Chapter M: J. Luvini, in an article inserted in ‘Les Mondes,’ March 7, (3)

Thus the steps of a fly walking on the stand are clearly heard, and give the sensation of a horse’s tread; and even a fly’s scream, especially at the moment of death, is said by Mr. Hughes to be audible. The rustling of a feather or of a piece of stuff on the board of the instrument, sounds completely inaudible in ordinary circumstances, are distinctly heard in the microphone. It is the same with the ticking of a watch placed upon the stand, which may be heard at ten or fifteen centimètres from the receiver. A small musical box placed upon the instrument gives out so much sound, in consequence of its vibratory movements, that it is impossible to distinguish the notes, and in order to do so it is necessary to place the box close to the instrument, without allowing it to come in contact with any of its constituent parts. It therefore appears that the instrument is affected by the vibrations of air, and the transmitted sounds are fainter than those heard close to the box. On the other hand, the vibrations produced by the pendulum of a clock, when placed in communication with the standard of the instrument by means of a metallic rod, are heard perfectly, and may even be distinguished when the connection is made by the intervention of a copper wire. A current of air projected on the system gives the sensation of a trickle of water heard in the distance. Finally, the rumbling of a carriage outside the house is transformed into a very intense crackling noise, which may combine with the ticking of a watch, and will often overpower it.

_Different Systems of Microphones._--The microphone has been made in several ways, but the one represented in fig. 39 is the arrangement which renders it the most sensitive. In this system, two small carbon cubes, A, B, are placed one above the other on a vertical wooden prism; two holes are pierced in the cubes to serve as sockets for a spindle-shaped carbon pencil, that is, with the points fined off at the two ends, and about four centimètres long: if of a large size, the inertia will be too great. One end of this pencil is in the cavity of the lower carbon, and the other must move freely in the upper cavity which maintains it in a position approaching to that of instable equilibrium, that is, in a vertical position. Mr. Hughes states that the carbons become more effective if they are steeped in a bath of mercury at red heat, but they will act well without undergoing this process. The two carbon cubes are also provided with metallic contacts which admit of their being placed in connection with the circuit of an ordinary telephone in which a Leclanché battery has been placed, or one, two, or three Daniell cells, with an additional resistance introduced into the circuit.

In order to use this instrument, it is placed on a table, with the board which serves to support it, taking care to deaden any extraneous vibrations by interposing between this board and the table several folds of stuff so arranged as to form a cushion, or, which is better, a belt of wadding, or two caoutchouc tubes: what is said by a person standing before this system is immediately reproduced in the telephone, and if a watch is placed on the stand, or a box with a fly enclosed in it, all its movements are heard. The instrument is so sensitive that words said in a low voice are most easily heard, and it is possible, as I have already said, to hear the speaker when he is standing nine yards from the microphone. Yet some precautions are necessary in order to obtain good results with this system, and besides the cushions placed beneath the instrument to guard it from the extraneous vibrations which might ensue from any movements communicated to the table, it is also necessary to regulate the position of the carbon pencil. It must always rest on some point of the rim of the upper cavity; but as the contact may be more or less satisfactory, experience alone will show when it is in the best position, and it is a good plan to make use of a watch to ascertain this. The ear is then applied to the telephone, and the pencil is placed in different positions until the maximum effect is obtained. To avoid the necessity of regulating the instrument in this way, which must be done repeatedly by this arrangement, MM. Chardin and Berjot, who are ingenious in the construction of telephones on this pattern, have added to it a small spring-plate, of which the pressure can be regulated, and which rests against the carbon pencil itself. This system works well.

M. Gaiffe, by constructing it like a scientific instrument, has given the instrument a more elegant form. Fig. 40 represents one of his two models. In this case, the cubes or carbon dice are supported by metallic holders, and the upper one E is made to move up and down a copper column G, so as to be placed in the right position by tightening the screw V. In this way the carbon pencil can be made to incline more or less, and its pressure on the upper carbon can be altered at pleasure. When the pencil is in a vertical position, the instrument transmits articulate sounds with difficulty, on account of the instability of the points of contact, and rustling sounds are heard. When the inclination of the pencil is too great, the sounds are purer and more distinct, but the instrument is less sensitive. The exact degree of inclination should be ascertained, which is easily done by experiment. In another model M. Gaiffe substitutes for the carbon pencil a very thin square plate of the same material, bevelled on its lower and upper surfaces, and revolving in a groove cut in the lower carbon. This plate must be only slightly inclined in order to touch the upper carbon, and under these conditions it transmits speech more loudly and distinctly.

I must also mention another arrangement, devised by Captain Carette of the French Engineers, which is very successful in transmitting inarticulate sounds. In this case the vertical carbon is pear-shaped, and its larger end rests in a hole made in the lower carbon; its upper and pointed end goes into a small hole made in the upper carbon, but so as hardly to touch it, and there is a screw to regulate the distance between the two carbons. Under such conditions, the contacts are so unstable that almost anything will put an end to them, and consequently the variations in the intensity of the transmitted current are so strong that the sounds produced by the telephone may be heard at the distance of several yards.

