Chapter M: J. Luvini, in an article inserted in ‘Les Mondes,’ March 7, (1)

1878, has suggested a system of rheotome by means of a current, for battery telephones, which, although complicated, possibly offers some advantages, since it produces currents alternately reversed. In this system, the vibrating disk of the sender, which should be in a vertical position, reacts on a moveable horizontal wire, turned back at a right angle, and supporting on each of its branches two platinum points which dip into two bulbs, filled with a liquid of moderate conducting capacity. The two branches of this wire, insulated from each other, are placed in communication with the two poles of the battery, and the four cups into which the platinum wire dips communicate inversely with the line and the earth by means of platinum wires immoveably fixed in the cups. It follows from this arrangement, that when the distances are duly regulated between the fixed and moveable wires, two equal currents will be opposed to each other across the line circuit when the diaphragm is motionless; but as soon as it vibrates, the respective distances of the wires will vary, and it follows from this that there will be a differential current, of which the intensity will correspond with the extent of the displacement of the system, or with the range of vibrations, and the direction will vary with the movements above or below the line of the nodes of vibration. In this way the advantage of the induced currents is obtained.

_Telephones with a battery and voltaic arcs._--In order to obtain variations of resistance of still greater sensitiveness than is the case with liquids or pulverised substances, the idea has been suggested of employing conductors of heated gas, and several arrangements of battery telephones have been made in which the circuit was completed by a stratum of air, separating the vibrating disk from a platinum point, which serves to excite an electric discharge of high tension. Under these conditions, the stratum of air becomes the conductor, and the intensity of the current which traverses it corresponds to its thickness. This problem has been solved, either by means of voltaic currents of high tension, or by a Ruhmkorff coil.

The former system was arranged by M. Trouvé, and he writes as follows on the subject in the journal ‘La Nature’ of April 6, 1878: ‘A metallic vibrating membrane forms one of the poles of a high tension battery; the other pole is fastened before the disk by a micrometer screw which can be adjusted so as to vary the distance from the disk according to the tension of the battery, but without ever coming in contact with it. The distance must not in any case exceed that to which the discharge of the battery can extend. Under these conditions, the membrane which vibrates under the influence of the waves of sound has the effect of constantly modifying the distance between the two poles, and thus of continually varying the intensity of the current: consequently the receiving instrument (a Bell telephone, or telephone with an electro-magnet) is subjected to magnetic variations, corresponding to the variations of the current which affect it, and this has the effect of making the receiving instrument vibrate at the same moment. This kind of telephonic instrument relies, therefore, on the possibility of varying within wide limits the resistance of the outer circuit of a high-tension battery, in which the poles are not in contact. In order to vary the conditions of this resistance, it is also possible to interpose some vapour or other medium, such as air, or gas of greater or less rarity.’

M. Trouvé thinks that he was successful with his battery of small disks, moistened with sulphate of copper and sulphate of zinc, arranging these elements, to the number of five or six hundred, in glass tubes of small diameter. It is well known that it is unnecessary for the elements to be of large size in order to obtain tension currents.

M. de Lalagade has suggested an analogous mode by employing for the formation of the arc a current of which the tension is increased by inserting a strong electro-magnet into the circuit. This electro-magnet acts on a Hughes magnet in order to produce induction currents capable of making the receiving instrument act. M. de Lalagade says that a Bunsen battery, or one of six cells with bichromate of potash, will be enough to produce a continuous voltaic arc between the vibrating plate of a telephone and a platinum point which is sufficiently remote to avoid contact. It is necessary, however, to begin with a contact, in order to produce the formation of this arc. In M. de Lalagade’s system, the vibrating plate should have in its centre a small platinum plate, in order to obviate the oxidising effects of the spark. The inventor asserts that sounds transmitted in this way, and reproduced in a telephone of which the electro-magnetic system is set upon a sounding-box, will have greater intensity than the sounds transmitted by an ordinary telephone, and the speaker will appear to be close to the ear.

