TELEPHONIC EXPERIMENTS.
Since Mr. Bell’s experiments of which an account has been given in the early part of this work, much study has been given by men of science and inventors to the effects produced in this curious instrument, so as to ascertain its theory and deduce improvements in its construction. We will take a glance at these researches in succession.
Experiments on the Effects produced by Voltaic and Induced Currents.—The comparative study of the effects produced in the telephone by voltaic and induced currents was one of the first and most important. In 1873, as we have seen, Mr. Elisha Gray converted the voltaic currents, which he employed to cause the vibrations of his transmitting plate, into induced currents by means of an induction coil, such as Ruhmkorff’s. The voltaic currents then traversed the primary helix of the coil, and the induced currents reacted on the receiving instrument, producing on its electro-magnetic system the vibrations excited at the sending station. When Mr. Edison designed his battery telephone, he had recourse to the same means to work his receiving telephone, since he had ascertained that induced currents were superior to voltaic currents. But this peculiarity of Mr. Edison’s arrangement was not clearly understood from the descriptions which reached Europe, so that several persons believed that they had invented this arrangement—among others, Colonel Navez and MM. Pollard and Garnier.
Colonel Navez, in an interesting paper on the new telephonic system, presented to the Belgian Royal Academy, February 2, 1878, only suggests this arrangement as a mode of reproducing speech at a great distance; but he quotes no experiment which distinctly shows the advantages of this combination. Twenty days later, MM. Pollard and Garnier, unacquainted with Colonel Navez’s researches, sent to me the results they had obtained by similar means, and these results appeared to me so interesting that I communicated them to the Académie des Sciences, February 25, 1878. In order that the importance of these results may be clearly understood, I will repeat the text of M. Pollard’s letter, addressed to me on February 20, 1878:
‘With the object of increasing the variations of electric intensity in the Edison system, we induce a current in the circuit of a small Ruhmkorff coil, and we fix the receiving telephone to the extremities of the induced wire. The current received has the same intensity as that of the inducing current, and consequently the variations produced in the current which works the telephone have a much wider range. The intensity of the transmitted sounds is strongly increased, and the value of this increase depends upon the relative number of spirals in the inducing and induced circuits. Our attempts to determine the best proportions have been laborious, since it is necessary to make a coil for each experiment; we have hitherto obtained excellent results with a small Ruhmkorff coil reduced to its simplest form, that is, without condenser or contact-breaker. The inducing wire is No. 16, and is wound in five layers; the induced wire is No. 32, and in twenty layers. The length of the coil is seven centimètres.
‘The following is the most remarkable and instructive experiment: When setting the sender to work with a single Daniell cell, there is no appreciable effect at the receiving station, at least in the telephone which I have made, when it is in immediate connection with the circuit; after inserting the small induction coil, sounds become distinctly audible, and their intensity equals that of good ordinary telephones. Since the battery current is only moderately intense, the points of plumbago are not worn down, and the regulating apparatus lasts for a long while. When a stronger battery is used, consisting of six cells of bichromate of potash (in tension) or twelve Leclanché cells, sufficient intensity is obtained by the direct action to make sounds nearly as audible as in ordinary telephones; but when the induction coil is inserted, the sounds become much more intense, and may be heard at a distance of from fifty to sixty centimètres from the mouthpiece. Songs may, under such circumstances, be heard at a distance of several yards; but the relative increase does not appear to be so great as in the case of the single Daniell cell.’
On the other hand, ‘Les Mondes,’ March 7, 1878, contains an account of a series of experiments made by Signor Luvini, Professor of Physics at the Military Academy of Turin, which proved that the introduction of electro-magnets into the circuit which connects the two telephones sensibly increases the intensity of sound. The maximum effect is produced by placing one close to the transmitting, and the other close to the receiving telephone, and the introduction of other magnets is of no use. The inducing wire of a Ruhmkorff coil, when introduced into such a circuit, excited no sensible effects of induction in the induced circuit, and consequently could not set the telephone in connection with this circuit at work. But the current of a Clarke machine produces sounds resembling the beats of a drum, which are deafening when the ear is applied to the instrument: they become very faint, however, at the distance of a mètre. The currents of a Ruhmkorff machine are still more energetic, and the sound fills a whole room. By modifying the position of the lever of the coil, the sound passes through different tones, which are always in unison with the breaks of the current, at least up to a certain pitch.
This property of currents induced by the Ruhmkorff coil has enabled M. Gaiffe to obtain by their means a very simple mode of regulating telephones, so as to produce in them the maximum amount of sensibility. For this purpose he places the telephone he proposes to regulate in the circuit of an induction instrument with moveable helices and graduated intensities. The sounds which result from the vibrator are then reverberated from the telephone, and are audible at a distance from the instrument; by using a screw-driver, it is possible to adjust the screw to which the free end of the bar magnet of the instrument is fixed. It can be tightened or loosened, so as to advance or withdraw the other end of the magnet from the vibrating plate of the telephone, and the process is repeated until the maximum intensity of sound is obtained.
On the other hand, as the sounds given out by the two telephones in correspondence are intense in proportion to the degree of unison in the vibrations produced by them, it is necessary to select those which emit the same sounds for the same given note; and the mode we have just described may be employed with advantage, since it will be enough to observe what instruments give the same note in the condition of maximum sensibility, when regulated in the same way by the induction machine.
