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Collected Physical Papers/On the Change of Conductivity of Metallic Particles under Cyclic Electromotive Variation

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XVII

ON THE CHANGE OF CONDUCTIVITY OF METALLIC PARTICLES UNDER CYCLIC ELECTROMOTIVE VARIATION

A mass of conducting particles has its conductivity changed when subjected to rapid electric vibration, as for instance, when acted on by electric waves. I have, in previous papers, adduced reasons leading to the conclusion that electric radiation produces a molecular change, and that conductivity variation is the expression of the induced change. We know that the physical properties of a given substance depend on its molecular condition. Any molecular change that may be induced may therefore be expected to be attended by changes in the physical properties of the substance, electric conductivity being one of these. The wide difference in electric conductivity of the same substance under different molecular conditions is seen in the case of carbon, in its two allotropic forms of graphite and diamond. The effectiveness of radiation in producing allotropic changes is seen in the conversion of the yellow into the red variety of phosphorus under the action of light.

That the variation of conductivity induced by electric radiation is due to some atomic or molecular action is seen from the fact that it is dependent on the chemical nature and the molecular condition of the substance. Thus, under electric radiation, the positive class of substances, e.g., Fe, Mg, etc., exhibit an increase of conductivity, whereas the negative class of substances, e.g., K, Br, I, show a diminution. The effect of molecular conditions in determining the sign of response to electric radiation (i.e., increase or diminution of conductivity) is exhibited by the two molecular varieties of silver, the response being positive in one case and negative in the other.

The conductivity variation is thus the expression of some molecular change. In many cases (potassium, Ag, brominated lead, etc.) we find that the change persists only during the action of radiation. On the cessation of the stimulus the substance at once shows elastic self-recovery. It would thus appear that the conductivity variation is due to molecular strain produced by radiation. This view is further supported by the fact that in working with self-recovering receivers of various types, both positive and negative, I have not only found that for each intensity of radiation there is a corresponding conductivity variation, but that under the continued action of radiation, the conductivity variation attains a maximum value, which remains constant, balanced by a force of restitution; when the radiation is stopped, the substance is found to recover its original conductivity. With an increase of intensity of radiation, the corresponding conductivity variation also increases, and vice versa. I have, in a Paper "On the Continuity of Effect of Light and Electric Radiation on Matter" given an account of self-recovering receiver, of negative type, made of silver particles, in which the conductivity variation is almost exactly proportional to the intensity of radiation. Self-recovery, in general, is merely a question of time; with certain substances it takes place immediately, with others, after a short or after a comparatively long period. This view of conductivity variation, as the expression of some molecular change, will receive independent support from the experiments on a self-recovering receiver to be described later.

The conductivity changes take place not only under very rapid Hertzian oscillations, but also under much slower electromotive variations. Thus an electromotive variation, whether quick or slow, is the effective cause of induced change of conductivity. In substances which exhibit recovery from the action of radiation so protracted, as to appear practically non-recovering, the induced molecular change is attended by a more or less permanent variation of conductivity. Recovery in such cases can be hastened by molecular vibration, by mechanical tape, or by the application of gentle heat.

After a rapid cyclic variation of E. M. F. the substance is thus transformed from its primitive condition; in other words a more or less irreversible molecular change is induced at the end of the process as exhibited by a hysteresis of conductivity. If this view be correct, then we can study the molecular change step by step, by observing the conductivity variations undergone by the substance as it is carried through a complete cycle of electromotive variation. The rapidity of this process must be just sufficiently slow to allow the successive changes to be recorded. The suddenness of electric variation no doubt exerts an influence on the amount of change; but this is a question of degree only.

The investigation thus resolves itself into the determination of the variation of conductivity of the sensitive particles, as the mass is subjected (1) to a continuously increasing E. M. F. from zero to a maximum, and then (2) to a continuously diminishing E. M. F. from the maximum back to zero. The required information may be obtained by the interpretation of the characteristic curve, in which the abscissæ represent the impressed E. M. F., and the ordinates give the corresponding values of the current. A continuously increasing E. M. F. can be made to act on the sensitive substance, by the movement of a slider over a potentiometer wire, say, to the right. The scale readings of the potentiometer give the values of the E. M. F. The readings of the galvanometer in circuit give the corresponding values of the current; movement of the slider to the left produces a continuous diminution of E. M. F. The characteristic curve can thus be obtained from the observed values of E. M. F. and the corresponding currents.