_Mercury Telephones._--These systems are based on the physical principle discovered by M. Lippmann, that if a layer of acidulated water is placed above mercury, and connected with it by an electrode and wire, every mechanical action which exerts pressure on the surface of the mercury, and alters the form of its meniscus, will cause an electric reaction, capable of producing a current with a force which corresponds to the mechanical action exerted. Conversely, every electric action produced on the circuit of such a system will occasion a displacement of the meniscus, and consequently its movement, which will be more marked in proportion to the smallness of the tube in which the mercury is placed, and to the greatness of the electric action. This electric action may result from a difference of potential in the electric condition of the two extremities of the circuit, which communicate with the electric source employed, or with some electric generator.[8]

In accordance with these effects, it is intelligible that if two tubes T T, pointed at the end, and containing mercury, are plunged into two vessels V V (fig. 34) containing acidulated water and mercury, and metallic wires, P P, Q Q, are used, first to connect the columns of mercury in the tubes, and secondly the layers of mercury at the bottom of the two vessels, the tubes being a little removed from the surface of the mercury in the vessels, we shall then have a metallic circuit, completed by two electrolytes, one of which will be subjected to the mechanical or electrical effects produced in the other. If two vibratory plates B B are placed above the tubes, and one of these is caused to vibrate, the other will reproduce these vibrations, influenced by the vibratory movements communicated by the corresponding column of mercury. The vibrations themselves will be in connection with the electrical discharges resulting from the movements of the column of mercury in the first tube, which are mechanically produced. If an electric generator is introduced into the circuit, the effect which we have just analysed will be caused by modifications in the potential of this generator, in consequence of electro-capillary effects. But if no generator is employed, the action will result from electric currents determined by the electro-capillary attraction itself. In the latter case, however, the instrument must be more delicately made, in order to obtain more sensitive electric reaction, and M. A. Bréguet describes his instrument as follows.

‘The instrument consists of a tube of thin glass, a few centimètres in length, containing alternate drops of mercury and acidulated water, so as to constitute so many electro-capillary elements, connected in tension. The two ends of the tube are fused together, yet so as to allow a platinum wire to touch the nearest drop of mercury on each side. A small circle of thin deal is fixed at right angles to the tube by its centre, thus providing a surface of some extent, which can be applied to the ear when the instrument is a receiver, and to make the tube more mobile under the influence of the voice when the instrument is a sender. The following are the advantages offered by instruments of this construction:--

‘1. They do not involve the use of a battery.

‘2. The disturbing influence of the resistance of a long line is almost destroyed in these instruments, although it is still appreciable in the Bell telephone.

‘3. Two mercury telephones, coupled together as we described above, are absolutely correlative, in this sense, that even different positions in the equilibrium of the mercury in one of them produce different positions of equilibrium in the opposite instrument. It is therefore possible to reproduce at a distance, without a battery, not merely faithful indications of oscillatory movements, which is done by the Bell telephone, but also the exact image of the most general movements.’

_Friction Telephones._--Mr. E. Gray has quite recently applied the principle of producing sounds by the friction of animal tissues to the construction of a speaking telephone which may be heard through a whole room, like the singing condenser. He obtains this result by means of clockwork, which causes the rotation of the metallic disk of which we have spoken (p. 23), and on which a piece of skin is so arranged as to produce friction. A carbon or liquid telephone is placed at the sending station, in such a way as to react on an induction coil, as in the systems of Edison, Navez, or Pollard, and speech is reproduced on the rotating disk, and is audible, as we have said, without the necessity of approaching the ear to the instrument.

The best arrangement of the metallic disk on which the animal tissue rubs is that of a cylindrical box, of which the outer lid is made of a thin sheet of zinc with a highly polished, slightly oxidised surface; for the agent of friction, glove-leather slightly moistened with acidulated water may be used, or a sinew of an ox, or skin taken from the ear or tail of a pig.

MODIFICATIONS INTRODUCED IN THE CONSTRUCTION OF THE BELL TELEPHONES.

The modifications which we have been considering relate to the principle of the instrument; those which we have now to consider are only modifications in the form and arrangement of the different organs which form the Bell telephone itself, and which have been designed with the object of increasing the intensity and distinctness of the sounds produced.

