Part 11
In Mr. Hughes’s instrument which we are now considering, the sounds, instead of reaching the receiving stations much diminished, which is the case with ordinary telephones, and even with that of Mr. Edison, are often remarkably increased, and it is for this reason that Mr. Hughes has given to this telephonic system the name of Microphone, since it can be employed to discover very faint sounds. Yet we must add that this increase really takes place only when the sounds result from mechanical vibrations transmitted by solid substances to the sending instrument. The sounds propagated through the air are undoubtedly a little more intense than in the ordinary system, but they lose some of their force, and therefore it cannot be said that in this case the microphone has the same effect upon sounds as the microscope has on objects on which light is thrown. It is true that with this system it is possible to speak at a distance from the instrument, and I have even been able to transmit conversation in a loud voice, when standing at a distance of nine yards from the microphone. When close to the instrument, I was also perfectly able to make myself heard at the receiving station while speaking in a low voice, and even to send the sounds to a distance of ten or fifteen centimètres from the mouthpiece of the receiving telephone by raising the voice a little; but the increase of sound is not really very evident unless it is produced by a mechanical action transmitted to the standard of the instrument.
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.
[Illustration: FIG. 39.]
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.
[Illustration: FIG. 40.]
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.
[Illustration: FIG. 41.]
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_.
[Illustration: FIG. 42.]
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.
[Illustration: FIG. 43]
[Illustration: FIG. 44.]
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.
[Illustration: FIG. 45.]
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.