It is very important that the telephones in correspondence should be well matched, not only to ensure clear transmissions, but also with reference to the tone of voice of those who are to use it. The sound becomes more audible when the tone of voice corresponds to the telephonic tone; and for this reason some telephones repeat the voices of women and children better than those of men, and with others the reverse takes place.
The telephonic vibrations vary in different instruments, and these variations may be noted in the way we have indicated.
The advantages of induced currents in telephonic transmissions may be easily understood, if we consider that the variations of resistance in the circuit, resulting from the greater or less range in the vibrations of the transmitting plate, are of constant value, and can only manifest their effects distinctly in short circuits; consequently the articulate sounds which result from them can only be really appreciable in circuits of great resistance. According to Mr. Warren de la Rue’s experiments (reported in the ‘Telegraphic Journal,’ March 1, 1878), the currents produced by the vibrations of the voice in an ordinary telephone represent in intensity those of a Daniell cell traversing 100 megohms of resistance (or 10,000,000 kilomètres); and it is plain that the simple question of greater or less intensity in the currents acting on the receiving telephone is not the only thing we have to consider. With an energetic battery, it is evident, in fact, that the differential currents will always be more intense than the induced currents produced by the action of the instrument. I myself am inclined to believe that induced currents owe the advantages they possess to the succession of inverse currents and their brief duration. These currents, of which M. Blaserna considers that the duration does not exceed 1/200 of a second, are much more susceptible than voltaic currents of the multiplied vibrations which are characteristic of phonetic vibrations, and especially since the succession of inverse currents which take place discharge the line, reverse the magnetic effects, and contribute to make the action more distinct and rapid. We cannot therefore be surprised that the induced currents of the induction coil, which can be produced under excellent conditions at the sending station, since the circuit of the voltaic current is then very short, are able to furnish results, not only more effective than the voltaic currents from which they take their origin, but even than the induced currents resulting from the action of the Bell telephone, since they are infinitely more energetic.
As for the effects produced by the currents of Bell telephones, which are relatively great when we consider their size, they are easily explained from the fact that they are produced under the influence of the vibrations of the telephone plate, so that their variations of intensity always maintain the same proportion, whatever may be the resistance of the circuit, and consequently they are not effaced by the distance which divides the two telephones.
Experiments on the part taken by the different telephonic organs in the transmission of speech.—In order to introduce all the improvements of which a telephone is capable, it is important to be quite decided as to the effects produced in the several parts of which it is composed, and as to the part taken by the several organs which are at work. To attain this object several men of science and engineers have undertaken a series of experiments which have produced very interesting results.
One of the points on which it was most important to throw light was that of ascertaining whether the vibrating plate used in their telephone receivers by Messrs. Bell and Gray is the only cause of the complex vibrations which reproduce speech, or if the different parts of the electro-magnetic system of the instrument all conduce to this effect. The experiments made by Mr. Page in 1837 on the sounds produced by the resonant electro-magnetic rods, and the researches pursued in 1846 by Messrs. de la Rive, Wertheim, Matteucci, &c., on this curious phenomenon, allow us to state the question, which is certainly more complex than it at first appears.
In order to start from a fixed point, it must first be ascertained whether a telephone can transmit speech without a vibrating plate. Experiments made by Mr. Edison9 in November 1877, with telephones provided with copper diaphragms, which produced sounds, make the hypothesis credible; and it received greater weight from the experiments made by Mr. Preece and Mr. Blyth. The fact was placed beyond a doubt by Mr. Spottiswoode (see the ‘Telegraphic Journal’ of March 1, 1878), who assures us that the vibrating plate of the telephone may be entirely suppressed without preventing the transmission of speech, provided that the polar extremity of the magnet be placed quite close to the ear; and it was after this that I presented to the Académie des Sciences my paper on the theory of the telephone, which led to an interesting discussion of which I shall speak presently. At first the authenticity of these results was denied, and then an attempt was made to explain the sounds heard by Mr. Spottiswoode as a mechanical transmission of the vibrations, effected after the manner of string telephones; but the numerous experiments which have subsequently been made by Messrs. Warwick, Rossetti, Hughes, Millar, Lloyd, Buchin, Canestrelli, Wiesendanger, Varley, and many others, show that this is not the case, and that a telephone without a diaphragm can transmit speech electrically.
Colonel Navez himself, who had first denied the fact, now admits that a telephone without a diaphragm can emit sounds, and even, under certain exceptional conditions, can reproduce the human voice; but he still believes that it is impossible to distinguish articulate words.
This uncertainty as to the results obtained by the different physicists who have studied the matter shows that at any rate the sounds thus reproduced are not clearly defined, and that in physical phenomena, only appreciable to our senses, the appreciation of an effect so undefined must depend on the perfection of our organs. We shall presently see that this very slight effect can be largely increased by the arrangement adopted by Messrs. Bell and Gray, and we shall also see that, by a certain mode of magnifying the vibrations, it has been decisively proved that a telephone without a diaphragm can readily reproduce speech. I proceed to give the description of such a telephone, which was shown by Mr. Millar at the meeting of the British Association at Dublin in August 1878.