Fig. 41 represents another arrangement, devised by M. Ducretet. The two carbon blocks are at D D′, the moveable carbon pencil is at C, the telephone at T, and the binding screws at B B′. An enlarged figure of the arrangement of the carbons is given on the left. The arm which holds the upper carbon D is fastened to a rod, bearing a plate P′, of which the surface is rough, and a little cage C′, made of wire netting, can be placed upon the plate, so as to enable us to study the movements of living insects.

When speech is to be transmitted with a force which can make the telephone audible in a large room, the microphone must have a special arrangement, and fig. 42 represents the one which Mr. Hughes considers the most successful, to which he has given the name of _speaker_.

In this new form, the moveable carbon which is required to produce the variable contacts is at C, at the end of a horizontal bar B A, properly balanced so as to move up and down on its central point. The support on which the bar oscillates is fastened to the end of a spring plate in order that it may vibrate more easily, and the lower carbon is placed at D below the first. It consists of two pieces laid upon each other, so as to increase the sensitiveness of the instrument, and we represent the upper piece at E, which is raised so as to show that when it is desired only one of these carbons need be used. For this purpose the carbon E is fastened to a morsel of paper, which is fixed to the little board and contributes to the articulation. A spring R, of which the tension can be regulated by the screw _t_, serves to regulate the pressure of the two carbons. Mr. Hughes recommends the use of metallised charcoal prepared from deal.[14] The whole is then enclosed in a semi-cylindrical case H I G, made of very thin pieces of deal, and the system is fixed, together with another similar system, in a flat box, M J L I, which, on the side M I, presents an opening before which the speaker stands, taking care to keep his lower lip at a distance of two centimètres from the bottom of the box. If the two telephones are connected for strength, and if the battery employed consists of two cells of bichromate of potash, it is possible to act so strongly on the current, that, after traversing an induction coil only six centimètres long, a telephone of Bell’s square model can be made to speak, so as to be heard from all parts of a room; a speaking tube, about a yard long, must indeed be applied to it. Mr. Hughes asserts that the sounds produced by it are nearly as loud as those of the phonograph, and this is confirmed by Mr. Thomson.

M. Boudet de Paris has lately invented a microphone speaker of the same kind, with which it is possible to make a small telephone utter a loud sound. An induction coil, influenced by a single Leclanché cell, must be employed.

Suppose that a very small carbon rod with pointed ends is placed at the bottom of a box, of about the size of a watch. One end of the rod rests against a morsel of carbon, which is fastened to a very thin steel diaphragm, placed before a mouthpiece which acts as a lid to the box, and is screwed above it. Next suppose that a small piece of paper, folded in two, in the shape of the letter V, is fixed above that part of the carbon in contact with the carbon of the diaphragm. This constitutes the instrument, and in order to work it, it must be held in a vertical position before the mouth, at a distance of about three centimètres, and it is necessary to speak in the ordinary tone. If the telephone is placed in direct communication with this instrument, it will send the voice to a distance. Without employing a Leclanché cell, the voice may be heard at the distance of ten yards, if one of the carbons used for the phonograph is placed before the mouthpiece of the telephone.

In this system, the sensitiveness of the instrument is entirely due to the slightness of the contact between the two carbons, and the slight elasticity of the folded paper constitutes the contact. Perhaps the paper itself has some influence; at any rate the most delicate spiral spring is incapable of producing the same effect, and it is necessary to suspend the instrument vertically, in order that the weight of the moveable carbon may not affect it. It can be regulated by depressing or elevating that part of the paper which rests on the carbon rod.

Although it is possible to work all telephones with this instrument, some are more effective than others. The mouthpiece must be concave, and the diaphragm must be close to its rim, and must be made of a particular kind of tin. The ordinary diaphragm does not act well, and M. Boudet de Paris has tried several, so as to obtain the maximum effect.

It is certain that when the instruments are as well regulated as those which the inventor has deposited with me, their results are really surprising. It is even possible, by using several microphones at the sending station, to obtain the reproduction of duets, and even of trios, with remarkable effect.

With this kind of microphone speaker M. Boudet de Paris is able to transmit speech into a snuff-box telephone, merely consisting of a flat helix of wire, placed before a slightly magnetised steel plate, and without insertion of a magnetic core. A single Leclanché cell was enough. An experiment of the same nature was tried in England, but it was found necessary to use six Leclanché cells.

The microphone may also be made of morsels of carbon pressed into a box between two metallic electrodes, or enclosed in a tube with two electrodes represented by two elongated fragments of carbon. In the latter case the carbons ought to be as cylindrical as possible, and those made by M. Carré for the Jablochkoff candles are very suitable. Fig. 43 represents an instrument of this kind which M. Gaiffe arranged for me, and which, as we shall see, serves as a thermoscope (fig. 44). It is composed of a quill filled with morsels of carbon, and those at the two ends are tipped with metal. One of these metal tips ends in a large-headed screw which, by means of its supports A B, is able to press more or less on the morsels of carbon in the tube, and consequently to establish a more or less intimate contact between them. When the instrument is properly regulated, speech can be reproduced by speaking above the tube. It is therefore a microphone as well as a thermoscope. Mr. Hughes has remarked one curious fact, namely, that if the different letters of the alphabet are pronounced separately before this sort of microphone, some of them are much more distinctly heard than others, and it is precisely those which correspond to the breathings of the voice.