This is a bare outline of the principle of the investigation described in this Paper. Various experimental modifications have to be introduced in order to overcome certain difficulties and render the method practical.

Before entering upon the subject of experimental arrangements, I may repeat what I pointed out in my paper on Electric Touch that in regard to their response to electric radiation, there are three types of substances, positive, negative and neutral, differentiated "by the characteristic curves of variation of current with E. M. F." The positive exhibits an increase of conductivity; the negative shows a diminution of conductivity: and the neutral does not exhibit any conductivity change. The first two of these classes again fall into two subdivisions, of substances which exhibit self-recovery and those which do not. The characteristic curves of the three classes of substances thus belong to three distinct types, so that it is possible to determine the class to which the sensitive substance belongs by its characteristic curve.

These curves also throw much light on the obscure subject of the action of radiation on various sensitive substances. In the present Paper I shall describe in some detail the reaction of the first class of substances, i.e., those which show a diminution of resistance, and amongst these I shall take typical instances of non-recovering and self-recovering substances, and demonstrate the peculiarities of their cyclic characteristic curves. A brief reference will also be made of the characteristics of the second class which exhibit an increase of resistance.


Experimental Arrangements

The electric circuit consists of (l) the sensitive substance, (2) a sensitive dead-beat D'Arsonval galvanometer, and (3) a potentiometer arrangement for a gradual and continuous increase or diminution of E. M. F.

1. The Sensitive Substance.—Experiments were made with both single and multiple contacts. In the former case the pressure of contact was adjusted by means of a fine micrometer screw, or by means of springs. In some cases the contact ends were both rounded; in others, a pointed end pressed against a flat piece.

In the case of multiple contacts, a small quantity of metallic filings was put in a glass tube, and the fragments compressed between two electrodes, the pressure being regulated by a perfectly easy-running micrometer screw. The pressure was adjusted till a feeble initial current flowed through the circuit. The micrometer screw, together with the tube of filings, were appropriately fixed on a heavy base. This rested in turn on a steady pedestal, with one or two sets of pneumatic tyres interposed. Experiments were carried out with the sensitive substance in air, or immersed in kerosine.

2. The Galvanometer was appropriately shunted so as to give a deflection of one division of scale for a definite small fraction of an ampere. In some experiments, for example, one division of the scale was equal to 10-6 ampere. The calculation of resistance of the sensitive substance is very much facilitated if the scale values of the galvanometer and the potentiometer were suitably adjusted beforehand. The resistance of the shunted galvanometer and the potentiometer was practically negligible in comparison with the resistance of the sensitive substance, which usually varied from about 1,000 to 50,000 ohms.

3. The Potentiometer consisted of a thin nickeline wire of uniform section, 20in. in length. Each inch of the scale was further sub-divided into tenths. A storage cell with an interposed resistance was applied to the terminals of the potentiometer, and the resistance adjusted till the terminal E. M. F. attained a suitable value. This was found by the help of an auxiliary voltmeter applied to the terminals of the potentiometer.

The E. M. F. acting on the main circuit was derived from the potentiometer; one end of the circuit was connected with one terminal of the potentiometer and the other connected with a slider, the movement of which to the right or to the left increased or diminished the impressed E. M. F. The sliding contact was made by means of a bent flat metallic spring which uniformly pressed against the potentiometer wire. Care was taken to maintain a uniformly good electrical contact. Any sudden introduction of the E. M. F. was avoided with special care, as, owing to the self-induction of the circuit, an unknown induced E. M. F. might act on the sensitive substance. The slide-contact was therefore never broken; starting from zero of the scale (E. M. F.=0) the slider was moved at a uniform rate to the right, thus continuously increasing the E. M. F. without any sudden variation. Movement of the slider to the left produced a continuous diminution of E. M. F. The voltaic cell was never cut off from action till the slider returned to zero.


