration were applied at intervals of one minute to A and B alternately. Response was completely abolished twenty-four minutes after application of NaOH.
Effect of dose.—It is sometimes found that while a reagent acts as a poison when given in large quantities, it may act as a stimulant in small doses. Of the two following recordsfig. 48shows the slight stimulatingeffect of very dilute KOH, andfig. 49exhibits nearly complete abolition of response by the action of the same reagent when given in stronger doses.
Fig. 48.—Stimulating Action of very dilute KOHFig. 48.—Stimulating Action of very dilute KOH
Fig. 48.—Stimulating Action of very dilute KOH
So we see that, judged by the final criterion of the effect produced by anæsthetics and poisons, the plant response fulfils the test of vital phenomenon. In previous chapters we have found that in the matter of response by negative variation, of the presence or absence of fatigue, of the relation between stimulus and response, of modification of response by high and low temperatures, and even in the matter of occasional abnormal variations such as positive response in a modified tissue, they were strictly correspondent to similar phenomena in animal tissues. The remaining test, of the influence of chemical reagents, having now been applied, a complete parallelism may be held to have been established between plant response on the one hand, and that of animal tissue on the other.
Fig. 49.—Nearly Complete Abolition of Response by Strong KOHFig. 49.—Nearly Complete Abolition of Response by Strong KOHThe two vertical lines are galvanometer deflections due to ·1 volt, before and after the application of reagent. It will be noticed that the total resistance remains unchanged.
Fig. 49.—Nearly Complete Abolition of Response by Strong KOH
The two vertical lines are galvanometer deflections due to ·1 volt, before and after the application of reagent. It will be noticed that the total resistance remains unchanged.
We have now seen that the electrical sign of life is not confined to animals, but is also found in plants. And we have seen how electrical response serves as an index to the vital activity of the plant, how with the arrest of this vital activity electrical response is also arrested temporarily, as in the case amongst others of anæsthetic action, and permanently, for instance under the action of poisons. Thus living tissues—both animal and vegetable—may pass from a responsive to an irresponsive condition, from which latter there may or may not be subsequent revival.
Hitherto, as already said, electrical response in animals has been regarded as a purely physiological phenomenon. We have proved by various tests that response in plants is of the same character. And we have seen that by physiological phenomena are generally understood those of which no physical explanation can be offered, they being supposed to be due to the play of some unknown vital force existing in living substances and giving rise to electric response to stimulation as one of its manifestations.
Is response found in inorganic substances?[14]—It is now for us, however, to examine into the alleged super-physical character of these phenomena by stimulating inorganic substances and discovering whether they do or do not give rise to the same electrical mode of response which was supposed to be the special characteristic of living substances.We shall use the same apparatus and the same mode of stimulation as those employed in obtaining plant response, merely substituting, for the stalk of a plant, a metallic wire, say ‘tin’(fig. 50). Any other metal could be used instead of tin.
Experiment on tin, block method.—Let us then take a piece of tin wire[15]from which all strains have been previously removed by annealing, and hold it clamped in the middle atC. If the strains have been successfully removedAandBwill be found iso-electric, and no current will pass through the galvanometer. IfAandBare not exactly similar, there will be a slight current. But this will not materially affect the results to be described presently, the slight existing current merely adding itself algebraically to the current of response.
If we now stimulate the endAby taps, or betterstill by torsional vibration, a transitory ‘current of action’ will be found to flow in the wire fromBtoA, from the unstimulated to the stimulated, and in the galvanometer from the stimulated to the unstimulated. Stimulation ofBwill give rise to a current in an opposite direction.
Fig. 50.—Electric Response in MetalsFig. 50.—Electric Response in Metals(a) Method of block; (b) Equal and opposite responses when the ends A and B are stimulated; the dotted portions of the curves show recovery; (c) Balancing effect when both the ends are stimulated simultaneously.
Fig. 50.—Electric Response in Metals
(a) Method of block; (b) Equal and opposite responses when the ends A and B are stimulated; the dotted portions of the curves show recovery; (c) Balancing effect when both the ends are stimulated simultaneously.