This instrument consists of a small bar magnet, three inches in length and 5/16 of an inch in width and thickness, and a copper helix (No. 30) of about six mètres in length is wound round the bar. It is fixed in a box of rather thick pasteboard, fitted above and below with two zinc plates, which render it very portable. With a telephonic battery sender and a single Leclanché cell, speech can be perfectly transmitted; the whistling of an air, a song, and even the act of respiration become audible. It seems also that the instrument can act without a magnet, merely with a piece of iron surrounded by the helix; but the sounds are then much fainter.
Signor Ignace Canestrelli obtained the same results by making one of the carbon telephonic senders react on a telephone without a diaphragm, by means of an induction coil influenced by two Bunsen cells. He writes as follows on the subject:
‘With this arrangement I was able to hear the sound of any musical instrument on a telephone without a diaphragm: singing, speaking, and whistling were perfectly audible. Whistling could be heard, even when the telephone without a diaphragm was placed at some distance from the ear. In some cases, depending on the pitch of the voice, on the distance of the sending station, and on the joint pressure exerted by the carbons, I could even distinguish words.
‘I finally discharged the currents of the transmitter into the coils of insulated copper wire with which the two poles of a magnet were provided. This magnet was placed on a musical box, made of very thin slips of wood, and on placing the ear at the opening of the box I obtained the same results as with the ordinary telephones without a diaphragm.’
M. Buchin, after repeating experiments of the same kind as the above, intimates that it is easy to hear the sounds produced by a telephone without a diaphragm, by introducing into the ear the end of an iron rod, of which the other end is applied to the active pole of the bar magnet of the telephone. (See ‘Le Journal d’Electricité,’ October 5, 1878.)
I repeat finally the account of some experiments made by Mr. Hughes and M. Paul Roy which are interesting from our present point of view.
1. If an armature of soft iron is applied to the poles of an electro-magnet, with its two branches firmly fixed on a board, and if pieces of paper are inserted between this armature and the magnetic poles, so as to obviate the effects of condensed magnetism; if, finally, this electro-magnet is connected with a speaking microphone, of the form given in fig. 39, it is possible to hear the words spoken in the microphone on the board which supports the electro-magnet.
2. If two electro-magnets are placed in communication with a microphone, with their poles of contrary signs opposite to each other, and if their poles are separated by pieces of paper, speech will be distinctly reproduced, without employing armature or diaphragm. These experiments are, however, delicate, and demand a practised ear.
3. If, instead of causing the current produced by a microphone to pass through the helix of a receiving telephone, it is sent directly into the bar magnet of this telephone in the direction of its axis—that is, from one pole to another—the words pronounced in the microphone may be distinctly heard. This experiment by M. Paul Roy indicates, if it is exact, that the electric pulsations which traverse a magnet longitudinally will modify its magnetic intensity. The experiment, however, demands verification.
Another point was obscure. It was important to know whether the diaphragm of a telephone really vibrates, or at least if its vibrations could involve its displacement, such as occurs in an electric vibrator, or in wind instruments which vibrate with a current of air. M. Antoine Bréguet has made some interesting experiments on the subject, which show that such a movement cannot take place, since speech was reproduced with great distinctness from telephones with vibrating plates of various degrees of thickness, and he carried the experiment so far as to employ plates fifteen centimètres in thickness.10 When pieces of wood, caoutchouc, and other substances were laid upon these thick plates, the results were the same. In this case it cannot be supposed that the plates were moved to and fro. I have moreover ascertained, by placing a layer of water or of mercury on these plates, and even on thin diaphragms, that no sensible movement took place, at least when the induced currents produced by the action of speaking were used as the electric source. No ripples could be seen on the surface of the liquid, even when luminous reflectors were employed to detect them. And indeed it can hardly be admitted that a current not more intense than that of a Daniell element, which has traversed 10,000,000 kilomètres of telegraphic wire—a current which can only show deviation on a Thomson galvanometer—should be powerful enough to make an iron plate as tightly stretched as that of a telephone vibrate by attraction, even if we grant that the current was produced by laying a finger on the diaphragm.
Very nice photographic experiments do, however, show that vibrations are produced on the diaphragm of the receiving telephone; they are indeed excessively slight, but Mr. Blake asserts that they are enough to cause a very light index, resting on the diaphragm, to make slight inflections on a line which it describes on a register. Yet this small vibration of the diaphragm does not show that it is due to the effect of attraction, for it may result from the act of magnetisation itself in the centre of the diaphragm.11 An interesting experiment by Mr. Hughes, repeated under different conditions by Mr. Millar, confirms this opinion.
If the magnet of a receiving telephone consists of two magnetised bars, perfectly equal, separated from each other by a magnetic insulator, and they are so placed in the coil as to bring alternately the poles of the same and of contrary signs opposite to the diaphragm, it is known that the telephone will reproduce speech better in the latter case than in the former. Now, if the effects were due to attraction, this would not be the case; for the actions are in disagreement when the poles of contrary signs are subjected to the same electric influence, while they are in agreement when these poles are of like signs.
On the other hand, it is known that if several iron plates are put together in order to form the diaphragm of the receiver, the transmission of sounds is much stronger than with a simple diaphragm; and yet the attraction, if it has anything to do with it, could only be exerted on one of the diaphragms.