Method of Observation

One observer moved the slider at a uniform rate and called out the successive potentiometer readings; the corresponding deflections of the galvanometer were read off by the second observer, and a third took down in parallel columns the applied E. M. F. and the corresponding galvanometer readings. From these data the characteristic curves could be subsequently obtained. This method of work was, however, very tedious. It took a long time and the co-operation of three persons to complete one experiment, and the curves could only be obtained from a series of observations which were not absolutely continuous. I had, therefore, to devise a recording apparatus by which a continuous curve could be obtained in a simple and direct manner.


Recorder of Conductivity Variation

If a platform be fixed to the slider of the potentiometer, then the movement of the platform in one direction would continuously increase the E. M. F. acting on the circuit containing the sensitive substance (fig. 51). If the galvanometer spot of light is thrown down on the platform by a mirror, suitably inclined, and if

Collected Physical Papers Fig. 51.png
Fig. 51. Diagrammatic Representation of Conductivity Recorder. P, Moving Platform. AB, Potentiometer. R, Sensitive Receiver. r, Rheostat. V, Voltmeter. SC, Sliding Contact. S, Secondary Cell. M, Mirror.

the movement of the spot of light, due to variation of current, takes place at right angles to the motion of platform, then a continuous curve would be traced on the platform, of which the abscissa would represent the E. M. F. and the ordinates the currents. This curve could be fixed if we laid a sheet of photographic paper on the platform, or we might obtain it by the simpler expedient of following the track of the galvanometer spot with a pencil.

I used squared paper for recording the curves. It is divided into inches, and these further sub-divided into tenths. The potentiometer wire is 20in. long, and the length of the platform is the same. As the platform moves through its own length past a fixed index, the sliding contact moves through the whole length of the potentiometer. If the applied E. M. F. at the terminals A and B be, for example, 1 volt, each inch of paper along the platform would then represent 1/20th or 0·05 volt, the smaller sub-division representing 0·005 volt. This or any other suitable terminal value of E. M. F. can be obtained with the help of the storage cell S, a rheostat r and an auxiliary voltmeter V, applied to the terminals A and B.

The galvanometer, as was said before, is also adjusted by means of a shunt, so that a deflection of one division is a convenient small fraction of an ampere. The absolute values of the E. M. F. and current being known, the resistance of the sensitive substance at any point of the curve can easily be determined.

In the completed apparatus the platform is mounted on small wheels and moves on rails with perfect smoothness. A string connected with one end of the platform is carried round a large winding wheel and a second string connected with the other end of the platform carries a counterpoise. When the wheel is turned at a uniform rate in one direction, the platform carrying the paper also moves at a uniform rate, at the same time producing a continuous increase of E. M. F. acting on the circuit. When the wheel is turned in the opposite direction, the counterpoise reverses the motion, and the impressed E. M. F. undergoes a continuous diminution. In this way it is possible to produce either an increase, or a decrease, or a rapid cyclic variation of E. M. F. I shall first describe in detail the rising part of the curve thus obtained, and then deal with peculiarities of complete curve when carried through a cycle.

Characteristic Curve of a Single Point Iron Receiver

In order to reduce the conditions of experiment to their simplest, I first studied the effect of E. M. variation on Contact at a single point. The change induced was thus confined to the molecular layers at the point of contact. This consisted of a sharp point of iron, pressing against a convex iron surface, the pressure being capable of very delicate adjustment by means of a micrometer. Five different experiments were carried out with the same receiver.

The initial adjustments were made with an E. M. F. of 0·05 volt. The only difference made in the several experiments is as regards the initial current, caused by change in the pressure of contact.

In curve A (fig. 52) the initial current was the lowest value of the series. The pressure of contact was adjusted till the initial current at 0·05 volt was 2/105 ampere. The E. M. F. was now continuously increased by the turning of the winding wheel, and the curve obtained in the manner previously described.

It will be seen that the curve is not straight, but concave to the axis representing the current. As the E. M. F. is increased, the current increases at a greater ratio. I may say here, in anticipation, that this appears to be a characteristic of the positive class of substances, i.e., of those which, like iron, exhibit a diminution of resistance under electric radiation.

The resistance of the receiver is thus not constant, but undergoes a continuous decrease with increasing E. M. F. The conductivity is therefore increased with the rise of E. M. F. The curve becomes steeper as the E. M. F. is increased, the conductivity undergoing a rapid and continuous increase.