Experiment to exhibit the balancing effect.—If the wire has been carefully annealed, the molecular condition of its different portions is found to be approximately the same. If such a wire be held at the ‘balancing point’ (which is at or near the middle) by the clamp, and a quick vibration, say, of 90° be given toA, an upward deflection will be produced; if a vibration of 90° be given toB, there will be an equal downward deflection. If now both the endsAandBare vibrated simultaneously, the responsive E.M. variation at the two ends will continuously balance each other and the galvanometer spot will remain quiescent (fig. 30,A,B,R). This balance will be still maintained when the block is removed and the wire is vibrated as a whole. It is to be remembered that with the length of wire constant,the intensity of stimulus increases with the amplitude of vibration. Again, keeping the amplitude constant, the intensity of stimulus is increased by shortening the wire. Hence it will be seen that if the clamp be shifted from the balancing point towardsA, simultaneous vibration ofAandBthrough 90° will now give a resultant upward deflection, showing that theAresponse is now relatively stronger. Thus keeping the rest of the circuit untouched, merely moving the clamp from the left, past the balancing point to the right, we get either a positive, or zero, or negative, resultant effect.
In tin the current of response is from the less to the more excited point. In the retina also, we found the current of action flowing from the less stimulated to the more stimulated, and as that is known as a positive response, we shall consider the normal response of tin to be in like manner positive.
Just as the response of retina or nerve, under certain molecular conditions, undergoes reversal, the positive being then converted into negative, and negative into positive, so it will be shown that the response in metallic wires under certain conditions is found to undergo reversal.
Anomalies of present terminology.—When there is no current of injury, a particular current of response can hardly be called a negative, or positive,variation. Such nomenclature is purely arbitrary, and leads, as will be shown, to much confusion. A more definite terminology, free from misunderstanding, would be, as already said, to regard the current towards the more stimulated as positive, and that towards the less stimulated, in tissue or wire, as negative.The stimulated end of tin, say the endA, thus becomeszincoid, i.e. the current through the electrolyte (non-polarisable electrodes with interposed galvanometer) is fromAtoB, andthrough the wire, from the less stimulatedBto the more stimulatedA. Conversely, whenBis stimulated, the action current flows round the circuit in an opposite direction. This positive is the most usual form of response, but there are cases where the response is negative.In order to show that normally speaking a stimulated wire becomes zincoid, and also to show once more the anomalies into which we may fall by adopting no more definite terminology than that of negative variation, I have devised the following experiment (fig. 51). Let us take a bar, one half of which is zinc and the other half copper, clamped in the middle, so that a disturbance produced at one end may not reach the other; the two ends are connected to a galvanometer through non-polarisable electrodes. The current through the electrolyte (non-polarisable electrodes and interposed galvanometer) will then flow from left to right. We must remember that metals under stimulation generally become, in an electrical sense, more zinc-like. On vibrating the copper end (inasmuch as copper would then become more zinc-like) the difference of potential between zinc and copper ought to be diminished, and the current flowing in the circuit would therefore be lessened. But vibration of the zinc end ought to increase the potential difference, and there ought to be then an increase of current during stimulation of zinc.
Anomalies of present terminology.—When there is no current of injury, a particular current of response can hardly be called a negative, or positive,variation. Such nomenclature is purely arbitrary, and leads, as will be shown, to much confusion. A more definite terminology, free from misunderstanding, would be, as already said, to regard the current towards the more stimulated as positive, and that towards the less stimulated, in tissue or wire, as negative.
The stimulated end of tin, say the endA, thus becomeszincoid, i.e. the current through the electrolyte (non-polarisable electrodes with interposed galvanometer) is fromAtoB, andthrough the wire, from the less stimulatedBto the more stimulatedA. Conversely, whenBis stimulated, the action current flows round the circuit in an opposite direction. This positive is the most usual form of response, but there are cases where the response is negative.
In order to show that normally speaking a stimulated wire becomes zincoid, and also to show once more the anomalies into which we may fall by adopting no more definite terminology than that of negative variation, I have devised the following experiment (fig. 51). Let us take a bar, one half of which is zinc and the other half copper, clamped in the middle, so that a disturbance produced at one end may not reach the other; the two ends are connected to a galvanometer through non-polarisable electrodes. The current through the electrolyte (non-polarisable electrodes and interposed galvanometer) will then flow from left to right. We must remember that metals under stimulation generally become, in an electrical sense, more zinc-like. On vibrating the copper end (inasmuch as copper would then become more zinc-like) the difference of potential between zinc and copper ought to be diminished, and the current flowing in the circuit would therefore be lessened. But vibration of the zinc end ought to increase the potential difference, and there ought to be then an increase of current during stimulation of zinc.
