Part 7
_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.
[Illustration: FIG. 35.]
_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.
[Illustration: FIG. 36.]
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.
[Illustration: FIG. 37.]
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.
[Illustration: FIG. 38.]
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.