A microphone of this kind may be made by substituting for the carbon powders of more or less conductivity, or even metal filings. I have shown in my paper on the action of substances of moderate conductivity, that such power varies considerably with the pressure and the temperature; and as the microphone is based on the differences of conducting power which result from differences of pressure, we can understand that these powders may be used as a means of telephonic transmission. In a recent arrangement of this system Mr. Hughes has made the powder adhere together with a sort of gum, and has thus made a cylindrical pencil which, when connected with two electrodes which are good conductors, can produce effects analogous to those we have just described. As I have said, it is possible to use metal filings, but Mr. Hughes prefers powdered charcoal.

Mr. Blyth states that a flat box, about 15 inches by 9, filled with coke, and with two tin electrodes fixed to the two ends, is one of the best arrangements for a microphone. He says that three of these instruments, hung like pictures against the wall of a room, would suffice, when influenced by a single Leclanché cell, to make all the sounds produced in a telephone audible, and especially vocal airs. Mr. Blyth even asserts that a microphone capable of transmitting speech can be made with a simple piece of coke, connected with the circuit by its two ends, but it must be coke: a retort carbon, with electrodes, will not act.

It is a remarkable property of these kinds of microphones that they can act without a battery, at least when they are so arranged as to form a voltaic element for themselves, and this can be done by throwing water on the carbons. Mr. Blyth, who was the first to speak of this system, does not distinctly indicate its arrangement, and we may assume that his instrument did not differ from the one we have already described, to which water must have been added. In this way, indeed, I have been able to transmit not only the ticking of a watch and the sounds of a musical box, but speech itself, which was often more distinctly expressed than in an ordinary microphone, since it was free from the sputtering sound which is apt to accompany the latter. Mr. Blyth also asserts that sounds may be transmitted without the addition of water, but in this case he considers that the result is due to the moisture of the breath. Certainly much moisture is not required to set a voltaic couple in action, especially when a telephone is the instrument of manifestation. The ordinary microphone may be used without a battery, if the circuit in which it is inserted is in communication with the earth by means of earthen cakes; the currents which then traverse the circuit will suffice to make the tickings of a watch placed upon the microphone perfectly audible. M. Cauderay, of Lausanne, in a paper sent to the Académie des Sciences, July 8, 1878, informs us that he made this experiment on a telegraphic wire which unites the Hôtel des Alpes at Montreux with a _châlet_ on the hill--a distance of about 550 yards.

_The Microphone used as a Speaking Instrument._--The microphone can not only transmit speech, but it can also under certain conditions reproduce it, and consequently supply the place of the receiving telephone. This seems difficult to understand, since a cause for the vibratory motion produced in part of the circuit itself can only be sought in the variations in intensity of the current, and the effects of attraction and magnetisation have nothing to do with it. Can the action be referred to the repulsions reciprocally exerted by the contiguous elements of the same current? Or are we to consider it to be of the same nature as that which causes the emission of sounds from a wire when a broken current passes through it, so that an electric current is itself a vibratory current, as Mr. Hughes believes? It is difficult to reply to these questions in the present state of science; we can only state the fact, which has been published by Messrs. Hughes, Blyth, Robert Courtenay, and even by Mr. Edison himself. I have been able to verify the fact myself under the experimental conditions indicated by Mr. Hughes, but I was not so successful in the attempt to repeat Mr. Blyth’s experiments. This gentleman stated that in order to hear speech in a microphone it would be enough to use the model made from fragments of carbon, as we have described, to join to it a second microphone of the same kind, and to introduce into the circuit a battery consisting of two Grove elements. If anyone then speaks above the carbons of one of the microphones, what is said should be distinctly heard by the person who puts his ear to the other, and the importance of the sounds thus produced will correspond with the intensity of the electric source employed. As I have said, I was unable by following this method to hear any sound, still less articulate speech; and if other experiments had not convinced me, I should have doubted the correctness of the statement. But this negative experiment does not in fact prove anything, since it is possible that my conditions were wrong, and that the cinders which I employed were not subject to the same conditions as Mr. Blyth’s fragments of coke.

With respect to Mr. Hughes’s experiments, I have repeated them with the microphone made by MM. Chardin and Berjot, using that by M. Gaiffe as the sender, and I ascertained that with a battery of only four Leclanché cells, a scratch made on the sender, and even the tremulous motion and the airs played in a little musical box placed on the sender, were reproduced--very faintly, it is true--in the second microphone; in order to perceive them, it was enough to apply the ear to the vertical board of the instrument. It is true that speech was not reproduced, but of this Mr. Hughes had warned me; it was evident that with this arrangement the instrument was not sufficiently sensitive.