Hence conduction in such cases does not obey Ohm's law. The resistance is not independent of the E. M. F.

Collected Physical Papers Fig. 52.png
Fig. 52. Characteristic Curves of a Single Point Iron Receiver. A, B, C, D, E are different curves for different initial currents. C′ is the curve for a constant resistance.

but varies with it. A remarkable analogy is here presented with the phenomenon of magnetic conduction. Just as the magnetic permeability of iron is not constant, but varies with the magnetic force acting on it, so the conductivity of metallic particles is not constant, but varies with the E. M. F. acting on them. The characteristic curves given above bear also a remarkable resemblance to the curves of magnetisation. Other parallelisms will be noted later. These similarities are probably due to the fact that in both cases we are dealing ultimately with phenomena of molecular deflection and rearrangement, taking place in one, under increasing magnetising force, and in the other, under increasing electromotive force.

As the absolute values of the current and the E. M. F. are known, it is easy to find the resistance of the receiver for any given E. M. F. The different values of the resistance can also be determined by the method of substitution. After the curve is drawn, leaving the paper still on the platform, a resistance box is substituted for the receiver, and the whole operation is repeated, with this difference, that now we have to change the resistance continuously in order to keep the galvanometer spot on the curve. The values of these resistances at definite points of the curve now correspond to the resistances of the receiver at those points. We can thus find the value of the resistance of the receiver at any point in the curve. This way of finding the resistance was used to supplement that of direct calculation , from the known values of the E. M. F. and the corresponding current; the two results were found practically identical.

I give below a table showing the variation of resistance with E. M. F. for the curve A (fig. 52).

Table I.—Showing the Variation of Resistance with E. M. F.

E. M. F. (volt). Current (in 1/l05amps) Resistance (ohms).
0·05 2·0 2,500
0·10 4·1 2,440
0·15 6·6 2,270
0·20 9·2 2,l00
0·25 14·0 1,770
0·30 20·0 1,500
0·35 28·5 1,230
0·40 44·6 860
0·45 69·0 640

It will be seen from the above table that as the E. M. F. increased from 0·05 volt to 0·45 volt the resistance decreased continuously from 2,500 to 640 ohms, i.e., about one-fourth its original value, and this diminution of resistance or increase of conductivity is not abrupt, but continuous.

In the lower portion of the curve, where the E. M. F. is low, the resistance is great and its variation small; but the curve soon becomes steep with the rise of the electromotive force. With a higher E. M. F. the change, to which the conductivity variation is due, proceeds very rapidly. This change is so great that at a certain critical value of E. M. F. it is almost abrupt. This is well seen in curve A, a little beyond 0·45 volt.

When the E. M. F. is adjusted to fall just short of the critical point, say, at 0·4 volt, then a slight increase of E. M. F., say +0·1 volt, will cause a very great variation of current; whereas if the E. M. F. had been so adjusted as to fall very much below the critical point, say, at 0·3 volt, an additional electromotive force of the same amount 0·1 volt, produces a relatively small variation of current. In the former case, conditions stood as it were on the brink of a precipice, and a slight additional impetus precipitated a fall. It will thus be seen that if the E. M. F. acting on the receiver be so adjusted as to be near the critical point, a slight electromotive variation will produce a great change, exhibited by a very considerable increase of conductivity. It is interesting to note in this connection that I found the sensitiveness of receivers to electric radiation could be greatly enhanced by raising the E. M. F. acting on the circuit to a point as high as they could bear, just short of electric instability.


Influence of Intensity of Initial Current in Modifying the Characteristic Curve

When at the beginning of an experiment the receiver is adjusted at a definite low E. M. F., say, 0·05 volt, we can have any initial current, according to the adjusted pressure of contact, the current increasing with increase of pressure. We may thus start with a large or a small initial current, the E. M. F. at starting being constant.

In the curve B (fig. 52) the initial current is a little more than double that for A. It will be seen that the curve has become distinctly steeper; the critical point is at the same time lowered, from 0·46 volt as in the last case, to 0·43 volt. The sudden bend in the curve is now less pronounced. In connection with the lowering of the critical point by pressure, it is also interesting to note that by increasing the pressure within certain limits, I succeeded in greatly enhancing the sensitiveness of the receiver to the action of electric radiation.

