Part 9

‘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.