A different arrangement of the microphone is required for the transmission and the reproduction of speech by this system, and a section of the one which Mr. Hughes found most successful is given in fig. 45. It somewhat resembles Mr. Hughes’s microphone speaker, placed in a vertical position, and the fixed carbon is fastened to the centre of the stretched membrane of a string telephone. The ear or mouth tube is at A, the membrane at D D, the carbon just mentioned at C: this carbon is of metallised charcoal prepared from deal, and so also is the double carbon E, which is in contact with it and is fastened to the upper end of the little bar G I. The whole is enclosed in a small box, and the pressure exerted on the contact of the two carbons is regulated by a spring R and a screw H. The tube of the telephone serves as an acoustic tube for the listener, and Mr. Hughes’s speaker, described above, acts as sender. It is hardly necessary to say that the two instruments are placed at each end of the circuit, that the carbons are connected with the two poles of a battery of one or two cells of bichromate of potash, or two Bunsen or six Leclanché cells, and the two instruments are connected by the line wire. Under such conditions, conversation may be exchanged, but the sounds are always much less distinct than they are in a telephone.

I was able to ascertain this fact with a roughly made instrument brought from England by Mr. Hughes. MM. Berjot, Chardin, and de Méritens, who were also present at the experiments, were able with me to hear speech perfectly, and I have since successfully repeated the experiment alone, but it does not always succeed, and under its present conditions the instrument has no importance in a scientific point of view. It is evident that the instrument can dispense with any support, and the little box then forms the handle of the instrument; in this case the two binding screws are placed at the end of this handle, as in a telephone. The microphone speaker with a disk, represented in fig. 5, which acts as sender to the singing condenser, can be used, when properly regulated, as a receiving microphone. M. Berjot has obtained good results from a little instrument of the same kind as that in fig. 45, but with a metal diaphragm, and the microphonic system consists of a cylindrical piece of carbon resting on a small disk of the same substance, which is galvanised and soldered to the diaphragm. The whole is enclosed in a small round box, with its upper part cut in the form of a mouthpiece.

It seems that all microphone senders with disks can reproduce speech more or less perfectly; it is a question of adjusting and refining the carbon points of contact. A weak battery, consisting of a single Leclanché cell, is better for these instruments than a strong battery, precisely because of the effects of oxidation and polarisation, which are so energetically produced at these points of contact when the battery is strong.

The effects of the microphone receiver explain the sounds, often very intense, produced by the Jablochkoff candles when they are influenced by electro-magnetic machines. These sounds always vibrate in unison with those emitted by the machine itself, and they result, as I have already shown, from the rapid magnetisations and demagnetisations which are effected by the machine. These effects, observed by M. Marcel Deprez, were particularly marked in M. de Méritens’ first machines.

_Other Arrangements of Microphones._--An arrangement such as we have just described has been employed by M. Carette to form an extremely powerful microphone speaker. The only difference is that the stretched membrane is replaced by a thin metallic disk: he fastens one of the carbons to the centre of this disk, and applies to it the other carbon, which is pointed, and held by a _porte-carbon_ with a regulating screw, so that the pressure which takes place between the two carbons may be regulated at pleasure. By this arrangement speech may be heard at a distance from the telephone. In other respects it resembles the telephone sender represented in fig. 5.

M. de Méritens has executed the system represented, fig. 45, on a large scale, forming the tube A B of a zinc funnel a yard in length, and in this way he has been able to magnify the sounds, so that a conversation held in a low voice, three or four yards from the instrument, has been produced in a telephone with more sonorous distinctness. The instrument was placed on the floor of the apartment, with the opening of the funnel above, and the telephone was in the cellars of the house.

The form of the microphone has been varied in a thousand ways, to suit the purposes to which it was to be applied. In the ‘English Mechanic and World of Science,’ June 28, 1878, we see the drawings of several arrangements, one of which is specially adapted for hearing the steps of a fly. It is a box, with a sheet of straw paper stretched on its upper part; two carbons, separated by a morsel of wood, and connected with the two circuit wires, are fastened to it, and a carbon pencil, placed crosswise between the two, is kept in this position by a groove made in the latter. A very weak battery will be enough to set the instrument at work, and when the fly walks over the sheet of paper it produces vibrations strong enough to react energetically on an ordinary telephone. The instrument must be covered with a glass globe. When a watch is placed on the membrane, with its handle applied to the morsel of wood which divides the two carbons, the noise of its ticking may be heard through a whole room. Two carbon cubes placed side by side, and only divided by a playing-card, may also be used instead of the arrangement of carbons described above. A semicircular cavity, made in the upper part of the two carbons, in which are placed some little carbon balls, smaller than a pea and larger than a mustard seed, will make it possible to obtain multiple contacts which are very mobile and peculiarly fit for telephonic transmissions. This arrangement has been made by Mr. T. Cuttriss.

Several other arrangements of microphones have been devised by different makers and inventors, such as those of Messrs. Varey, Trouvé, Vereker, de Combettes, Loiseau, Lippens, de Courtois, Pollard, Voisin, Dumont, Jackson, Paterson, Taylor, &c., and they are more or less satisfactory. The instruments of MM. Varey, Trouvé, Lippens, and de Courtois are the most interesting, and we will describe them.