In the curves C, D, and E the initial currents were increased with resulting increase of steepness of the curves.

In order to test further the accuracy of the adjustments, I have taken on the same chart, the curve for a constant resistance. In C, the resistance of the receiver at 0·15 volt was found to be 830 ohms. A resistance box having the above resistance was substituted for the receiver, and a curve traced in the usual manner. This curve C′ is seen to be a straight line.

I give below a table showing the results of four sets of experiments on the variation of the resistance r and of conductivity c of the receiver under increasing E. M. F.

Table II.—Showing the Diminution of Resistance of the Receiver, with increasing E. M. F.

B C D E
Current in 1/105 amp. Resist in ohms. c r c r c r
0·10 9·0 1,136 12·0 830 25 400 43 232
0·15 14·2 1,060 18·0 830 40 375 69 217
0·20 20·5 970 26·0 770 59 340 102 196
0·25 29·5 850 40·0 625 83 300
0·30 40·0 750 63·5 472 118 254
0·35 56·0 620 105·0 333
0·40 85·0 470

It will be seen that in all the above cases the resistance undergoes a continuous diminution with the increase of E. M. F., and that the diminution of resistance due to electromotive variation is not an abrupt, but essentially a continuous process.

Experiments with Receiver Immersed in Kerosine

In order to find out whether the exclusion of atmospheric action would change the general character of the result, I took observations with a single point iron receiver immersed in kerosine.


Table III.—Showing the Diminution of Resistance with Increasing E. M. F. in a Receiver immersed in Kerosine

E. in volt. Current in 1/106 amp. Resistance in ohms.
0·05 15 3,333
0·10 35 2,857
0·15 56 2,678
0·20 78 2,564
0·25 124 2,016
0·30 182 1,648

Collected Physical Papers Fig. 53.png
Fig. 53. Effect of Lag.

It will be observed that under the above condition also, there is a continuous diminution of resistance with an increase of E. M. F.


Time-lag

The changes which give rise to the conductivity variation take a short time for their completion. The curves were taken when the electromotive variation was produced at a moderate and uniform speed. But if the electromotive variation is produced quickly and step by step, each step being followed by a pause, then

Collected Physical Papers Fig. 54.png
Fig. 54. Curves for Filings.

there is an immediate effect, followed by a permanent effect, the galvanometer deflection creeping up to this permanent value. This creeping effect becomes more marked with higher E. M. F. I give above a record where this effect is shown (fig. 53). The vertical portions of the curve represent the creeping effect during pause. The conductivity variation thus lags slightly behind the impressed electromotive variation. Certain effects due to this will be noticed later.


Characteristic Curves given by a Mass of Iron Filings

Owing to the multiplicity of contacts the conditions here are not so simple as in the case described above. It will, however, be seen from the curves given in figure 54, that the results are of the same general nature. The resistance undergoes diminution with increasing E. M. F. The curves are steeper with stronger initial currents. Greater intensity of initial current appears also to have the effect of lowering the critical point. I obtained similar results with Mg, and Ni.

It is difficult in one curve to represent adequately the variation of conductivity caused by small, moderate, and excessive increase of E. M. F. Broadly speaking, the curve may be divided into three parts. In the first, when the E. M. F. is low, the change is slight. The curve then becomes very steep in the second part, the conductivity variation being rapid. The increase of conductivity, however, reaches a limit, after which there is little further change. The whole curve is thus somewhat S-shaped.


Conductivity Variation with Cyclic Variation of E. M. F.

When conducting particles of the non-recovering, positive, or, iron type are subjected to sudden electric variatian due to radiation, a residual effect is observed indicative of an increase of conductivity or diminution of resistance.

Collected Physical Papers Fig. 55.png
Fig. 55. Cyclic Curves showing Conductivity Hysteresis. In each curve the right half is due to increasing, the left half to decreasing E. M. F.

Experiments were next carried out to find whether similar residual effects could be observed when the sensitive substance was carried through a cycle of electromotive variation. I tried both single and multiple contacts.