_Telephones with several diaphragms._--When we remember that the induced currents caused in a magnet result from the vibratory movements of the diaphragm, and that these are produced by the vibrations of the stratum of air interposed between this diaphragm and the vocal organ, it necessarily follows that if these vibrations of the air react on several diaphragms, each attached to its electro-magnetic organ, several induced currents might be caused simultaneously, and if these were properly connected, their effects on the receiver would be so much the more intense, since the sounds produced would result from the combination of several sources of sound. Several inventors, starting from this argument, have planned instruments of varying ingenuity, which we will now describe, but without being able to declare who was the first to realise this idea. It is in fact so simple, that it probably suggested itself to the minds of several inventors at the same time, and we see that while M. Trouvé proposed this improvement in France in November 1877, it was tried in America and discussed in England, where indeed it was not expected to produce very favourable results. Mr. Preece wrote on the subject in a paper entitled ‘On some Physical Points connected with the Telephone,’ which was published in April 1878. He observes that all the attempts to improve the telephone have ended in disappointment and failure. One of the first attempts of the kind was made by Mr. Wilmot, who expected to obtain favourable results by augmenting the number of diaphragms, helices, and magnets, connecting the helices in a series, and causing them to act simultaneously, so as to increase the energy of the currents developed by the influence of the voice; but experience showed that when the instrument acted directly, the vibratory effect of each of the diaphragms decreased in proportion to their number, and the general effect remained the same as with a single diaphragm. Mr. Wilmot’s instrument was made in the beginning of October 1877, and that of M. Trouvé was only an imitation of it.

On the other hand, we see that if the telephones with several membranes were not successful in England, this was not the case in America, for the telephones which experience has shown to give the best results in that country are those of Mr. Elisha Gray and Mr. Phelps, and these have several diaphragms. It is evident that there are details of construction in these instruments which may appear insignificant in theory, and which are notwithstanding very important from a practical point of view, and we believe that it is to this circumstance that instruments of this kind owe their success or failure. Thus, for example, it seems that the vibrations of air caused in the mouthpiece ought to be immediately directed on the surface of the diaphragms by means of distinct channels; it is necessary that the empty space round each diaphragm should be sufficiently limited to prevent echoes and interruptions, unless the case is so large that there is no danger of such effects. Above all, it is necessary that the organs should be fixed in some material unsusceptible of reverberation, and for this reason a preference is given to iron or ebonite. It is certain that when the instrument is properly made, its effects are superior to those of the Bell telephones, and it is asserted in the ‘Telegraphic Journal’ that experiments were made with one of these instruments before the Royal Society, in London, May 1, 1878, and that the intensity of sound was in proportion to the number of diaphragms. This instrument was designed by Mr. Cox Walker, of York, and possessed eight diaphragms. He considers that this is the arrangement which gives the best results.

_Mr. Elisha Gray’s System._--Mr. Elisha Gray’s last system, which we represent in fig. 35, is one of those which have given the best effects. It is made, as we see, of two telephones, side by side, to which correspond two tubes, issuing from a common mouthpiece E. One of these telephones is seen in section in the plate, the other in elevation, and they correspond to the two branches of a nickel-plated horseshoe magnet N U S, which may serve as a suspension ring. In that part of the plate which represents the section, the induction coil is shown in B, and the magnetic core, of soft iron, in A, which is screwed to the polar end of the magnet S; the vibrating plate is in L L, and, as we see, the tube of the mouthpiece terminates on its surface.

In another model there are four telephones side by side, instead of two, and the effects produced are still more marked.

_Mr. Phelps’s System._--This system is only deduced from the last, but there are two models of it. In the larger one, which makes it possible to hear as distinctly as if the person with whom conversation is held were speaking in a loud voice in the same room, the two telephones are placed parallel to each other, and so as to present their diaphragms vertically; the space between these two diaphragms is occupied by a vertical tube, terminating at its lower end in a horizontal tube corresponding to the centres of the two diaphragms, and on this tube the mouthpiece is fitted, which projects outside the box in which the instrument is enclosed. The induction coils, and the magnetic cores which traverse them, follow the axis of the system, and seem to constitute the axis of a wheel which is polarised by the poles of a horseshoe magnet, of which the position with reference to the surface of the diaphragms can be regulated by moveable screws. The appearance of the instrument somewhat resembles a gyroscope, resting by a horizontal axis on two shafts which issue from a flattened horseshoe magnet.

Above this system there is the electro-magnetic apparatus of the call-bell, in which there is nothing peculiar, and which is like the German alarums of which we shall speak at the end of this account. This instrument is remarkable for strength and clearness of sound, and especially for its freedom from the Punch and Judy voice so displeasing in other telephones.