M. Varey’s microphone consists of a sounding box of deal, mounted in a vertical position on a stand, and two microphones are arranged on either side of it, with vertical carbons united for tension. A small Gaiffe cell of chloride of silver, without liquid, is applied to the standard of the instrument, and is enough to make it work perfectly. This system is extremely sensitive.

M. Trouvé’s microphones, represented in figs. 46, 47, 48, are extremely simple, so that he is able to sell them at a very moderate price. They generally consist of a small vertical cylindrical box, as we see in the figure, with disks of carbon at its two ends, which are united by a carbon rod, or by a metallic tube tipped with carbon. This rod or tube turns freely in two cavities made in the carbons, and the box, while acting as a sounding box, becomes at the same time a prison for the insects whose movements and noises are the objects of study.

These boxes may be suspended on a cross-bar (fig. 47) by the two communicating wires, so as to be completely insulated. In this case the ticking of a watch placed upon the board, friction, and external shocks are hardly heard, but on the other hand the sound vibrations of the air alone are transmitted, and they acquire great distinctness. We have often repeated these experiments, and have always found that the tones of the voice were perfectly preserved.

The model represented fig. 48 is still more simple, and appears to be the latest development of this kind of instrument. It consists of a stand and a disk united by a central rod. The upper disk moves round the central rod, and permits the vertical carbon to assume any inclination which is desired. It is evident that the instrument will become less sensitive when the carbon is more oblique.

We must also mention a very successful microphone devised by M. Lippens. It is a slightly made box, like that of M. Varey, and on its opposite faces there are applied, on two frames left empty for the purpose, two thin plates of hardened caoutchouc, in the centre of which inside the box, two carbons are fastened, and on their outer surface a half-sphere is hollowed.

The interval between the two carbons hardly amounts to two millimètres, and a carbon ball is inserted into the two cavities which form its spherical case. This ball is supported by a spiral spring which can be extended more or less by means of a wire wound on a windlass which is fixed above the instrument, like the spring of an electric telegraph instrument. By means of this spring, the pressure of the carbon ball against the sides of the cavity which contains it can be regulated at pleasure, and the sensitiveness of the instrument and its capacity for transmitting speech can be adjusted. Under these conditions, the vibrations of the caoutchouc plates directly affect the microphone, and the currents of air have no influence on it, so that the effects are more distinct. It is so sensitive that it is best for the speaker to place himself at the distance of at least 50 centimètres from the instrument. M. Lippens’ instrument is a pretty one, mounted on a wooden stand, which is neatly turned.

In order to put an end to the sputtering usual in microphones, it occurred to M. de Courtois to prevent any cessation of contact between the carbons by keeping them close together, and to effect the variations of resistance necessary for articulate sounds by making them slide over each other, so as to insert a shorter or longer portion of the carbon in the circuit. For this purpose a vibrating disk is placed in a vertical position in a rigid frame, and a small conducting rod, terminated by a pointed carbon, is applied to it, with this carbon point resting on another flat piece of carbon placed below it. Influenced by the vibrations of the disk, the carbon point moves to and fro, effecting more or less extensive contacts with the lower carbon, and thus producing variations of resistance which almost correspond to the range of vibrations on the disk.

_Experiments made with the Microphone._--I must now mention the interesting experiments which led Mr. Hughes to the invention of the remarkable instrument of which we have spoken, as well as those undertaken by other scientific men, either from a scientific or a practical point of view.

Believing that light and heat can modify the conductivity of bodies, Mr. Hughes went on to consider whether sound vibrations, transmitted to a conductor traversed by a current, would not also modify this conductivity by provoking the contraction and expansion of the conducting molecules, which would be equivalent to the shortening or lengthening of the conductor thus affected. If such a property existed, it would make it possible to transmit sounds to a distance, since variations in the conductivity would result from variations corresponding to the intensity of the current acting on the telephone. The experiment which he made on a stretched metal wire did not, however, fulfil his expectation, and it was only when the wire vibrated so strongly as to break, that he heard a sound at the moment of its fracture. When he again joined the two ends of the wire, another sound was produced, and he soon perceived that imperfect contact between the two broken ends of wire would enable him to obtain a sound. Mr. Hughes was then convinced that the effects he wished to produce could only be obtained with a divided conductor, and by means of imperfect contacts.

He then sought to discover the degree of pressure which it was most expedient to exert between the two adjacent ends of the wire, and for this purpose he effected the pressure by means of weights. He ascertained that when the pressure did not exceed the weight of an ounce on the square inch at the point of connection, the sounds were reproduced with distinctness, but somewhat imperfectly. He next modified the conditions of the experiment, and satisfied himself that it was unnecessary to join the wires end to end in order to obtain this result. They might be placed side by side on a board, or even separated (with a conductor placed crosswise between them), provided that the conductors were of iron, and that they were kept in metallic connection by a slight and constant pressure. The experiment was made with three Paris points, and arranged as it appears in fig. 49, and it has since been repeated under very favourable conditions by Mr. Willoughby Smith with three of the so-called rat-tail files, which made it possible to transmit even the faint sound of the act of respiration.[15]