As stated before, the E. M. F. can be continuously increased by turning the winding wheel which moves the platform uniformly in one direction; the E. M. F. is continuously diminished by turning the wheel in the opposite direction, and thus reversing the motion of the platform. By this means the receiver can be subjected to a cyclic electromotive variation through large or small range at will. I reproduce curves of cyclic variation taken with filings, when the range of elctromotive variation is increased from (0·1—0·25 volt) to (0·1—0·81 volt). They will give a good general idea of the phenomena (fig. 55). A series of readings for cyclic variations taken with a single point receiver will be given later.

First, I tried the effect of cyclic variation through a small range, from 0·1 to 0·25 volt. It will be seen that the forward and return curves do not coincide, but enclose a small area. The receiver, in as far as conductivity is a criterion of its physical state, does not regain its original condition. There remains a residual conductivity variation, just as in iron there is a residual magnetism after it has been subjected to a cyclic variation of magnetising force. The residual magnetism disappears on tapping, just as the effect of residual conductivity can be dissipated by the same means.

In curve B, where the range of electromotive variation is still larger, from 0·1 to 0·43 there is a greater divergence between the forward and return curves, and the area enclosed is further increased. An interesting effect, due to lag, will be noticed at its far end; though the platform was on its return course, producing a diminution of E. M. F., the current nevertheless continued for a short time to rise. It is thus seen that whatever be the change to which the conductivity variation is due, it lags behind the impressed electromotive variation.

The curves C, D and E, further show that by increasing the range of electromotive variation, the area enclosed between the forward and return curves becomes considerably increased.

I give below detailed readings taken on two occasions with a single point iron receiver, (1) when the range of cyclic variation was small, (2) when the range was comparatively large (c. f. Tables IV and V).

Table IV.—Showing the Variation of Current and Resistance with Cyclic Electromotive Variation of Small Range

E. M. F. in volt. Galvanometer readings. 1dn=1/106 amp. Resistance in ohms
Direct. Return. Direct. Return.
0·05
0·10
0·15
0·20
0·25
0·30
10·0
25·5
47·0
70·0
103·0
171·0
12·0
27·0
49·5
75·5
114·5
197·5
5,000
3,920
3,191
2,856
2,427
1,754
4,166
3,703
3,030
2,648
2,183
1,520
 

It has been shown that the resistance of the receiver at the higher E. M. F. is less than that at the lower. As the E. M. F. is reduced by the reversed motion of the slider, there is a tendency towards the recovery of the higher resistance which it had at the lower E. M. F. The recovery, however, is not perfect, owing to hysteresis. The recovery is more incomplete if the critical E. M. F. had been exceeded.

There is thus a residual after-effect. In the case given, the initial resistance was 5,000 ohms, and the resistance after cyclic variation through a range of 0·25 volt was 4,166 ohms, or 8/10ths of its original value.

The lag and creeping effects are also noticeable in the forward and return readings.

While the slider moved up to 0·30 volt, the immediate galvanometer reading was 171 divisions, and by the time the slider had commenced its backward movement to the left (producing a diminution of E. M. F.), the current value had risen from 171 to 197·5; the conductivity had gone on increasing, the resistance having fallen from 1,754 to 1,520 ohms.

I now give a table of results for electromotive variation of wider range from 0·1 to 0·6 volt.

Table V .—Showing the Current and Conductivity Variation of a Single-point Iron Receiver, due to Cyclic Electromotive Variation of Comparatively Large Range but Short of Critical Point

E in volt. G. deflection, 1 dn=1/106 amp. Resistance in ohms.
Direct. Return. Direct. Return.
0·05
0·20
0·35
0·50
0·60
1·5
8·0
34·0
154·0
320·0
13
58
145
346
 
33,333
25,000
10,294
3,246
1,875
3,846
3,448
2,414
1,445
 
 

In the previous case it was shown that with a cyclic variation of the moderate range of 0·25 volt, the resistance was reduced to 8/10ths; in this case, at double the range, the reduction is very much greater, for the resistance falls to nearly 1/10th of its original value.

This confirms the conclusion previously made that the greater the range of electromotive variation the greater is the reduction of resistance.