Fig. 51.—Current of Response towards the Stimulated EndFig. 51.—Current of Response towards the Stimulated EndHence when Cu stimulated: action current →, normal E.M.F. diminished (·85-·009) V.When Zn stimulated: action current ←, normal E.M.F. increased (·85 + ·013) V.
Fig. 51.—Current of Response towards the Stimulated End
Hence when Cu stimulated: action current →, normal E.M.F. diminished (·85-·009) V.
When Zn stimulated: action current ←, normal E.M.F. increased (·85 + ·013) V.
In the particular experiment offig. 51, the E.M.F. between the zinc and copper ends was found to be ·85 volt. This was balanced by a potentiometer arrangement, so that the galvanometer spot came to zero. On vibrating the zinc wire, a deflection of 33 dns. was obtained, in a direction whichshowed anincreaseof E.M.F. On stopping the vibration, the spot of light came back to zero. On now vibrating the copper wire, a deflection of 23 dns. was obtained in an opposite direction, showing adiminutionof E.M.F. This transitory responsive variation disappeared on the cessation of disturbance.By disturbing the balance of the potentiometer, the galvanometer deflection due to a known increase of E.M.F. was found from which the absolute E.M. variation caused by disturbance of copper or zinc was determined.It was thus found that stimulation of zinc had increased the P.D. by fifteen parts in 1,000, whereas stimulation of copper had decreased it by eleven parts in 1,000. According to the old terminology, the response due to stimulation of zinc would have been regarded as positive variation, that of copper negative. The responses however are not essentially opposite in character, the action current in the bar being in both cases towards the more excited. For this reason it would be preferable, as already said, to employ the terms positive and negative in the sense I have suggested, i.e. positive, when the current in the acted substance is towards the more excited, and negative, when towards the less excited. The method of block is, as I have already shown, the most perfect for the study of these responses.
In the particular experiment offig. 51, the E.M.F. between the zinc and copper ends was found to be ·85 volt. This was balanced by a potentiometer arrangement, so that the galvanometer spot came to zero. On vibrating the zinc wire, a deflection of 33 dns. was obtained, in a direction whichshowed anincreaseof E.M.F. On stopping the vibration, the spot of light came back to zero. On now vibrating the copper wire, a deflection of 23 dns. was obtained in an opposite direction, showing adiminutionof E.M.F. This transitory responsive variation disappeared on the cessation of disturbance.
By disturbing the balance of the potentiometer, the galvanometer deflection due to a known increase of E.M.F. was found from which the absolute E.M. variation caused by disturbance of copper or zinc was determined.
It was thus found that stimulation of zinc had increased the P.D. by fifteen parts in 1,000, whereas stimulation of copper had decreased it by eleven parts in 1,000. According to the old terminology, the response due to stimulation of zinc would have been regarded as positive variation, that of copper negative. The responses however are not essentially opposite in character, the action current in the bar being in both cases towards the more excited. For this reason it would be preferable, as already said, to employ the terms positive and negative in the sense I have suggested, i.e. positive, when the current in the acted substance is towards the more excited, and negative, when towards the less excited. The method of block is, as I have already shown, the most perfect for the study of these responses.
In the experimentfig. 50, if the block is abolished and the wire is struck in the middle, a wave of molecular disturbance will reachAandB. The mechanical and the attendant electrical disturbance will at these points reach a maximum and then gradually subside. The resultant effect in the galvanometer will be due toEA-EBwhenEAandEBare the electrical variations produced atAandBby the stimulus. The electric changes atAandBwill continuously balance each other, and the resultant effect on the galvanometer will be zero: (a) ifthe exciting disturbance reachesAandBat the same time and with the same intensity; (b) if the molecular condition is similar at the two points; and (c) if the rate of rise and subsidence of excitation is the same at the two points. In order that a resultant effect may be exhibited in the galvanometer, matters have to be so arranged that the disturbance may reach one point, sayA, and notB, andvice versa. This was accomplished by means of a clamp, in the method of block. Again a resultant differential action may be obtained even when the disturbance reaches bothAandB, if the electrical excitability of one point is exalted or depressed by physical or chemical means. We shall in Chap. XVI study in detail the effect of chemical reagents in producing the enhancement or depression of excitability. There are thus two other means of obtaining a resultant effect—(2) by the method of relative depression, (3) by the method of relative exaltation.