It further appears that it is not merely the magnetic core which emits sounds, but that they are also produced with some distinctness by the helices. Signor Rossetti had already ascertained this fact, and had even remarked that they could be animated by a slight oscillatory movement along the bar magnet, when they were not fixed upon it. Several observers, among others M. Paul Roy, Herr Wiesendanger, and Signor Canestrelli, have since mentioned similar facts, which are really interesting.
‘If,’ writes M. Paul Roy, ‘a coil of fine wire, which is at the extremity of the bar magnet of a Bell telephone, receives the pulsatory currents transmitted by a carbon telephone, it is only necessary to bring the coil close to the ear in order to hear the sounds.
‘The sounds received in this way are very faint, but become much stronger if a piece of iron is introduced into the circuit coil. A magnet acts with still greater force, even when it consists of a simple magnetised needle. Finally, the sound assumes its maximum intensity when an iron disk is inserted between the ear and the coil.
‘By placing the end of the coil to the ear, and sending a current through it from the bar magnet, it is ascertained that the sound is at its minimum when the neutral line of the magnet is enclosed by the coil, and that it increases until attaining its maximum, when the magnet is moved until one of its poles corresponds to the coil.
‘This fact of the reproduction of sounds by a helix is universal. Every induction coil and every electro-magnet are capable of reproducing sound when the currents of the sender are of sufficient intensity.’
Signor Canestrelli writes as follows: ‘With the combination of a carbon telephone and one without diaphragm or magnet—that is, with only a simple coil—I was able to hear whistling through the coil, placed close to the ear. This coil was of very fine copper wire, and the currents were produced through an induction coil by two Bunsen elements. The contacts of the telephone were in carbon, and it was inserted in the primary circuit.
‘I fastened the coil to the middle of a tightly stretched membrane which served as the base of a short metal cylinder. When a magnet was placed near this part of the coil, the sounds were intensified, and when I fixed the magnet in this position, I could hear what was said.
‘I afterwards substituted for the magnet a second coil, fastened to a wooden bar, and on causing the induced currents to pass into both coils at once I was able to hear articulate speech, although not without difficulty.
‘Under these latter conditions I found it possible to construct a telephone without a magnet, but it required a strong current, and it was necessary to speak into the sender in a special manner, so as to produce strong and concentrated sounds.’
Another very interesting experiment by M. A. Bréguet shows that all the constituent parts of the telephone—the handle, the copper rims, and the case, as well as the diaphragm and the electro-magnet—can transmit sounds. M. Bréguet ascertained this fact by the use of string telephones, which he attached to different parts of the telephone on which the experiment was made. In this way he was not only able to establish a correspondence between the person who worked the electric telephone and the one who was listening through the string telephone, but he also made several string telephones act, which were attached to different parts of the electric telephone.
These two series of experiments show that sounds may be obtained from different parts of the telephone without any very appreciable vibratory movements. But Signor Luvini wished for a further assurance of the fact, by ascertaining whether the magnetisation of any magnetic substance, followed by its demagnetisation, would involve a variation in the form and dimensions of this substance. He consequently caused a large tubular electro-magnet to be made, which he filled with a quantity of water, so that, when its two ends were corked, the liquid should rise in a capillary tube fitted to one of the corks. In this way the slightest variations in the capacity of the hollow part of the electro-magnet were revealed by the ascent or descent of the liquid column. He next sent an electric current of varying intensity through the electro-magnet, but he was never able to detect any change in the level of the water in the tube; although by this arrangement he could measure a change of volume of 1/30 of a cubic millimètre. It appears from this experiment that the vibrations produced in a magnetic substance under the influence of successive magnetisations and demagnetisations, are wholly molecular. Yet other experiments made by M. Canestrelli seem to show that these vibrations are so far sensible as to produce sounds which can be detected by the microphone. He writes as follows on the subject:
‘When the broken currents of an induction coil are discharged into a coil placed on a sounding-box, it is possible to hear at a little distance the sounds produced by the induced currents thus generated. On approaching the magnet to the opening of the coil, these sounds are intensified, and the vibrations of the magnet become sensible to the touch; this vibration might even be made visible by suspending the magnet inserted into the coil to a metallic wire, which is fitted to a membrane stretched on a drum, and the latter will then reproduce sounds. When the same magnet is suspended to a microphone, it is possible, with the aid of a telephone, to ascertain the same effects, which are then increased.’
We shall presently consider how these different deductions are to be interpreted, so as to render the true theory of the telephone intelligible; but, before doing so, we will mention some other experiments which are not without interest.
We have seen that the experiments of Messrs. Edison, Blyth, and Preece, show that sounds may be reproduced by a telephone with a diaphragm made of some unmagnetic substance, and they also show, which is still more curious, that these sounds may be transmitted under the influence of induced currents produced by these diaphragms when they are placed in vibration before the magnet. Messrs. Edison and Blyth had already adduced this fact, which was received with incredulity, but it has been confirmed by Mr. Warwick in an article published in the ‘English Mechanic.’ He writes that in order to act upon the magnet, so as to produce induced currents, something possessed of greater energy than gas must first be made to vibrate. It is not, however, necessary that this substance should be magnetic, for diamagnetic substances act perfectly.12 Mr. Preece sought for the cause in the induced currents developed in any conducting body when a magnet is moved before it, currents which give rise to the phenomenon discovered by Arago and known by the name of magnetism by rotation. Yet these facts do not appear to us to be sufficiently well established to make the theory worthy of serious consideration, and it is possible that the effects observed resulted from simple mechanical transmissions.