He afterwards tried different combinations of the same nature, which offered several solutions of continuity, and a steel chain produced fairly good results, but slight inflections, like those caused by the _timbre_ of the voice, were not reproduced, and he tried other arrangements. He first sought to apply metallic powders to the points of contact; powdered zinc and tin, known in commerce under the name of white bronze, greatly increased the effects obtained; but they were unstable, on account of the oxidation of the contacts; and it was in seeking to solve this difficulty, as well as to discover the most simple means of obtaining a slight and constant pressure on the contacts, that Mr. Hughes was led to the arrangement, previously described, of carbons impregnated with mercury, and he thus obtained the maximum effect.[16]

Mr. Hughes considers that the successful effects of the microphone depend on the number and perfection of the contacts, and this is doubtless the reason why some arrangements of the carbon pencil in the instrument described above were more favourable than others.

In order to reconcile these experiments with his preconceived ideas, Mr. Hughes thought that, since the differences of resistance proceeding from the vibrations of the conductor were only produced when it was broken, the molecular movements were arrested by the lateral resistances which were equal and opposite, but that if one of these resistances were destroyed, the molecular movement could be freely developed. He considers that an imperfect contact is equivalent to the suppression of one of these resistances, and as soon as this movement can take place, the molecular expansions and contractions which result from the vibrations must correspond to the increase or diminution of resistance in the circuit. We need not pursue Mr. Hughes’s theory further, since it would take too long to develope it, and we must continue our examination of the different properties of the microphone.[17]

Carbon, as we have said, is not the only substance which can be employed to form the sensitive organ of this system of transmission. Mr. Hughes has tried other substances, including those which are good conductors, such as metals. Iron afforded rather good results, and the effect produced by surfaces of platinum when it was greatly subdivided was equal, if not superior, to that furnished by the mercurised carbon. Yet since the difficulty of making instruments with this metal is greater, he prefers the carbon, which resembles it in being incapable of oxidation.

We have already said that the microphone may be used as a thermoscope, in which case it must have the special arrangement represented in fig. 43. Under these conditions, heat, reacting on the conductivity of these contacts, may cause such variations in the resistance of the circuit that the current of three Daniell cells will be annulled by approaching the hand to the tube. In order to estimate the relative intensity of the different sources of heat, it will be enough to introduce into the circuit of the two electrodes A and B, fig. 43, a battery P, of one or two Daniell cells, and a moderately sensitive galvanometer G. For this purpose one of 120 turns will suffice. When the deviation decreases, it shows that the source of heat is superior to the surrounding atmosphere; and conversely, that it is inferior when the deviation increases. Mr. Hughes says that the effects resulting from the intervention of sunshine and shadow are shown on the instrument by considerable variations in the deviations of the galvanometer. Indeed it is so sensitive to the slightest variations of temperature that it is impossible to maintain it in repose.

I have repeated Mr. Hughes’s experiments with a single Leclanché cell, and for this purpose I employed a quill, filled with five fragments of carbon, taken from the cylindrical carbons of small diameter which are made by M. Carré for the electric light. I have obtained the results which are mentioned by Mr. Hughes, but I ought to say that the experiment is a delicate one. When the pressure of the fragments of carbon against each other is too great, the current traverses them with too much force to allow the calorific effects to vary the deviation of the galvanometer, and when the pressure is too slight, the current will not pass through them. A medium degree of pressure must therefore be effected to ensure the success of the experiment, and when it is obtained, it is observed that on the approach of the hand to the tube, a deviation of 90° will, after a few seconds, diminish, so that it seems to correspond with the approach or withdrawal of the hand. But breathing produces the most marked effects, and I am disposed to believe that the greater or less deviations produced by the emission of articulate sounds when the different letters of the alphabet are pronounced separately, are due to more or less direct emissions of heated gas from the chest. It is certain that the letters which require the most strongly marked sounds, such as A, F, H, I, K, L, M, N, O, P, R, S, W, Y, Z, produce the greatest deviations of the galvanometric needle.

In my paper on the conductivity of such bodies as are moderately good conductors, I had already pointed out this effect of heat upon divided substances, and I also showed that after a retrograde movement, which is always produced at once, a movement takes place in an inverse direction to the index of the galvanometer when heat has been applied for some instants, and this deviation is much greater than the one which is first indicated.

In a paper published in the ‘American Scientific Journal,’ June 28, 1878, Mr. Edison gives some interesting details on the application of his system of a telephonic sender to measuring pressures, expansions, and other forces capable of varying the resistance of the carbon disk by means of greater or less compression. Since his experiments on this subject date from December 1877, he again claims priority in the invention of using the microphone as a thermoscope; but we must observe that according to Mr. Hughes’s arrangement of his instrument, the effect produced by heat is precisely the reverse of the effect described by Mr. Edison. In fact, in the arrangement adopted by the latter, heat acts by increasing the conductivity acquired by the carbon under the increased pressure produced by the expansion of a body sensitive to heat: in Mr. Hughes’s system, the effect produced by heat is precisely the contrary, since it then acts only on the contacts, and not by means of pressure. Therefore the resistance of the microphone-thermoscope is increased under the influence of heat, instead of being diminished. This contrary effect is due to the division of some substance which is only a moderate conductor, and I have shown that under such conditions these bodies, when only slightly heated, always diminish the intensity of the current which they transmit. I believe that Mr. Edison’s arrangement is the best for the thermoscopic instrument, and makes it possible to measure much less intense sources of heat. Indeed he asserts that by its aid the heat of the luminous rays of the stars, moon, and sun may be measured, and also the variations of moisture in the air, and barometric pressure.

