Willoughby Smith (1828-1891)

Journal of the Society of Telegraph-Engineers and Electricians, Vol. XII. - 1883

The One Hundred and Twenty-first Ordinary General Meeting of the Society was held at the Institution of Civil Engineers, 25, Great George Street, Westminster, on Thursday evening, April 12th, 1883 – Mr. WILLOUGHBY SMITH, President, in the Chair.

NOTE ON THE INFLUENCE OF SURFACE-CONDENSED GAS UPON THE ACTION OF THE MICROPHONE.

By I. PROBERT and ALFRED W. SOWARD, Associates.

One of the theories advanced to account for the action of the microphone states that the layer of air between the microphonic surfaces acts as an ordinary resistance; that the effect of sonorous vibrations is to cause the carbons to approach and to recede from one another, and so to alter the thickness, and consequently the resistance, of the interposed layer. This variation of resistance produces the variation of current essential for the reproduction of sound.

There is in physics a phenomenon known as heterogenous adhesion, one phase of which is the surface condensation of gases. A study of this phenomenon shows that every surface exposed to a gas is coated with a condensed layer of that gas. The more readily the gas is liquefiable by pressure, the greater is the quantity condensed; and in the case of the very readily liquefiable gases the exposed surface is actually coated with a layer of liquid.

Our atmosphere, as is well known, consists mainly of nitrogen, oxygen, carbon dioxide, and water vapour. Careful experiments with charcoal (a substance which, owing to its porous nature, possesses a very large surface relatively to its mass) show that the coefficients of surface condensation of these gases (that of hydrogen being taken as unity) are 3.5, 4, 15, and 80 respectively. These figures, multiplied by the figures representing the proportions in which the gases exist in the air, viz., 79, 21, .03, and (say) 1.5, give the proportions in which the substances exist in the condensed layer. They are- nitrogen, 276.5; oxygen, 84; carbon dioxide, .45; and water vapour, 120. Of these four gases, the first two, oxygen and nitrogen, would be condensed without liquefaction, and the effect of this condensation would be to slightly reduce the electrical conductivity of the layer; for it is known that the conductivity of a gas is decreased by condensation. The carbon dioxide would probably be liquefied, and the effect of its liquefaction would be (slightly, on account of the small quantity acting) to increase the conductivity; the water vapour would certainly be liquefied, and the effect of its liquefaction would be to very considerably increase the electrical conductivity of the layer. The effect, then, of this phenomenon of surface condensation is to coat the microphonic surfaces with a layer of far better electrical conductivity than ordinary air.

In order to test the influence of this layer of condensed air upon the action of the microphone, we have lately made some experiments with the instrument in different gases. The gases used were hydrogen, carbon dioxide, wet air, and dry air, and our method of experimentation was as follows:-

A microphone of a well-known pattern, consisting of a glass tube containing cylindrical carbon blocks, and having many contact surfaces, was attached to the inside of a clock-case, and placed in circuit with a Hughes audiometer, a Morse key, and three Leclanché cells. The carbon blocks of the microphone were pressed well together, so as to avoid, as far as possible, alteration of contact due to chance vibrations. A current of air, dried by passage over pumice stone soaked in strong sulphuric acid, and then over pentoxide of phosphorus, was urged through the microphone for half an hour. The resistance of the microphone was then measured, and the point of the audiometer scale determined at which the beating of the clock became inaudible in the telephones. A current of similarly dried hydrogen was then passed through the microphone for half an hour, and the resistance again measured, and the point of silence determined in the audiometer. The experiment was next repeated with carbon dioxide, the dried gas being passed through the microphone for a similar period; and finally, for fifteen minutes a current of air was forced through water contained in a Woulfe’s bottle (in order to load the air with water vapour), and then through the microphone. The resistances and points of silence in the audiometer were determined as before. Our results are expressed in the following table, and it may be noted that the scale of the audiometer was graduated from 0° in the centre to 100°:-

Name of gas.	Resistance of microphone.	Point on scale of audiometer at which silence was reached.
Wet air	Ohms. 386	51°
Carbon dioxide	435	56°
Hydrogen	600	58°
Dry air	520	63°

It will be seen that the best result was obtained with wet air, which calculation shows should give a good conducting surface layer. Next in order is carbon dioxide, which in the liquid state is a moderate conductor. Hydrogen comes next, as would be expected, for neither it nor dry air is reducible to the liquid state by ordinary surface condensation, and the best result is to be looked from the less condensable of the two – that is, hydrogen. The resistance of the hydrogen-charged microphone, is, however, anomalous.