To conclude the account of these experiments, we will add that Mr. W. F. Barrett thinks it somewhat difficult to define the mode of vibration of the diaphragm, since, while a certain amount of compression exerted on the iron destroys the sounds resulting from the peculiar effects of magnetisation, a still stronger compression causes them to reappear. It is certain that the question is full of obscurity, and demands great research: it is enough to have shown that the theory hitherto held is insufficient.
On the other hand, Colonel Navez considers that the intensity of sound reproduced in a telephone depends not only on the range of vibrations, but also on the vibrating surface and the effect it produces on the stratum of air which transmits the sound. (See paper by Colonel Navez in the ‘Bulletin de l’Académie de Belgique,’ July 7, 1878.)
Experiments on the Effects which result from Mechanical Shocks communicated to different parts of a Telephone.—If a piece of iron is applied to the screw which holds the magnet of the ordinary telephone, it is observed that the transmitted sounds are more distinct, owing to the force supplied to the active pole of the magnet; but at the moment when the piece of iron is applied to the screw a distinct noise is heard, which seems to be due to the mechanical vibrations caused in the magnet at the moment of the shock. M. des Portes, a lieutenant in the French navy, has lately made some interesting experiments on this class of phenomena. He has observed that if, in a telephonic circuit of 90 yards completed by the earth, the sending telephone is reduced to a simple magnet, provided with the coil which constitutes its electro-magnetic organ, and if this magnet is suspended vertically by a silken thread, with the coil above it, a blow struck upon the magnet, either by a copper rod or a piece of wood, will cause distinct sounds to be produced in the receiving telephone—sounds which will increase in intensity when the blow is struck close to the coil, and which will become still stronger, but less clear, if a vibrating plate of soft iron is placed in contact with the upper pole of the magnet.
When the striking instrument is made of iron, the sounds in question are more strongly marked than if it is of wood, and when the magnet has a vibrating disk applied to its active pole, a vibration of the disk takes place at the moment when the shock is heard.
If the striking body is a magnet, the sounds produced resemble those obtained when it is of iron, if the effect is produced between poles of the same nature; but if the poles are of contrary natures, a second noise is heard after each blow, which is produced by drawing away the magnet, and which appears to be a blow struck with much less force. The sound is of course increased if the magnet is provided with its vibrating disk.
If words are uttered on the vibrating disk of the sending telephone, when it is applied to the pole of the magnet, various sounds are heard on the receiving telephone, somewhat similar to those produced by vibrating one of the strings of a violin, and the sound made in withdrawing the disk from contact with the magnet is distinctly heard in the receiver.
The person who applies his ear to the vibrating disk of the sender when it is arranged as above, may hear the voice of anyone who speaks into the receiver, but cannot distinguish the words, owing, no doubt, to the condensed magnetism at the point of contact between the magnet and the vibrating disk, which slackens the magnetic variations and makes it more difficult for them to take place.
A coil is not necessary in order to perceive the blows struck upon the magnet with a rod of soft iron. It is enough to wind three turns of naked conducting wire, which acts as line wire, round one end of the magnet, and the sounds perceived cease, as in other experiments, when the circuit is broken, plainly showing that they are not due to mechanical transmission. It is a still more curious fact that if the magnet is placed in the circuit, so as to form an integral part of it, and if the two ends of the conducting wire are wound round the ends of the magnet, the blows struck upon the latter with the soft iron rod are perceived in the telephone as soon as one pole of the magnet is provided with a vibrating disk.
I have myself repeated M. des Portes’ experiments by simply striking on the screw which, in ordinary telephones, fastens the magnet to the instrument, and I have ascertained that, whenever the circuit was complete, the blows struck with an ivory knife were repeated by the telephone: they were, it is true, very faint when the vibrating disk was removed, but very marked when the disk was in its place. On the other hand, no sound was perceived when the circuit was broken. These sounds were louder when the blows were struck upon the screw than when they were struck on the pole of the magnet itself above the coil: for this reason, that in the first case the magnet could vibrate freely, while in the second the vibrations were stifled by the fixed position of the bar magnet.
These effects may be to some extent explained by saying that the vibrations caused in the magnet by the shock produce undulatory displacements of the magnetised particles in the whole length of the bar, and that induced currents would, according to Lenz’s law, result in the helix from these displacements—currents of which the force would increase when the power of the magnet was further excited by the reaction of the diaphragm, which acts as an armature, and also by that of the striking instrument when it also is magnetic. Yet it is more difficult to explain M. des Portes’ later experiments, and the effect may be produced by something more than the ordinary induced currents.