This instrument, which we give fig. 50, with its several details, and with the rheostatic arrangement employed for measuring, consists of a metallic piece A fixed on a small board C, and on one of its sides there is the system of platinum disks and carbons shown in fig. 28. A rigid piece G, furnished with a socket, serves as the external support of the system, and into this socket is introduced the tapering end of some substance which is readily affected by heat, moisture, or barometric pressure. The other extremity is supported by another socket I, fitted to a screw-nut H, which may be more or less tightened by a regulating screw. If this system is introduced into a galvanometric circuit _a, b, c, i, g_, provided with all the instruments of the electric scale of measure, the variations in length of the substance inserted are translated by greater or less deviations of the galvanometric needle, which follow from the differences of pressure resulting from the lengthening or shortening of the surface capable of expansion which is inserted in the circuit.

The experiments on the microphone made in London at the meeting of the Society of Telegraphic Engineers on May 25, 1878, were wonderfully successful, and they were the subject of an interesting article in the ‘Engineer’ of May 31, which asserts that the whole assembly heard the microphone speak, and that its voice was very like that of the phonograph. When the meeting was informed that these words had been uttered at some distance from the microphone, the Duke of Argyll, who was present, while admiring the important discovery, could not help exclaiming that this invention might have terrible consequences, since, for instance, if one of Professor Hughes’s instruments were placed in the room in Downing Street, in which Her Majesty’s ministers hold their cabinet council, their secrets might be heard in the room in which the present meeting took place. He added that if one of these little instruments were in the pocket of Count Schouvaloff, or of Lord Salisbury, we should at once be in possession of the secrets for which all Europe was anxiously waiting. If these instruments were able to repeat all the conversations held in the room in which they stood, they might be really dangerous, and the Duke thought that Professor Hughes, who had invented such a splendid yet perilous instrument, ought next to seek an antidote for his discovery. Dr. Lyon Playfair, again, thought that the microphone ought to be applied to the aërophone, so that by placing these instruments in the two Houses of Parliament, the speeches of great orators might be heard by the whole population within five or six square miles.

The experiments lately made with the microphone at Halifax show that the Duke of Argyll’s predictions were fully justified. It seems that a microphone was placed on a pulpit-desk in a church in Halifax, and connected by a wire about two miles long with a telephone placed close to the bed of a sick person, who was able to hear the prayers, the chanting, and the sermon. This fact was communicated to me by Mr. Hughes, who heard it from a trustworthy source, and it is said that seven patients have subscribed for the expense of an arrangement by which they may hear the church services at Halifax without fatigue.

The microphone has also lately been applied to the transmission of a whole opera, as we learn from the following account in the ‘Journal Télégraphique,’ Berne, July 25, 1878:--

‘A curious micro-telephonic experiment took place on June 19 at Bellinzona, Switzerland. A travelling company of Italian singers was to perform Donizetti’s opera, “Don Pasquale,” at the theatre of that town. M. Patocchi, a telegraphic engineer, took the opportunity of making experiments on the combined effects of Hughes’s carbon microphone as the sending instrument, and Bell’s telephone as the receiver. With this object he placed a Hughes microphone in a box on the first tier, close to the stage, and connected it by two wires, from one to half a millimètre in thickness, to four Bell receivers, which were placed in a billiard-room above the vestibule of the theatre, and inaccessible to sounds within the theatre itself. A small battery of two cells, of the ordinary type used in the Swiss telegraphic service, was inserted in the circuit, close to the Hughes microphone.

‘The result was completely successful. The telephones exactly reproduced, with wonderful purity and distinctness, the instrumental music of the orchestra, as well as the voices of the singers. Several people declared that they did not lose a note of either, that the words were heard perfectly; the airs were reproduced in a natural key, with every variation, whether _piano_ or _forte_, and several amateurs assured M. Patocchi that by listening to the telephone they were able to estimate the musical beauty, the quality of the singers’ voices, and the general effect of the piece, as completely as if they had been among the audience within the theatre.

‘The result was the same when resistances equivalent to 10 kilomètres were introduced into the circuit, without increasing the number of cells in the battery. We believe that this is the first experiment of the kind which has been made in Europe, at least in a theatre, and with a complete opera; and those who are acquainted with the delicacy and grace of the airs in “Don Pasquale” will be able to appreciate the sensitiveness of the combined instruments invented by Hughes and Bell, which do not suffer the most delicate touches of this music to be lost.’