Self-Recovering Receiver

It has been shown that receivers which exhibit a diminution of resistance under electric radiation give a characteristic curve which is concave to the axis representing the current, and that in the non-recovering type noticed hitherto, the forward and return curves do not coincide, but enclose an area. I was now interested in finding out what would be the peculiarities of the characteristic curve of receivers showing self-recovery.

I have, as previously mentioned, described a self-recovering receiver of the negative type, made of silver particles. I also succeeded in constructing a self-recovering single point receiver of the positive type,

Collected Physical Papers Fig. 56.png
Fig. 56. Response Curves to Single Flashes of Radiation in Self-Recovering Receiver.

the successive responses of which to brief flashes of radiation are given in figure 56.

In working with receivers which have been at rest for long periods it is often noticed that the response is at first feeble. The sensitiveness soon improves, and may even become excessive; it finally settles down to a moderate and uniform sensitivity. All these phases are well seen in fig. 56 where (a) exhibits the preliminary stages, and (b) the steadier condition, with more uniform response. The up-curves show the increase of current under the flash of radiation, and the falling and concave portions, the recovery on the cessation of radiation. In self-recovering receivers of the negative type the response curves are the reverse of these, in the sense of the reflected image in a mirror.


The Characteristic Curve of the Self-recovering Receiver of Positive Type

I now subjected the self-recovering receiver to a continuously increasing electromotive variation, and

Collected Physical Papers Fig. 57.png
Fig. 57. Characteristic Curve of Self-Recovering Receiver of Positive Type.

traced the curve. I was not prepared, however, for results so remarkably perfect. In the present case I was not only able to obtain the most important and characteristic effects with great ease and certainty, but also to repeat them, the results of successive experiments being practically identical.

The extreme regularity of the observed effect is seen at a glance in figure 57. I give below a table which shows how continuously the resistance is diminished under increasing E. M. F.


Table VI.-Showing Variation of Current and Resistance in Self-recovering Positive Receiver with Increasing E. M. F.

E. M. volt. Current. 5 dn=l/105 amp. Resistance in ohms.
0·2
0·3
0·4
0·5
0·6
0·7
0·8
0·9
1·0
1·1
1·2
2
5
10
17
27
40
55
73
94
115
141
50,000
30,000
20,000
14,700
11,100
8,750
7,270
6,160
5,320
4,780
4,245

It will be seen that whereas at 0·2 volt the resistance is 50,000 ohms, at 1·2 volt it has undergone an uninterrupted fall to 4,245 ohms, or 1/12th of its original value. There has been no sudden breakdown at any intermediate point.

Figure 58 exhibits the variation of resistance under increasing E. M. F. described in the table VI.

Collected Physical Papers Fig. 58.png
Fig. 58. Curve showing the Variation of Resistance with Increasing E. M. F.


Effect of Cyclic Variation of E. M. F.

But the most astonishing thing about the action of the receiver was that in taking the return curve, I found it practically coinciding in every part with the forward curve, so that when the E. M. F. was brought back to its initial value of 0·1 volt, the receiver had completely returned by its original path to its first condition, there being no residual effect. I repeated the experiment many times in succession, but the curves obtained, whether forwards or backwards, were in every case merely superposed on the original. Since the condition of the receiver is exactly the same after many cycles as at the beginning, it is evident that the conductivity variation could not be due essentially to any chemical change, for such changes are irreversible.

The increase of conductivity with increase of E. M. F. is not due to any sudden breakdown, such as fusion between the contact surfaces, for the process described above is anything but abrupt. Again, fusion would have produced a permanent conductivity change, but in the case under review the change is not permanent. Thus, for example, when the applied E. M. F. was 0·2 volt, the current was represented by a deflection of two divisions, the resistance being 50,000 ohms. When the E. M. F. was raised to 0·7 volt, the current increased to 40 divisions, indicating a diminution of resistance which remained at the definite value of 8,750 ohms. If the E. M. F. was again reduced to its original value of 0·2 volt the current was once more two divisions, and the resistance rose to exactly its original value of 50,000 ohms. It would thus appear that—

(1) For a particular E. M. F. there is a definite value of conductivity.

(2) When in a given part of the curve the E. M. F. is increased by a definite amount, the conductivity is also increased in a definite manner. The increased stress produces a definite conductivity distortion, and on the removal of the stress there is a quasi-elastic recovery of its original conductivity.