In order to obtain our layers of condensed gas, we considered it sufficient to pass a stream of the desired gas over the microphone for some time, because it is known that when a piece of carbon charged with one gas is placed in an atmosphere of another, the two gases diffuse into one another, with a result that the carbon remains charged with a mixture in the proportions indicated by multiplying their percentage volumes by their respective condensation coefficients. In our experiments the quantity of gas passed through the microphone in any one experiment was vastly greater than the residual gas from previous experiments.

We did not attempt to rigorously exclude water in any of our experiments; for it has been shown that the last trace of water so obstinately clings to a surface, that to perfectly dry a glass tube it must be raised to the softening point, and so kept for some hours while a stream of dry air is urged through it.

In these experiments with different gases there is a possible source of error which must not be overlooked. We have elsewhere shown (Chemical News, Vol.47, p.157) that the resistance of a piece of porous carbon is not a constant for a given temperature, but varies with the chemical nature and the density of the gas with which the pores of the carbon are filled. It follows from this, that if a constant electro-motive force be used, the current flowing and the sounds obtained will be altered by any alteration in the nature of the gas absorbed in the body of the microphone, irrespective of the contact surfaces. But the carbon used for our microphone had very little absorptive power, and its resistance was practically constant at constant temperature.

We think that from these experiments it is fair to infer that the layer of condensed gas with which every microphone surface is cevered (sic) is concerned to some extent in the regulation of microphonic action.

As bearing upon the effect of a layer of moisture we may notice the following experiment:-

A common tin canister, joined through a rheotome to one pole of a 4-cell Grove’s battery, and held in the hand by an insulating handle, was pressed against the ear; a wire attached to the second pole of the battery was held against the tongue. Sounds were heard corresponding to the working of the rheotome, and a burning sensation was experienced at the ear, such as one might imagine would be produced by innumerable small electric discharges. Similar results were obtained with a 10-cell Daniell’s battery (chamber pattern). The surface of the canister, which, having been exposed to the air, was coated, as all similar exposed surfaces are, with an invisible layer of moisture, was then well wetted. The burning sensation became more marked, but the sounds of the make and break were no longer audible. We may add that a solid brass ball, whether wet or dry, gave no sounds in this experiment.

A hearty vote of thanks was unaminously accorded to Messrs. Probert and Soward for their joint interesting paper.

The PRESIDENT : In opening the discussion on the papers, I may say that Mr. Bidwell has asked some very pertinent questions, and no doubt, from the number present, there are many gentlemen here who will be able to answer them satisfactorily, and favour us with their views on this interesting subject.

Mr. A. STROH : Mr. Bidwell having referred in his paper to my experiment showing the repellant action at the microphonic contact, which I described at our last meeting, I wish to say a few words in answer to his remarks.

At first, he said, in carrying out a similar experiment to mine, he obtained the same results as I did; but after a time he found that, when he placed a small weight upon the loose carbon, the deflection he obtained was almost the same as before, while the microphone failed in recording sound-waves. My answer to this is, simply, that the microphonic condition still existed when the little weight was placed on the loose carbon; and, if Mr. Bidwell had employed stronger sounds than the ticking of a watch, I believe the microphone would have still acted as such.

Mr. Bidwell’s paper deals with a very difficult subject, and the amount of work it represents is considerable. His experiments are certainly highly interesting, and, as I can bear out some of the facts he obtained, I will give a short account of some experiments I have made since our last meeting.

The microphone I used was again of the hammer-and-anvil pattern. This form is most suitable for experiment.

On a little board, A, Fig. 1, was fixed, by means of a brass holder, a, a thin carbon rod, b. Against the end of the latter rested loosely another carbon rod, c, which was mounted on a little spindle, d, and a small spiral spring, e, served to vary the pressure at the contact between the two carbons b and c.

The object of this arrangement was that the points of contact might be brought under a microscope, and for that reason the carbons, where they came into contact with one another, were made as flat as possible, and also thin at the edges, so that they might be fairly brought into the focus of the microscope.