Electric response by method of depression.—We may thus by reducing or abolishing the excitability of one end by means of suitable chemical reagents (so-called method of injury) obtain response in metals without a block. The entire length of the wire may then be stimulated and a resultant response will be produced, owing to the difference between the excitability of the two ends. A piece of tin wire is taken, and one normal contact is made atA(strip of cloth moistened with water, or very dilute salt solution). The excitability ofBis depressed by a few drops of strong potash or oxalic acid. By the application of the latter there will be a small P.D. betweenAandB; this will simplyproduce a displacement of zero. By means of a potentiometer the galvanometer spot may be brought back to the original position. The shifting of the zero will not affect the general result. The effect of mechanical stimulus is to produce a transient electro-motive response, which will be superposed algebraically on the existing P.D. The deflection will take place from the modified zero to which the spot returns during recovery. On now stimulating the wire as a whole by, say, torsional vibration, the current of response will be found towards the more excitable, i.e. fromBtoA(fig. 52,a).
Fig. 52.—Response by Method of Depression (Without Block)Fig. 52.—Response by Method of Depression (Without Block)When the wire is stimulated as a whole the current of response is towards the more excitable.In (a) A is a normal contact, B has been depressed by oxalic acid; current of response is towards the more excitable A.In (b) the same wire is used, only A is depressed by oxalic acid and a normal contact is made at a fresh point B′, a little to the left of B in (a). Current of response is now from A towards the more excitable B′.
Fig. 52.—Response by Method of Depression (Without Block)
When the wire is stimulated as a whole the current of response is towards the more excitable.
In (a) A is a normal contact, B has been depressed by oxalic acid; current of response is towards the more excitable A.
In (b) the same wire is used, only A is depressed by oxalic acid and a normal contact is made at a fresh point B′, a little to the left of B in (a). Current of response is now from A towards the more excitable B′.
A corroborative reversal experiment may next be made on the same piece of wire. The normal contact, through water or salt solution, is now made atB′, a little to the left ofB. The excitability ofAis now depressed by oxalic acid. On stimulation of the whole wire, the current of response will now be found to flow in an opposite direction—i.e. fromAtoB′—but still from the relatively less to the relatively more excitable (fig. 52,b).
From these experiments it will be seen how in one identical piece of wire the responsive current flows now in one direction and then in the other, in absolute conformity with theoretical considerations.
Fig. 53.—Method of ExaltationFig. 53.—Method of ExaltationThe contact B is made more excitable by chemical stimulant (Na2CO3). The current of response is towards the more excitable B.
Fig. 53.—Method of Exaltation
The contact B is made more excitable by chemical stimulant (Na2CO3). The current of response is towards the more excitable B.
Method of exaltation.—A still more striking corroboration of these results may, however, be obtained by the converse process of relative exaltation of the responsiveness of one contact. This may be accomplished by touching one contact, sayB, with a reagent which like Na2CO3exalts the electric excitability. On stimulation of the wire, the current of response is towards the more excitableB(fig. 53).
I give four records (fig. 54) which will clearly exhibit the responses as obtained by the methods of relative depression or exaltation. In (a)Bis touched with the excitant Na2CO3, a permanent current flows fromAtoB, response to stimulus is in the same direction as the permanent current (positive variation). In (b)Bis touched with a trace of the depressant oxalic acid, the permanent current is in the same direction as before, but the current of response is in the opposite direction (negative variation). In (c)Bis touched with dilute KHO, the response is exhibited by a positive variation. In (d)Bis touched with strong KHO, the response is now exhibited by a negative variation. The last two results, apparently anomalous, are due to the fact, which will be demonstrated later,that KHO in minute quantities is an excitant, while in large quantities it is a depressant.
Fig. 54Fig. 54PermanentCurrentCurrentofResponseB treated with sodium carbonate.→→B treated with oxalic acid→←B treated with very dilute potash→→B treated with strong potash→←Current of response is always towards the more excitable point.(a) Response when B is treated with sodium carbonate.—An apparent positive variation.(b) Response when B is treated with oxalic acid.—An apparent negative variation.(c) Response when B is treated with very dilute potash.—Positive variation.(d) Response when B is treated with strong potash.—Negative variation.The response is up when B is more excitable, and down when A is more excitable.Lines thus ------ indicate deflection due to permanent current.
Fig. 54
Current of response is always towards the more excitable point.
The response is up when B is more excitable, and down when A is more excitable.