These are not the only experiments which show the effects produced under the influence of molecular disturbance of various kinds. Mr. Thompson, of Bristol, has observed that if a piece of iron and a tin rod placed perpendicularly on the iron are introduced into the circuit of an ordinary telephone, it is enough to strike the tin rod in order to produce a loud sound in the telephone. He has also shown that if the two ends of a bar magnet are enclosed by two induction coils which are placed in connection with the circuit of a telephone, and if the flame of a spirit lamp is moved below the magnet in the space dividing the two coils, a distinct sound is heard as soon as the flame exerts its influence on the bar magnet. This effect is undoubtedly due to the weakening of the magnetic force of the bar which is produced by the action of heat. I have myself observed that a scratching sound on one of the wires which connect the telephones is heard in both of them, at whatever point in the circuit the scratch is made. The sounds produced are indeed very faint, but they can be distinctly heard, and they become more intense when the scratch is made on the binding-screws of the telephone wires. These sounds cannot result from the mechanical transmission of vibrations, since they are imperceptible when the circuit is broken. From these experiments it appears that some sounds which have been observed in telephones tried on telegraph stations may arise from the friction of the wires on their supports—a friction which produces those very intense sounds which are sometimes heard on telegraphic wires.
Theory of the Telephone.—It appears from the several experiments of which we have spoken that the explanation generally given of the effects produced in the telephone is very imperfect, and that the transmission of speech, instead of resulting from the repetition by the membrane of the receiving telephone (influenced by electro-magnetism) of vibrations caused by the voice on the membrane of the transmitting membrane, is due to molecular vibrations produced in the whole electro-magnetic system, and especially on the magnetic core contained in the helix. These vibrations must be of the same nature as those which have been observed in resonant electro-magnetic rods by MM. Page, de la Rive, Wertheim, Matteucci, &c., and these have been employed in telephones by Reiss, by Cecil and Leonard Wray, and by Vanderweyde.
According to this hypothesis, the principal office of the vibrating plate consists in its reaction, in order to produce the induced currents when the voice has placed it in vibration, and by this reaction on the polar extremity of the bar magnet it strengthens the magnetic effects caused in the centre of the bar when it vibrates under the electro-magnetic influence, or at least when it is affected by the magnet. Since the range of these vibrations for a single note is great in proportion to the flexibility of the note, and since, on the other hand, the variations in the magnetic condition of the plate are rapid in proportion to the smallness of its mass, the advantage of employing, as Mr. Edison has done, very thin and relatively small plates is readily understood. In the case of transmission, the wider range of vibration increases the intensity of the induced currents transmitted. In the case of reception the variations in the magnetising force which produces the sounds are rendered clearer and more distinct, both in the armature membrane and in the bar magnet: something is gained, therefore, in each case. This hypothesis by no means excludes the phonetic effects of the mechanical and physical vibrations which may be produced in the armature plate under the influence of magnetisation and demagnetisation to which it is subjected, and these join their influence to that of the magnetic core.
What is the nature of the vibrations sent into the receiving telephone? This question is still obscure, and those who have studied it are far from being in agreement: as early as 1846 it was the subject of an interesting discussion between MM. Wertheim and de la Rive, and the new discoveries render it still more complex. M. Wertheim considers that these vibrations are at once longitudinal and transverse, and arise from attractions exchanged between the spirals of the magnetising helix and the magnetic particles of the core. M. de la Rive holds that in the case we are considering the vibrations are simply longitudinal, and result from molecular contractions and expansions produced by the different combinations assumed by the magnetic molecules under the influence of magnetisation and demagnetisation. This appears to us to be the most natural explanation, and it seems to be confirmed by the experiment made by M. Guillemin in 1846. M. Guillemin ascertained that if a flexible iron rod, surrounded by a magnetising helix, is kept in position by a vice at one end, and bent back by a weight at the other, it can be made to return instantly to its normal position by sending a current through the magnetising helix. This recovery can in such a case be due to nothing but the contraction caused by the magnetic molecules, which, under the influence of their magnetisation, tend to produce intermolecular attractions, and to modify the elastic conditions of the metal. It is known that when iron is thus magnetised it becomes as hard as steel, and a file makes no impression on its surface.
It is at any rate impossible to dispute that sounds are produced in the magnetic core, as well as in the armature, under the influence of intermittent electric action. These sounds may be musical or articulate; for as soon as the sender has produced the electric action required, there is no reason why vibrations which are effected in a transverse or longitudinal direction should transmit the one more than the other. These vibrations may, as we have seen, be termed microscopic.
Signor Luvini, who shares our opinion of the foregoing theory, does not, however, think it wholly satisfactory, unless account is taken of the reaction caused by the bar magnet on the helix which surrounds it. ‘There cannot,’ he says, ‘be action without reaction, and consequently the molecular action produced in the magnet ought to cause corresponding variations in the helix, and these two effects ought to contribute to the production of sounds.’ He supports this remark by a reference to Professor Rossetti’s experiment, of which we have spoken above.
We believe, however, that this double reaction of which Signor Luvini speaks is not indispensable, for we have seen that insulated helices can produce sounds; it is true that the spirals, reacting on each other, may be the cause of this.
The difficulty of explaining the production of sounds in an electro-magnetic organ destitute of armature caused the authenticity of the experiments we have described to be at first denied, and Colonel Navez started a controversy with us which is not likely to be soon terminated; yet one result of this controversy is that Colonel Navez was obliged to admit that the sound of the human voice may be reproduced by a telephonic receiver without a disk. But he still believes that this reproduction is so faint that it is not possible to recognise articulate words, and he maintains that the transverse vibrations of the disk, which are due to effects of attraction, are the only ones to reproduce articulate speech with such intensity as to be of any use.