The little board A, on which was placed a loud-ticking watch as a source of sound, was fixed at B on a stand apart from that of the microscope, so that touching and adjusting the latter might not interfere with the microhone. A hole in the board at h, under the contact, is necessary to admit light from below.

In the circuit with the microphone was a telephone, a make-and-break key, and a small battery.

Looking through the microscope, the edges of the carbon contact had a jagged appearance, as represented in Fig. 2., and only one or two projecting points were seen to touch. The tension of the spring for the first observation was such as I knew by experience to be necessary to insure microphonic contact. By depressing the key a current was now sent through, and I observed the effect closely by means of the microscope. What I noticed was a light, or small arc, or, what I afterwards concluded it was, a burning away of the carbon. The two carbons came closer together during that burning, and presently another contact was made, and another light became visible. The carbons still approached until I saw three or four points of contact, each of which was illuminated; and at last, when, as I presumed, there were sufficient points of contact for the greater portion of the current to pass, the burning ceased.

The PRESIDENT : What battery power had you?

Mr. STROH : Two small bichromate cells.

When the burning ceased I heard the watch tick.

This all took place in the course of a few seconds. While the microphonic condition remained, I could see no longer any light at the points of contact. But as soon as the battery power was increased the burning commenced again, and continued until the area of contact was sufficiently enlarged, and, consequently, the resistance so far reduced that the degree of heat which was still produced was below that which causes the burning of the carbon.

This, I believe, agrees entirely with Mr. Bidwell’s observation, viz., that the resistance of the contact falls with an increase of current.

But this was not the only thing I observed. I now let down the spring of the microphone, so that the two carbons were only just in contact, and no more. I then passed a current from the two cells, and saw a momentary burning and heard a short clock in the telephone; then followed an interruption, and the current would pass no more. By making contact now with the key many times, no trace of current could be detected with the telephone, showing that after the first passage of the current great resistance was established. This is also an effect observed and investigated by Mr. Bidwell.

I now screwed up the tension spring a little, and so increased the pressure: the insulating film or substance which prevented the current from passing was by the extra pressure forced away or broken through, and another current passed on making contact, but only for a moment, and then again the interruption occurred. This I could repeat several times, until the spring was wound so tight that the insulating substance was broken through as soon as it was formed, and then I could hear in the telephone the well-known boiling or hissing sound which we hear when the microphone is out of adjustment. Sometimes the noise was a perfectly clear musical note, and by adjustment of the spring it occasionally became as shrill as a railway whistle.

I have been able to adjust the microphone so, that while still listening to the whistling noise, I could hear the watch as well; and, whenever I have succeeded in doing that, the tick of the watch was always louder than I have obtained it under any other circumstances.

That experiment was made with a telephone which had a considerable resistance in the coil. I then took a telephone with very small resistance, only a few ohms, and obtained an entirely different effect. I let down the spring again, so that the carbons were only just in contact. I saw a momentary burning as before, but the moveable carbon was driven away from the fixed carbon, and apparently remained repelled at some distance. Each time the circuit was completed by the key, the moveable carbon was driven away, and I could see a continuous stream of sparks flying across the space. The sound produced in the telephone by this effect was like a miniature artillery bombardment. By tightening the spring the noise was changed into a musical note, the pitch of which rose with pressure.

By close observation I could see very fine dark lines, which reached from every projection of the jagged edge of the loose carbon, right across the space to the fixed carbon. This showed that the moveable or loose carbon was in a state of vibration, the amplitude of which was equal to the distance of the apparent separation of the two carbons. This distance, which was considerable when the rate of vibration was low, became rapidly less as the rate was increased by tightening the tension spring, and a point was soon reached when the separation could be no longer seen, but was likely still to exist during the production of the higher notes.

Another effect which I have occasionally observed with the microscope during the production of the various noises above mentioned was, that little fragments of carbon, which probably became detached by their expansion by heat, but still remained between the carbons, were in a state of agitation.

There can be no doubt that during the production of these singing, hissing, or boiling noises the current cannot be a constant one; but in the case of the former it must be intermitted, and of equal periods, while in the case of the latter it is probably undulatory, and of unequal or irregular periods.