Lines thus ------ indicate deflection due to permanent current.
We have thus seen that we may obtain response (1) by block method, (2) by the method of injury, or relative depression of responsiveness of one contact, and (3) by the method of relative exaltation of responsiveness of one contact. In all these cases alike we obtain a consistent action current, which in tin is normally positive, or towards the relatively more excited.
FOOTNOTES:[14]Following another line of inquiry I obtained response to electric stimulus in inorganic substances using the method of conductivity variation (see ‘De la Généralité des Phénomènes Moléculaires Produits par l’Electricité sur la Matière Inorganique et sur la Matière Vivante,’Travaux du Congrès International de Physique, Paris, 1900; and also ‘On Similarities of Effect of Electric Stimulus on Inorganic and Living Substances,’British Association 1900. SeeElectrician). To bring out the parallelism in all details between the inorganic and living response, I have in the following chapters used the method of electro-motive variation employed by physiologists.[15]By ‘tin’ is meant an alloy of tin and lead used as electric fuse.
[14]Following another line of inquiry I obtained response to electric stimulus in inorganic substances using the method of conductivity variation (see ‘De la Généralité des Phénomènes Moléculaires Produits par l’Electricité sur la Matière Inorganique et sur la Matière Vivante,’Travaux du Congrès International de Physique, Paris, 1900; and also ‘On Similarities of Effect of Electric Stimulus on Inorganic and Living Substances,’British Association 1900. SeeElectrician). To bring out the parallelism in all details between the inorganic and living response, I have in the following chapters used the method of electro-motive variation employed by physiologists.
[14]Following another line of inquiry I obtained response to electric stimulus in inorganic substances using the method of conductivity variation (see ‘De la Généralité des Phénomènes Moléculaires Produits par l’Electricité sur la Matière Inorganique et sur la Matière Vivante,’Travaux du Congrès International de Physique, Paris, 1900; and also ‘On Similarities of Effect of Electric Stimulus on Inorganic and Living Substances,’British Association 1900. SeeElectrician). To bring out the parallelism in all details between the inorganic and living response, I have in the following chapters used the method of electro-motive variation employed by physiologists.
[15]By ‘tin’ is meant an alloy of tin and lead used as electric fuse.
[15]By ‘tin’ is meant an alloy of tin and lead used as electric fuse.
We have already seen that metals respond to stimulus by E.M. variation, just as do animal and vegetable tissues. We have yet to see whether the similarity extends to this point only, or goes still further, whether the response-curves of living and in organic are alike, and whether the inorganic response-curve is modified, as living response was found to be, by the influence of external agencies. If so, are the modifications similar? What are the effects of superposition of stimuli? Is there fatigue? If there be, in what way does it affect the curves? And lastly, is the response of metals exalted or depressed by the action of chemical reagents?
Conditions of obtaining quantitative measurements.—In order to carry out these investigations, it is necessary to remove all sources of uncertainty, and obtain quantitative measurements. Many difficulties at first presented themselves in the course of this attempt, but they werecompletely removed by the adoption of the following experimental modification. In the simple arrangement for qualitative demonstration of response in metals previously described, successive experiments will not give results which are strictly comparable (1) unless the resistance of the circuit be maintained constant. This would necessitate the adoption of some plan for keeping the electrolytic contacts atAandBabsolutely invariable. There should then be no chance of any shifting or variation of contact. (2) There must also be some means of applying successive stimuli of equal intensity. (3) And for certain further experiments it will be necessary to have some way of gradually increasing or decreasing the stimuli in a definite manner.
Modification of the block method.—By consideration of the following experimental modifications of the block method (fig. 55), it will be found easy to construct a perfected form of apparatus, in which all these conditions are fully met. The essentials to be kept in mind were the introduction of a complete block midway in the wire, so that the disturbance of one half should be prevented from reaching the other, and the making of a perfect electrolytic contact for the electrodes leading to the galvanometer.
Starting from the simple arrangement previously described where a straight wire is clamped in the middle (fig. 55,a), we next arrive at (b). Here the wireA Bis placed in aUtube and clamped in the middle by a tightly fitting cork. Melted paraffin wax is poured to a certain depth in the bend of the tube. The twolimbs of the tube are now filled with water, till the endsAandBare completely immersed. Connection is made with the non-polarisable electrodes by the side tubes. Vibration may be imparted to eitherAorBby means of ebonite clip holders seen at the upper endsA Bof the wire.