It is certain that the articulation of speech requires a somewhat intense vibration which cannot easily be produced in a telephone without a diaphragm; for it must be remembered that in an instrument so arranged, the magnetic effects are reduced in a considerable ratio, which is that of the magnetic force developed in the magnet, multiplied by itself, and that so faint an action as that effected in a telephone becomes almost null when, in consequence of the suppression of the armature, it is only represented by the square root of the force which produced it. It is therefore possible that the sounds which are hardly perceptible in a telephone without a diaphragm become audible when the cause which provokes them is multiplied by itself, and when there are in addition the vibrations produced in the heart of the armature itself, influenced by the magnetisations and demagnetisations to which it is subjected.
In order to show that the action of the diaphragm is less indispensable than Colonel Navez seems to imagine, and that its vibrations are not due to electro-magnetic attractions, it will be enough to refer to Mr. Hughes’s experiments, which we have mentioned above. It is certain that if this were the effect produced, we should hear better when the two bar magnets present their poles of the same nature before the diaphragm, than when they present the poles of contrary natures, since the whole action would then converge in the same direction. Again, the more marked effects obtained with multiple diaphragms in juxtaposition completely exclude this hypothesis. It is, however, possible that in electro-magnetic telephones the iron diaphragm, in virtue of the rapid variations of its magnetic condition, may contribute to render the sounds clearer and more distinct; it may react in the way the tongue does; but we believe that the greater or less distinctness of the articulate sounds must be chiefly due to the range of vibrations. Thus Mr. Hughes has shown that the carbons of metallised wood employed in his microphonic speakers were to be preferred to retort carbons for the transmission of speech, for the very reason that they had less conductivity, so that the differences of resistance which result from differences of pressure are more marked, and consequently it is easier to seize the different degrees of vocal sounds which constitute articulate speech.
It must be clearly understood that what we have just said only applies to the Bell telephone, that is, to a telephone in which the electric currents have such a faint intensity that it cannot be supposed there is any external attractive effect. When these currents are so energetic as to produce such an effect, a transverse electro-magnetic vibration certainly takes place, which is added to the molecular vibration, and helps to increase the sounds produced. But it is no less true that this transverse vibration by attraction or by movement of the diaphragm is not necessary for the reproduction of sounds, whether musical or articulate.
We are not now concerned with the discussion of magnetic effects; there has been an advance in science since Colonel Navez started the controversy, and we must ask how his theory of the movements of the telephone diaphragm by attraction will explain the reproduction of speech by a receiving microphone destitute of any electro-magnetic organ, and I can assert that my experiments show that there can be no mechanical transmission of vibrations, since no sound is heard when the circuit is broken or deprived of its battery. Colonel Navez must therefore accept the molecular vibrations. This certainly gives us a new field for study; but it is because European men of science persist in remaining bound by incomplete theories that we have allowed the Americans who despise them to reap the glory of the great discoveries by which we have lately been astonished.
The experiments quoted above show that sounds may be reproduced not only by simple helices without an electro-magnetic organ, but also by the plates of a condenser, in spite of the pressure exerted upon them; and when we add to this the effects I have just pointed out, it may be supposed that vibrations of sound must result from every reaction between two bodies which has the effect of producing abruptly and at close intervals modifications in the condition of their electric or magnetic equilibrium. It is known that the presence of ponderable matter is necessary for the production of electric effects, and it is possible that the molecular vibrations of which I have spoken may be the result of molecular movements, due to the variations of the electric force which holds the molecules in a special condition of reciprocal equilibrium.
In conclusion, the theory of the telephone and microphone, considered as reproductive organs of speech, is still far from being perfectly clear, and it would be imprudent to be too positive on questions of such recent origin.
The theory of the electric transmission of sounds in electro-magnetic telephones is somewhat complex. It has been seen that they can be obtained from diaphragms of non-magnetic substance, and even from simple mechanical vibrations produced by shocks. Are we to ascribe them in the first case to the inductive reaction of the magnet on the vibrating plate, and in the second case to the movements of magnetic particles before the spirals of the helix? The matter is still very obscure; yet it is conceivable that the modifications of the inducing action of the magnet on the vibrating diaphragm may involve variations in the magnetic intensity, just as we can admit an effect of the same kind due to the approach and withdrawal of the magnetic particles of the spirals of the helix; M. Trève, however, believes that there is in the latter case a special action, which he has already had occasion to study under other circumstances, and he sees in the current thus caused the effect of the transformation of the mechanical labour produced amidst the magnetic molecules. The question is complicated by the fact that these effects are often produced by purely mechanical transmissions.
There is another point to consider, on which Colonel Navez has made some interesting remarks; that is, whether the effects in the receiver are stronger with permanent than with temporary magnets. In the first model of the telephone, exhibited by Mr. Bell at Philadelphia, the receiver was, as I have said, made of a tubular electro-magnet, furnished with a vibrating disk at its cylindrical pole; but this arrangement was abandoned by Mr. Bell, with the object, as he states in his paper, of rendering his instrument both a receiver and a sender.13 Yet Colonel Navez maintains that the magnet plays an important part, and is even indispensable under the present conditions of its form. ‘It is possible,’ he says, ‘under certain circumstances, and by making the instrument in a special way, to make a Bell receiver speak without a permanent magnet, yet with an instrument of the usual construction the sound ceases when the magnet is withdrawn and replaced by a cylinder of soft iron. In order to restore the voice of the telephone, it is enough to approach the pole of a permanent magnet to the cylinder of soft iron. It follows from these experiments that a Bell telephone cannot act properly unless the disk is subjected to an initial magnetic tension obtained by means of a permanent magnet. It is easy to deduce this assertion from a consideration of the theory.’