It also appears to me that a current crossing a microphonic contact has a strong tendency to cause these vibratory disturbances, and I am inclined to believe that during microphonic action, even when all is in good adjustment, the sound-waves which are transmitted are accompanied by other vibrations which are due to the passage of the current itself.

The following reasoning has led me to this supposition:- Whenever I succeeded to adjust the microphone so that I could hear the watch tick during the production of singing or hissing noises, the timbre or quality of the sound of the ticking was exactly that of the hissing, and any change in the character of the latter was always accompanied by a corresponding change of the former.

As it is a fact that sudden small changes in the quality of sound are sometimes observed even when the adjustment of the microphone is perfect, that is to say, when no singing or hissing noises are heard, it must be that whenever sound waves are transmitted these have superimposed upon them other vibrations possibly of a very high rate or pitch, which vary and so produce change of timbre or quality. The latter would be the vibrations or disturbances produced by the passage of the current across the contact.

I next substituted metallic contacts for the carbon ones. With platinum I have obtained very good microphonic effects whenever sticking could be avoided, and it occurred to me during an experiment to place a little oil on the platinum contacts. The oil by its capillary attraction remains always surrounding the contact, and, when the adjustment was so that I could hear the watch tick, I saw that the oil was in violent agitation, and little particles of dust or carbon which were in it I could see spin round with great rapidity. I was puzzled for a long time as to what caused the particles to be agitated in so violent a manner, but found that it was simply the effect of heat. The oil was boiling at the point of contact, even with a simple cell, and when I saw an exceedingly thin film of smoke arising from near the contact I was convinced that such was the case.

I then placed upon the surface of the oil a number of carbon particles or dust. When the current was again sent through, the whole of the carbon dust was violently rotated and started in all directions, and at last it accumulated round the points of contact and built up a pillar which forced the electrodes wide apart, and took the shape shown in Fig. 3.

When the pillar began to form, it appeared that the carbon dust and particles were attracted to the points of contact by some means or other, and that some, having once got there, could not get away, and one was built upon the other, and so the pillar was formed in the oil.

With a pair of steel contacts I made the same experiment with the oil, with equal success. A curious phenomenon which I especially observed with steel contacts was, however, that the revolutions of the black particles in the oil would continue their movements for a considerable time after the current had been interrupted by the key. In some cases it took quite two minutes before they came entirely to rest.

Finding that a pillar was also formed without carbon dust in the oil, I took the latter away altogether, and found that by a little humouring I could cause this pillar to grow even when the contacts were dry and clean.

This pillar, while in the course of formation, had the appearance of being in a state of low incandescence, but I came to the conclusion afterwards that it was surrounded by a dull red glow, which appeared to be really a succession of sparks running along the surface of the pillar.

I do not pretend to say that the formation of this pillar can have anything to do with microphonic action, but I think it right for us to consider all these little effects, and then, by collecting and compiling them, help some day to explain microphonic action.

At present we are not unanimous. Mr. Shelford Bidwell is in favour of variable mechanical pressure caused by sound-waves as being the principal agency. Mr. Preece is in favour of heat, and others favour the arc and other theories; but, though I can see the possibility that all these theories may take a fair share in explaining the mysterious action, I will be content with simply recording facts, and with the expression of a hope that the day may not be far removed when we shall understand the action of the microphone so completely that we can all join in the same opinion.

The PRESIDENT: As it is rather late, and we have a ballot this evening, I propose to adjourn this important discussion. The paper read tonight by Mr. Bidwell was itself important, and now Mr. Stroh has brought forward fresh points of considerable interest, which I think we shall do well to sleep on.

A ballot then took place, at which the following were elected:-

As Associates :

Burrows, Henry Alpha.	Macartney, H. J. Ellison.
Day, Russell Camper, B. A., Oxon.	Mondey, William.
Hinton, W. G.	Pidgeon, Randall.
Lanyon, Colonel Sir W. Owen, K. C. M. G., C. B.	Sealy, G. W. Smart, Edgar.
Legg, W. A.	Smith, Thos.
Wharton, Charles Joseph.

As Students :

Joyce, Samuel, jun.	Rider, John Hall.
Porter, Charles V.	Taylor, William.

The meeting then adjourned until Thursday evening, 26th April, 1883.

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