Although experiments with the microphone are of such recent date, they have been very various, and among other curious experiments we learn from the English newspapers that the attempt has been made to construct an instrument on the same principle as the telephone, which shall be sensitive to the variations of light. It is known that some substances, and particularly selenium, are electrically affected by light, that is, that their conductivity varies considerably with the greater or less amount of light which is shed upon them. If, therefore, a circuit in which a substance of this nature is inserted, is abruptly subjected to a somewhat intense light, the increase of resistance which results from it ought to produce a powerful sound in a telephone inserted in the circuit. This fact has been verified by experiment, and Mr. Willoughby Smith infers from it, as we have already suggested, that the effects produced in the microphone are due to variations of resistance in the circuit, which are produced by more or less close contacts between imperfect conductors.

In order to obtain this effect under the most favourable conditions, Mr. Siemens employs two electrodes, consisting of network of very fine platinum wire, fitting into each other like two forks, of which the prongs are interlaced. These electrodes are inserted between two glass plates, and a drop of selenium, dropped in the centre of the two pieces of network, connects them on a circular surface large enough to establish sufficient conductivity in the circuit. It is on this flattened drop that the ray of light must be projected.

APPLICATIONS OF THE MICROPHONE.

The applications of the microphone increase in number every day, and in addition to those of which we have just spoken, there are others of really scientific and even of practical interest. Among the number is the use which can be made of it as a system of relays for telegraphy, in science for the study of vibrations imperceptible to our senses, in medicine and surgery, and even in manufactures.

_Its application to Scientific Research._--We have seen that several physicists, including Messrs. Spottiswoode, Warwick, Rossetti, Canestrelli, Wiesendanger, Lloyd, Millar, Buchin, and Blyth, have been able to hear what is said in a telephone which has no iron diaphragm, but it was so difficult to establish the fact that it has been often disputed. More certain evidence was desirable, and the microphone is an opportune agent for affording it.

The ‘Telegraphic Journal’ of September 1, 1878, observes that M. du Moncel, in order to claim the victory in his controversy with Colonel Navez, had still to show that the sounds which appeared to be inarticulate in telephones without a diaphragm might become intelligible if they were intensified. This has been done for him by the use of Mr. Hughes’s microphone, and the following experiments were made for the purpose.

1. If a magnetising coil, surrounding a bar of soft iron, is inserted in the circuit of a microphone, with a battery of three cells, the ticking of a watch and other sounds of the same kind may be heard on approaching the ear to the electro-magnet which has been thus constituted. It is true that these sounds are very faint when they are not amplified, but if the electro-magnet is fastened to a board, and a second microphone is fixed to the same board, the sounds produced by the electro-magnet are magnified, and become distinctly audible in the telephone which is placed in connection with this second microphone.

2. These sounds may be further amplified by resting one of the extremities of the core of the electro-magnet on one of the poles of a permanent magnet, which is fixed upon the board. Articulate speech may then be heard in the telephone which is placed in connection with the microphone resting on the board, and the point at issue between MM. Navez and Du Moncel is completely decided in this way: for the auxiliary microphone can only propagate and amplify the vibration of articulate sounds, which are communicated by the bar magnet of the coil to the board on which the two instruments are placed. In this way it would be possible to render articulate sounds perceptible to M. Navez, when transmitted by the bar magnet of a telephone without a diaphragm.

3. When a second bar magnet rests on the free pole of the electro-magnet, so as to present to it a pole of the same nature as the one with which it is already in communication--in a word, if a bar is placed between the two poles of a horseshoe electro-magnet, the effects are still more marked, and hence it may be assumed that the bar reacts as an armature, by concentrating the lines of magnetic force in the vicinity of the helix.

4. When the two poles of a horseshoe magnet are inserted together inside a coil, their effects are equally energetic, although by this arrangement one of the poles might be expected to neutralise the effect of the other: but the most important effects have been obtained by placing an armature of soft iron across the poles of the magnet which has been already inserted in the coil. Under these conditions articulate sounds are distinctly heard.

These experiments were confirmed by Mr. F. Varley, in a letter published in the ‘Telegraphic Journal’ of September 15, 1878, and among the fresh experiments mentioned by him, we will quote those which he made with an iron tube inserted in a helix, in which the two opposite poles of two bar magnets are introduced. These poles are only separated from each other by the interval of an inch, so that the centre of the iron tube may be strongly magnetised.

Mr. Varley says that this last arrangement reproduces the articulate sounds which issue from a sending microphone, and this experiment is more decisive than that of Professor Hughes, in which case it might be supposed that the bar magnet, resting on the polar end of an electro-magnetic bar, was only a modification of the disk in the Bell telephone, set in vibration by the alternate currents passing through the helix, and that these vibrations were communicated to the board, and became sensible when enlarged by the microphone. But such an objection cannot be alleged in the case of the arrangement described above, for since the sound is produced between the current passing into the helix and the magnetic current of the bar, it can only be the result of a vibration produced by a disturbance of the reciprocal relations subsisting between these two elements. Mr. Varley adds that these experiments confirm M. du Moncel’s researches, which have thrown considerable light upon the causes which are at work in the action of the speaking telephone, and with which we have hitherto been imperfectly acquainted.