Mr. Phelps’s small model is in the form of an oblong or elliptical snuff-box, of which the two centres are occupied by two telephonic systems, influenced by the same magnet. This magnet is placed in a horizontal position below the snuff-box, and its poles correspond to the magnetic cores of the coils. These cores are made of iron tubes, split longitudinally in order to destroy irregular induction reactions, and the iron diaphragms rest on five spiral springs, which raise them above the magnetic system. On their other surface the diaphragms are provided with rings of some semi-elastic substance, which prevent the central vibrations of the disks from becoming complicated by those of their edges. The lid, hollowed out in very shallow cavities, is next placed upon the disks, and there are channels of communication in it to serve as a sounding-box. The mouthpiece corresponds to one of these cavities, and the other is closed by a small metallic stopper, which can be withdrawn to regulate the instrument when necessary. Since the vibrations of air are transmitted by the channels to both cavities, the two telephones act together, although at first sight only one of them seems to be required to produce the effect.

Mr. Phelps praises the simultaneous effects produced on the two instruments, which he ascribes, first, to the semi-elastic ring surrounding the rim of each disk, and acting as the hammer of the ear, that is, as a damper; then, to the longitudinal splits of the magnetic core, and lastly to the small size of the cavities left above the vibrating disks. The instrument is made of ebonite, grooved on the surface in order to give a better grasp to the hand.

Mr. Phelps has a new model, called _the crown telephone_, which is now in use in America, together with Mr. Edison’s carbon sender. In it each of the two systems of the large model we have described is worked by six horseshoe magnets radiating round the magnetic core, and so arranged that the north poles correspond to this core, and the other poles to the circular rim of the diaphragm. In this way the magnetic field is considerably enlarged, and the sound much intensified.

In experiments recently made at Dr. Wells’s church, New York, an assembly of three hundred people were able to hear speech and vocal or instrumental music distinctly in different parts of the hall.

_Mr. Cox Walker’s System._--This system, on which we have already said a few words, has exactly the arrangement of that by Mr. Elisha Gray. The magnets which act upon the diaphragms are horseshoe, and separate pipes, issuing from a common mouthpiece, direct the vibrations of air on the diaphragms. These, indeed, are only defined parts of one diaphragm, bounded in a circle by mouthpieces corresponding to the air-pipes, and sufficiently restricted on their edges to limit the field of vibration.

_M. Trouvé’s System._--M. Trouvé has simplified the arrangement of telephones with a double diaphragm, by designing the instrument so as to make Bell’s bar magnet react by both poles at once on several disks. For this purpose, he employs a tubular magnet, and winds a helix throughout its whole length, as we see in fig. 36. This magnet is maintained in a fixed position in the centre of a small cylindrical box, of which the base is slightly funnel-shaped, thus acting as a mouthpiece and acoustic tube. It is consequently pierced in the centre with a hole larger at _a_, the station for speaking, than on the opposite side _b_. Between the base and the poles of the magnet there are two vibrating iron plates, M, M′, one of which, M, is pierced with a hole _a_ of the same diameter as the hollow part of the magnet, and consequently smaller than that of the mouthpiece. Finally, several other plates _n_, _n_, _n_, are ranged in parallel lines between these two plates, so that the magnet and its helix may pass through them.

When anything is said before the mouthpiece _a_, the waves of sound encountering the edges of the plate M place it in vibration, and, continuing their passage inside the tubular magnet, they cause the plate M′ to vibrate at the same time as M. A double inducing action therefore takes place on the tubular magnet, and this is translated by the induced currents developed in the helix, which have greater energy since each of the plates intensifies the magnetic effects produced at the pole opposite to the one they influence, which is always the case with bar magnets when the inactive pole is provided with an armature. This advantage may even be obtained in the case of ordinary telephones, if the screw which holds the magnet is placed in contact with a mass of soft iron.

In M. Trouvé’s arrangement, the induced currents therefore possess greater energy; but he adds that the sounds reproduced will also be strengthened by the multiplicity of vibratory effects, and by the enlargement of the magnetic effects, which results from a better arrangement of the magnets.