Fig. 55.—Successive Modifications of the Block Method from the ‘Straight Wire’ (a) to ‘Cell Form’ (e)Fig. 55.—Successive Modifications of the Block Method from the ‘Straight Wire’ (a) to ‘Cell Form’ (e)When A is excited, current of response in the wire is from less excited B to more excited A. Note that though the current of response is constant in direction, the galvanometer deflection in (d) will be opposite to that in (b).
Fig. 55.—Successive Modifications of the Block Method from the ‘Straight Wire’ (a) to ‘Cell Form’ (e)
When A is excited, current of response in the wire is from less excited B to more excited A. Note that though the current of response is constant in direction, the galvanometer deflection in (d) will be opposite to that in (b).
It will be seen that the two limbs of the tube filled with water serve the purpose of the strip of moistened cloth used in the last experiment to make electric connections with the leading-out electrodes—with the advantage that we have here no chance of any shifting of contact or variation of surface, the contact betweenthe wire and the surrounding liquid being perfect and invariable.
On now vibrating the endAof the tin wire by means of the ebonite clip holder, a current will be found to flow fromBtoAthrough the wire—that is to say, towards the excited—and fromAtoBin the galvanometer.
The next modification (c) is to transfer the galvanometer from the electrolytic to the metallic part of the circuit, that is to say, it is interposed in a gap made by cutting the wireA B, the upper part of the circuit being directly connected by the electrolyte. Vibration ofAwill now give rise to a current of response which flows in the metallic part of the circuit with the interposed galvanometer fromBtoA. We see that though the direction of the current in this is the same as in the last case, yet the galvanometer deflection is now reversed, for the evident reason that we have it interposed in the metallic and not in the electrolytic part of the circuit.
The next arrangement (d) consists simply of the preceding placed upside down. HereAandBare held parallel to each other in an electrolytic bath (water). Mechanical vibration may now be applied toAwithout affectingB, andvice versa.
The actual apparatus, of which this is a diagrammatic representation, is seen in (e).
Two pieces, from the same specimen of wire, are clamped separately at their lower ends by means of ebonite screws, in anL-shaped piece of ebonite. The wires are fixed at their upper ends to two electrodes—leading to the galvanometer—and kept moderately and uniformly stretched by spiral springs. The handle, by which a torsional vibration is imparted to the wire, may be slipped over either electrode. The amplitude of vibration is measured by means of a graduated circle.
It will be seen from these arrangements:
(1) That the cell depicted in (e) is essentially the same as that in (a).
(2) That the wires in the cell being immersed to a definite depth in the electrolyte there is always a perfect and invariable contact between the wire and the electrolyte. The difficulty as regards variation of contact is thus eliminated.
Fig. 56.—Equal and Opposite Responses exhibited by A and BFig. 56.—Equal and Opposite Responses exhibited by A and B
Fig. 56.—Equal and Opposite Responses exhibited by A and B
(3) That as the wiresAandBare clamped separately below, we may impart a sudden molecular disturbance to eitherAorBby giving a quick to-and-fro (torsional) vibration round the vertical wire, as axis, by means of the handle. As the wireAis separate fromB, disturbance of one will not affect the other. Vibration ofAproduces a current in one direction, vibration ofBin the opposite direction. Thus we have means of verifying every experiment by obtaining corroborative and reversed effects. When the two wires have been brought to exactly the same molecular condition by theprocesses of annealing or stretching, the effects obtained on subjectingAorBto any given stimulus are always equal (fig. 56).
Usually I interpose an external resistance varying from one to five megohms according to the sensitiveness of the wire. The resistance of the electrolyte in the cell is thus relatively small, and the galvanometer deflections are proportional to the E.M. variations. It is always advisable to have a high external resistance, as by this means one is not only able to keep the deflections within the scale, but one is not troubled by slight accidental disturbances.
Graduation of intensity of stimulus.—If now a rapid torsional vibration be given toAorB, an E.M. variation will be induced. If the amplitude of vibration be kept constant, successive responses—in substances which, like tin, show no fatigue—will be found to be absolutely identical. But as ‘the amplitude of vibration’ is increased, response will also become enhanced (see Chap. XV).
Fig. 57.—Top View of the Vibration CellFig. 57.—Top View of the Vibration CellThe amplitude of vibration is determined by means of movable stops S S′, fixed to the edge of the graduated circle G. The index arm I plays between the stops. (The second index arm, connected with B, and the second circle are not shown.)