(3) As the seat of these changes is in the molecular layers at the definite single-point contact, it would appear that the conductivity variation and its recovery are due to molecular distortion and subsequent elastic recovery.

What has been said above of the conductivity change under electromotive variation and the complete recovery is true only when the cycle is completed at a moderate speed, during which, time is allowed for the completion of, or the recovery from, the induced change. But if the cyclic variation be carried out with very great rapidity, the phenomenon of lag comes into play. The conductivity variation then lags behind the impressed electromotive variation, and the receiver does not instantaneously recover its original resistance.

For example, in the case of the self-recovering receiver which at 0·2 volt gave a current represented by two galvanometer divisions, the resistance being equal to 50,000 ohms, the cyclic electromotive variation was quickly carried through the range from 0·2 volt to 1·2 volt and back to 0·2 volt, the immediate value of the current at this last point was not two, but six, divisions of the galvanometer. Thus the receiver, owing to lag, does not instantaneously recover its original resistance; the deflection, however, soon creeps back to two, exhibiting a complete recovery. This characteristic of self-recovery is also exhibited by rapid electromotive variation as under electric radiation (see fig. 56).

Space allows only a brief reference to the characteristic cyclic curve of negative class of substance exemplified by potassium. In this we are presented with the extraordinary phenomenon that an increase of E. M. F. is attended by a diminution of current, so that at a critical E. M. F. the current disappears altogether.


Summary

1. Under the action of electric radiation the conductivity of metallic particles exhibits variation. In the positive class, like iron, there is an increase, and in the negative, like K, a diminution, of conductivity. Each class again falls into two sub-classes, (a) sensitive substances which exhibit self-recovery, and (b) sensitive substances which do not. In the case of self-recovering substances the conductivity distortion varies with the intensity of radiation. Under the continued action of radiation, the distortion attains a maximum, balanced by a force of restitution, and on the cessation of radiation there is an elastic self-recovery.

2. The three classes of substances, positive, negative and neutral, may be distinguished by their characteristic curves.

3. The change produced in the sensitive substance by the action of radiation is not, normally speaking, chemical.

4. The conductivity change is produced, not only by very rapid, but also by comparatively slow electric variation. Generally speaking, all the conductivity variation effects produced by electric radiation can be reproduced by comparatively slow cyclic electromotive variation.

5. These conductivity changes under cyclic E. M. variation can be continuously recorded by means of the Conductivity Recorder.

6. Electric conduction in metallic particles sensitive to electric radiation does not obey Ohm's law. The conductivity is not constant and independent of the E. M. F., but varies with it. In the positive class the characteristic curve, in which the ordinates represent the currents, and the abscissa the E. M. F., is concave to the axis of the current. The conductivity increases continuously with the increasing E. M. F. The variation of the conductivity in the lower portion of the curve is small, but increases with great rapidity in the upper portion. In the negative class of substance the characteristic curve is convex to the axis of the current.

7. The curve obtained with strong, is steeper than that with feeble, initial current.

8. There is found, especially when the initial current is feeble, a critical E. M. F., at which the conductivity change becomes so rapid as to produce an almost abrupt bend in the curve. Stronger initial current appears not only to lower the critical point, but also to mitigate the abruptness of this change.

9. The effect of E. M. F. in modifying the conductivity of the surface layer is well seen in self-recovering substances. There is a definite conductivity corresponding to a definite E. M. F. As the E. M. F. is increased, the sensitive molecular layer is strained, and a definite increase of conductivity produced. When the increased stress is removed, the corresponding strain also disappears, and there is an elastic recovery to its former molecular and conductive state. Hence, when it is carried through a complete cycle of electro-motive variation, with moderate speed, the forward and return curves coincide, and the substance regains, at the end of the cycle, its original molecular condition.

10. This is the case where there is complete recovery on the removal of the stress. With non-recovering substances we find an outstanding residual effect. In a curve taken with cyclic electromotive variation, the forward and return curves do not coincide, but enclose an area. There is a hysteresis. The larger the range of the electromotive variation the greater is the area enclosed. There is a residual conductivity variation, at the end of the cycle, which may be dissipated by mechanical vibration.

(British Association at Glasgow, Section A, September 1901.)