‘When the ear is placed at _a_,’ M. Trouvé writes, ‘it perceives immediately the sounds produced by the first plate M, and those of the second plate reach the ear through the interior of the magnet. This new arrangement is well adapted for an experimental comparison of the results produced by a telephone with a single membrane (a Bell telephone), and those produced by a telephone with several membranes. It is in fact enough to listen at the two faces of the telephone alternately, in order to perceive at once the difference of intensity in the sounds produced. Those collected at _a_, on the side of the pierced iron plate, appear manifestly doubled in intensity compared with those collected at _b_ on the side of the simple membrane which forms the ordinary telephone.

‘The difference is still more striking if, in transmitting or receiving a sound of invariable intensity through a multiple telephone, the unbroken membrane M′ is repeatedly prevented from vibrating.’

Before making this arrangement M. Trouvé had planned another, which he presented to the Académie des Sciences, November 26, 1877, and which we have glanced at in the beginning of this chapter. He describes it in these terms:--

‘In order to increase the intensity of the effects produced in the Bell telephone, I have substituted for the single membrane a cubic chamber, of which each face is, with one exception, formed of a vibrating membrane. Each of these membranes, put in vibration by the same sound, influences a fixed magnet, which is also provided with an electric circuit. In this way, by connecting all the currents generated by the magnets, a single intensity is obtained, which increases in proportion to the number of magnets influenced. The cube might be replaced by a polyhedron, of which the faces might be formed of an indefinite number of vibrating membranes, so as to obtain the desired intensity.’

_M. Demoget’s System._--Several other systems of telephones with multiple membranes have been proposed. One of them, planned by M. Demoget, consists in placing before the vibrating disk of the ordinary Bell telephone, separated by the space of a millimètre, one or two similar vibrating disks, taking care to pierce in the centre of the first a circular hole of the same diameter as that of the bar magnet, and to pierce a larger hole in the second membrane. The inventor asserts that the distinctness as well as the intensity of sounds is increased in this way.

‘By this arrangement,’ says M. Demoget, ‘since the vibrating magnetic mass is larger in proportion to the magnet, the electro-motive force of the currents generated is increased, and consequently the vibrations of the disks of the second telephone are more perceptible.’

_Mr. McTighe’s Telephone._--In this telephone, which has several diaphragms, there is a horseshoe magnet, and instead of placing the coils upon the poles, there is a single coil fastened to an iron core, which is inserted between wide polar appendices fitted to the two poles of the magnet. These appendices consist of thin plates, which act as vibrating plates.

_Modifications in the arrangement of Telephonic Organs._--We see that the forms given to the Bell telephone are very varied, and this is still more the case with its constituent organs, without, however, producing any remarkable improvements. Mr. Preece observes that little has been gained by varying the size and strength of the magnets, and the best effects have been obtained by using the horseshoe magnets directed by Mr. Bell himself. The telephone was certainly introduced into Europe with the arrangement which is theoretically the best, although Mr. Bell is still occupied in improving it. This is also the opinion of M. Hellesen, who, like Mr. Preece, has made many experiments on this point; but this has not deterred several people from declaring that they have discovered the way of making a telephone speak so as to be audible to an assembly of people.

Of the different instruments made with this object, that of M. Righi seems to be the most important. It was lately tried with success at the Académie des Sciences, the Conservatoire des Arts et Métiers, and the Press pavilion of the Exhibition.

The receiver is only a Bell telephone of large size, with a diaphragm of parchment L L (fig. 37), in the centre of which there is a sheet-iron disk F. This membrane is stretched on a large funnel E, which is fixed on a box C C, containing the electro-magnetic coil B: and the magnet N S, much larger than in the ordinary instruments, issues from the box, and serves as its support.

The sender resembles the one represented in fig. 19, except that, instead of liquid, M. Righi employs plumbago mixed with powdered silver, and the platinum needle is replaced by a metallic disk D (fig. 38). The receiver I, which contains the powder, is supported on a spring R, which can be pushed up and down by a regulating screw V, and the whole is fitted into a box C C, and supported on a foot P. The speaker places himself above the mouthpiece E, and the vibrations transmitted to the membrane L L cause the variations of resistance in I which are necessary for the transmission of speech, as in the Edison system. Two Bunsen cells are enough to set the instrument at work, and it will make the sound of a trumpet or flute audible throughout a room. Vocal music, which is less intense, is necessarily transmitted to a rather less distance, and words spoken in the natural voice are heard by those standing about two yards and a half from the instrument.