Fig. 57.—Top View of the Vibration Cell
The amplitude of vibration is determined by means of movable stops S S′, fixed to the edge of the graduated circle G. The index arm I plays between the stops. (The second index arm, connected with B, and the second circle are not shown.)
Amplitude of vibration is measured by means of the graduated circle (fig. 57). A projecting index, in connection with the vibration-head, plays between fixed and sliding stops (SandS′), one at the zero point of the scale, and the other movable.The amplitude of a given vibration can thus be predetermined by the adjustment of the sliding stop. In this way we can obtain either uniform or definitely graduated stimuli.
Considerations showing that electric response is due to molecular disturbance.—The electromotive variation varies with the substance. With superposition of stimuli, a relatively high value is obtained in tin, amounting sometimes to nearly half a volt, whereas in silver the electromotive variation is only about ·01 of this value. The intensity of the response, however, does not depend on the chemical activity of the substance, for the electromotive variation in the relatively chemically inactive tin is greater than that of zinc. Again, the sign of response, positive or negative, is sometimes modified by the molecular condition of the wire (see Chap. XII).
As regards the electrolyte, dilute NaCl solution, dilute solution of bichromate of potash &c. are normal in their action, that is to say, the electric response in such electrolytes is practically the same as with water. Ordinarily I use tap-water as the electrolyte. Zinc wires in ZnSO4solution give responses similar in character to those given by, for example, Pt or Sn in water.
Test experiment.—It may be urged that the E.M. effect is due in some way (1) to the friction of the vibrating wire against the liquid; or (2) to some unknown surface action, at the point in the wire of the contact of liquid and air surfaces. This second objection has already been completely met in experimental modification,fig. 55,b, where the wire was shown to give response when kept completely immersed in water, variation of surface being thus entirely eliminated.
Both these questions may, however, be subjected to a definite and final test. When the wire to be acted on is clamped below, and vibration is imparted to it, a strong molecular disturbance is produced. If now it be carefully released from the clamp, and the wire rotated backwards and forwards, there could be little molecular disturbance, but the liquid friction and surface variation, if any, would remain. The effect of any slight disturbance outstanding owing to shaking of the wire would be relatively very small.
We can thus determine the effect of liquid friction and surface action by repeating an experiment with and without clamping. In a tin wire cell, with interposed external resistance equal to one million ohms, the wireAwas subjected to a series of vibrations through 180°, and a deflection of 210 divisions was obtained. A corresponding negative deflection resulted on vibrating the wireB. NowAwas released from the clamp, so that it could be rotated backwards and forwards in the water by means of the handle. On vibrating the wireAno measurable deflection was produced, thus showing that neither water friction nor surface variation had anything to do with the electric action. The vibration of the still clampedBgave rise to the normal strong deflection.
As all the rest of the circuit was kept absolutely the same in the two different sets of experiments, theseresults conclusively prove that the responsive electro-motive variation is solely due to the molecular disturbance produced by mechanical vibration in the acted wire.
A new and theoretically interesting molecular voltaic cell may thus be made, in which the two elements consist of thesame metal. Molecular disturbance is in this case the main source of energy. A cell once made may be kept in working order for some time by pouring in a little vaseline to prevent evaporation of the liquid.
It will be shown further, in succeeding chapters, by numerous instances, that any conditions which increase molecular mobility will also increase intensity of response, and conversely that any conditions having the reverse effect will depress response.
I shall now proceed to describe in detail the response-curves obtained with metals. The E.M. variations resulting from stimulus range, as has been said, from ·4 volt to ·01 of that value, according to the metal employed. And as these are molecular phenomena, the effect will also depend on the molecular condition of the wire.
Preparation of wire.—In order to have our results thoroughly consistent, it is necessary to bring the wire itself into a normal condition for experiment. The very fact of mounting it in the cell strains it, and the after-effect of this strain may cause irregularities in the response.
For the purpose of bringing the wire to this normal state, one or all of the following devices may be used with advantage. (1) The wires obtained are usually wound on spools. It is, therefore, advisable to straighten any given length, before mounting, by holding it stretched, and rubbing it up and down with a piece of cloth. On washing with water, they are now ready for mounting in the cell.