The assertion may be true in the case of Bell telephones, which are worked by extremely weak currents, but when these currents are relatively strong, all electro-magnets will reproduce speech perfectly, and we have seen that M. Ader made a telephone with the ordinary electro-magnet which acted perfectly.
The action of the currents sent through the helix of a telephone can be easily explained. Whatever may be the magnetic conditions of the bar, the induced currents of different intensity which act upon it produce modifications in its magnetic state, and hence the molecular vibrations follow from contraction and expansion. These vibrations are likewise produced in the armature under the influence of the magnetisations and demagnetisations which are produced by the magnetic action of the core, and they contribute to the vibrations of the core itself, while at the same time the modifications in the magnetic condition of the system are increased by the reaction of the two magnetic parts upon each other.
When the bar is made of soft iron, the induced currents act by creating magnetisations of greater or less energy, followed by demagnetisations which are the more prompt since inverse currents always succeed to those which have been active, and this causes the alternations of magnetisation and demagnetisation to be more distinct and rapid. When the bar is magnetised, the action is differential, and may be exerted in either direction, according as the induced currents corresponding to the vibrations which are effected pass through the receiving coil in the same or opposite direction as the magnetic current of the bar. If these currents are in the same direction, the action is strengthening, and the modifications are effected as if a magnetisation had taken place. If these currents are of opposite direction, the inverse effect is produced; but, whatever the effects may be, the molecular vibrations maintain the same reciprocal relations and the same height in the scale of musical sounds. If the question is considered from the mathematical point of view, we find the presence of a constant, corresponding with the intensity of the current, which does not exist in mechanical vibrations, and which may possibly be the cause of the peculiar tone of speech reproduced by the telephone, a tone which has been compared to the voice of Punch. M. Dubois Raymond has published an interesting paper on this theory, which appeared in ‘Les Mondes,’ February 21, 1878, but we do not reproduce it here, since his remarks are too scientific for the readers for whom this work is intended. We will only add that Mr. C. W. Cunningham asserts that the vibrations produced in a telephone cannot be manifested under precisely the same conditions as those which affect the tympanum of the ear, because the latter has a peculiar funnel-shaped form, which excludes every fundamental note, specially adapted to it, and this is not the case with the bars and magnetic plates which possess fundamental notes capable of greatly altering the half-tones of the voice. He considers the alteration of the voice observed in the telephone must be ascribed to these fundamental notes.
M. Wiesendanger’s Thermophone.—M. Wiesendanger, in an article inserted in the ‘English Mechanic and World of Science,’ September 13, 1878, ascribes the reproduction of speech in certain telephones to vibratory movements resulting from molecular expansions and contractions produced by variations of temperature, and these variations would follow from the currents of varying intensity which are transmitted through the telephonic circuits. He was conscious of one objection to this theory, namely, that the movements of expansion and contraction due to heat are slowly produced, and consequently are not capable of substantial action, rapid enough to produce vibrations; but he considers that molecular effects need not take place under the same conditions as those which are displayed in the case of material substances.
M. Wiesendanger believes that this hypothesis will explain the reproduction of speech in the receiving microphones of Mr. Hughes, and that it may even be applied to the theory of the electro-magnetic telephone, if we consider that a magnetising helix, as well as a magnetic core, round which an electric current circulates, is more or less heated, according to the intensity of the current which traverses it, especially when the wire of the helix and the core are bad conductors of electricity and of magnetism. Pursuing this idea, M. Wiesendanger has sought to construct telephones in which calorific effects are more fully developed, and with this object he used very fine wire of German silver and platinum to make the coils. He ascertained that these coils could produce sounds themselves, and, to increase their intensity, he put them between disks of iron, or on tin tubes, placed on resonant surfaces close to the disks. In this way he says that he was able to make a good receiving telephone without employing magnets. He afterwards arranged the instrument in different ways, of which the two following are the most noteworthy.
In the first, the electro-magnetic system was simply formed by a magnetic disk with a helix wound round it, of which the wire was in connection with the circuit of a microphone, and which was fastened to the centre of the parchment membrane of an ordinary string telephone; the disk consisted of two iron plates separated by a carbon disk of smaller diameter, and the whole was so compressed as to form a solid mass.
In the second, the helix was wound on a tin tube, six inches long and five-eighths of an inch in diameter, which was soldered by merely a point to the centre of the diaphragm of an ordinary telephone.
The inventor asserts that the tube and diaphragm only act as resonators, and that the sounds produced by this instrument are nearly the same as those obtained from the ordinary string telephone: the tunes of a musical box were heard, and the reproduction of speech was perfect, both in intensity and in distinctness of sound; it even appeared that telephonic sounds were audible with the tin tube alone, surrounded by the helix. M. Wiesendanger says that ‘these different receiving telephones show clearly that the diaphragm and magnet are not essential, but merely accessory, parts of a telephone.’