The maximum distance at which the instrument has been worked with the battery only is twenty-eight miles, the distance between Bologna and Ferrara, and for greater distances it is necessary to have recourse to induction coils.

In this case, an induction coil is introduced into the circuit at each station, and its primary wire is traversed by a current from the local battery, and so also is the sender, which is elsewhere connected with the receiver by a commutator. The secondary circuit of these coils is completed through the earth and line wire. From this arrangement it follows that the induced current which influences the receiver in correspondence, only produces its effect after a second induction, produced on the primary wire of the local coil, and it appears that this is a sufficient effect; but the advantage of this arrangement is, that it is possible to receive and transmit sounds without the aid of anything but the commutator.

Among other arrangements which have been suggested, we may mention one in which, instead of the bar magnet, a horseshoe magnet is used, with a vibrating plate placed between its poles. For this purpose the poles are tipped with iron, and one of them is pierced with a hole which corresponds to the mouthpiece of the instrument. The two branches of the magnet are also furnished with magnetising helices. When anything is spoken before the hole, the vibrating plate causes induced currents in the two helices: these currents would be of opposite direction if the poles were of like nature, but, since the magnetic poles are of contrary nature, they are in the same direction. The vibrating plate then acts like the two plates of M. Trouvé’s instrument, which we have described above.

In another arrangement, lately made by Ader, the receiver is only an ordinary two-branched magnet, of which the armature is supported, at about two millimètres from its poles, by a glass plate to which it is glued, and the plate itself is fastened to two rigid supports. In order to hear it is only necessary to apply the ear to the plate. The sender is a moveable rod of iron or carbon, which rests on a fixed piece of carbon, with no pressure except its own weight, and it supports a concave disk, to which the speaker applies his mouth. These two parts are so arranged as to move horizontally, so that, when the instrument is suspended, the circuit is forcibly disconnected by the fact of its position, and is therefore closed until anyone takes it up to speak. Speech is well reproduced by this system, and may be transmitted to some distance if it is made on a larger scale.

Again, an anonymous inventor, in a little note inserted in ‘Les Mondes,’ February 7, 1878, writes as follows: ‘Since the intensity of the currents produced in the telephone is in proportion to the mass of soft iron which vibrates before the pole of the magnet, and since, on the other hand, the plate is sensitive in proportion to its tenuity, I employ, instead of the ordinary plate, one reduced by nitric acid to the least possible thickness, and I fix it to a circle of soft iron, which keeps it stretched and forms part of the same substance. This circle is placed in a circular opening made inside the compartment. The intensity of a telephone is much increased when such a system replaces the ordinary plate, even at one end of the line.’

In order to obtain vibrating plates of extreme tenuity, M. E. Duchemin thought of employing very thin plates of mica, sprinkled with pulverised iron fixed to the plate by a layer of silicate of potash. The inventor asserts that it is possible to correspond in a low voice with the aid of this system; but it has this inconvenience, that the plate will be broken by speaking too loud.

Professor Jorgenson, of Copenhagen, has also made a Bell telephone which produces very intense sounds, and which has permitted him to observe some curious effects. In this instrument, the magnet is made in a mode analogous to Nicklès’ tubular magnets. There is first a cylindrical magnet with a core of soft iron at its upper end, to which the coil is fitted; next, a magnetised tube, formed of a steel ring, which encloses the first magnetic system, and is connected with it by an iron tube. Finally, above the polar extremities of this system, there is the vibrating disk, with the same arrangement as that of ordinary telephones, and of which the superficies is large. If this plate is only a millimètre in thickness, the words spoken can be heard throughout a room; but the sounds lose their clearness when the ear is approached to the vibrating plate, the words are confused, and there is the reverberation which is observed on speaking in a place apt to produce echoes: the listener is, in fact, stunned by the sounds produced. On using a thicker plate--one, for example, of three or four millimètres--the telephone only produces the effect of the ordinary instruments, and it is necessary to apply the ear to it.

M. Marin Maillet, of Lyons, has suggested that the sounds reproduced by the telephone might be increased by reflecting them through a certain number of reflectors, which, by concentrating them in a focus on a resonator, would considerably enlarge them. Since this idea was not accompanied by experiments, it can hardly be regarded as serious.

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. Edison[9] 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.