(2) The cell is usually filled with tap-water, and a period of rest after making up, generally speaking, improves the sensitiveness. These expedients are ordinarily sufficient, but it occasionally happens that the wire has got into an abnormal condition.
Fig. 58.—Effect of Annealing on increasing the Response of both A and B Wires (Tin)Fig. 58.—Effect of Annealing on increasing the Response of both A and B Wires (Tin)Stimuli (vibration of 160°) applied at intervals of one minute.
Fig. 58.—Effect of Annealing on increasing the Response of both A and B Wires (Tin)
Stimuli (vibration of 160°) applied at intervals of one minute.
In this case it will be found helpful (3) to have recourse to the process of annealing. For if response be a molecular phenomenon, then anything that increases molecular mobility will also increase its intensity. Hence we may expect annealing to enhance responsiveness. This inference will be seen verified in the record given infig. 58. In the case under consideration, the convenient method employed was by pouring hot water into the cell, and allowing it to stand and cool slowly. The first three pairs of responses were taken by stimulatingAandBalternately, on mounting in the cell, which was filled with water. Hot water was then substituted, and the cell wasallowed to cool down to its original temperature. The six following pairs of responses were then taken. That this beneficial effect of annealing was not due to any accidental circumstance will be seen from the fact thatbothwires have their sensitiveness equally enhanced.
(4) In addition to this mode of annealing, both wires may be short-circuited and vibrated for a time. Lastly (5) slight stretchingin situwill also sometimes be found beneficial. For this purpose I have a screw arrangement.
By one or all of these methods, with a little practice, it is always possible to bring the wires to a normal condition. The responses subsequently obtained become extraordinarily consistent. There is therefore no reason why perfect results should not be arrived at.
Fig. 59.—Uniform Responses in TinFig. 59.—Uniform Responses in Tin
Fig. 59.—Uniform Responses in Tin
Effect of single stimulus.—The accompanying figure (fig. 59) gives a series, each of which is the response curve for a single stimulus of uniform intensity, the amplitude of vibration being kept constant. The perfect regularity of responses will be noticed in this figure. The wire after a long period of rest may be in an abnormal condition, but after a short period of stimulation the responses become extremely regular, as may be noticed in this figure. Tin is, usually speaking, almost indefatigable, and I have often obtained several hundreds of successive responses showing practically no fatigue. In the figure it will be noticed that the rising portionof the curve is somewhat steep, and the recovery convex to the abscissa, the fall being relatively rapid in its first, and less rapid in its later, parts. As the electric variation is the concomitant effect of molecular disturbance—a temporary upset of the molecular equilibrium—on the cessation of the external stimulus, the excitatory state, and its expression in electric variation, disappear with the return of the molecules to their condition of equilibrium. This process is seen clearly in the curve of recovery.
Different metals exhibit different periods of recovery, and this again is modified by any influence which affects the molecular condition.
That the excitatory state persists for a time even on the cessation of stimulus can be independently shown by keeping the galvanometer circuit open during the application of stimulus, and completing it at various short intervals after the cessation, when a persisting electrical effect, diminishing rapidly with time, will be apparent. The rate of recovery immediately on the cessation of stimulus is rather rapid, but traces of strain persist for a short time.
We have seen that the stimulation of matter causes an electric variation, and that the acted substance gradually recovers from the effect of stimulus. We shall next study how the form of response-curves is modified by various agencies.
In order to study these effects we must use, in practice, a highly sensitive galvanometer as the recorder of E.M. variations. This necessitates the use of an instrument with a comparatively long period of swing of needle, or of suspended coil (as in a D’Arsonval). Owing to inertia of the recording galvanometer, however, there is a lag produced in the records of E.M. changes. But this can be distinguished from the effect of the molecular inertia of the substance itself by comparing two successive records taken with the same instrument, in one of which the latter effect is relatively absent, and in the other present. We wish, for example, to find outwhether the E.M. effect of mechanical stimulus is instantaneous, or, again, whether the effect disappears immediately. We first take a galvanometer record of the sudden introduction and cessation of an E.M.F. on the circuit containing the vibration-cell (fig. 60,a). We then take a record of the E.M. effect produced by a stimulus caused by a single torsional vibration. In order to make the conditions of the two experiments as similar as possible, the disturbing E.M.F., from a potentiometer, is previously adjusted to give a deflection nearly equal to that caused by stimulus. The torsional vibration was accomplished in a quarter of a second, and the contact with the potentiometer circuit was also made for the same length of time.