The Edison Telephone.—One of the earliest and most interesting improvements made in the Bell telephone is that introduced by Mr. Edison in the early part of the year 1876. This system is indeed more complicated than the one we have just considered, since it requires a battery, and the sending instrument differs from the receiving instrument; but it is less apt to be affected by external causes, and transmits sound to a greater distance.
The Edison telephone, like Mr. Gray’s, which we have already had occasion to mention, is based upon the action of undulatory currents, determined by the variations in the resistance of a conductor of moderate conducting power, which is inserted in the circuit, and the vibrations of a diaphragm before which the speaker stands react upon it. Only, instead of employing a liquid conductor, which is practically useless, Mr. Edison has attempted to use semi-conducting solid bodies. Those which were most suitable from this point of view were graphite and carbon, especially the carbon extracted from compressed lamp-black. When these substances are introduced into a circuit between two conducting plates, one of which is moveable, they are capable of modifying the resistance of the circuit almost in the same proportion as the pressure exerted upon them by the moveable plate,5 and it was seen that, in order to obtain the undulatory currents necessary for the production of articulate sounds, it was enough to introduce a disk of plumbago or of lamp-black between the vibrating plate of a telephone and a platinum plate placed in connection with the battery. When the telephone disk is placed in circuit, its vibrations before the disk of carbon produce a series of increasing and decreasing pressures, thus causing corresponding effects in the intensity of the transmitted current, and these effects react in an analogous manner on the undulatory currents determined by induction in the Bell system. In order to obtain good results, however, several accessory arrangements were necessary, and we represent in fig. 27 one of the arrangements made in this part of Mr. Edison’s telephonic system.
In this figure a section of the instrument is given, and its form greatly resembles that of Bell. L L is the vibrating disk; O′ O, the mouthpiece; M, the opening to the mouthpiece; N N N, the case for the instrument, which is, like the mouthpiece, made of ebonite, and below the disk it presents a rather large cavity, and a tubular hole which is scooped in the handle. In its upper part this tube terminates in a cylindrical rim, furnished with a worm on which is screwed a little rod with a ridge on its inner side, and the rheostatic system is placed within this tube. The system consists, first, of a piston E, fitted to the end of a long screw E F, and the turning of the button will move the piston up or down within a certain limit. Above this piston there is fitted a very thin platinum plate A, connected by a flexible chain and a wire with a binding-screw P′. Another plate B, exactly similar, is connected with the binding-screw P, and the carbon disk C is placed between these two plates. This disk is composed of compressed lamp-black and petroleum, and its resistance is one ohm, or 110 yards, of telegraphic wire. Finally, an ebonite disk is fastened to the upper platinum plate, and an elastic pad, composed of a piece of caoutchouc tube G, and of a cork disk H, is interposed between the vibrating plate L L and the disk B, in order that the vibrations of the plate may not be checked by the rigid obstacle formed by the whole rheostatic system. When these different parts are in position, the instrument is regulated by the screw F, and this is easily done by screwing or unscrewing it until the receiving telephone gives out its maximum of sound.
In another model, represented in fig. 28, which has produced the best results in the distinctness with which sounds are transmitted, the vibrating plate L L is supported on the disks of the secondary carbon conductor C by means of a little iron cylinder A, instead of the caoutchouc pad, and the pressure is regulated by a screw placed below e. The mouthpiece E of the instrument is more prominent, and its opening is larger. Finally, the instrument, which is cased in nickel silver, is without a handle. The rigid disk b, resting on the first platinum plate p, is of aluminium instead of ebonite.
The receiving telephone somewhat resembles that of Mr. Bell, yet it presents some differences which can be understood from the examination of fig. 29. The magnet N S is horseshoe in form, and the magnetising coil E only covers one of the poles, N: this pole is precisely in the centre of the vibrating plate L L, while the second pole is near the edge of this plate. The size of the plate itself is considerably reduced: its superficies is about the same as that of a five-franc piece, and it is enclosed in a kind of circular groove, which keeps it in a definite position. In consequence of this arrangement the handle of the instrument is of solid wood, and the vacant space for the electro-magnetic system is somewhat larger than in the Bell model; but an arrangement is made for subduing the echo, and there is a kind of sounding-box to magnify the sound. It is evident that the relation which the electro-magnetic system bears to the vibrating disk must increase the sensitiveness of the instrument; for as the pole S is in close contact with the disk L L, the latter is polarised, and becomes more susceptible to the magnetic influence of the second pole N, which is separated from it by an interval not exceeding the thickness of a sheet of coarse paper. In Mr. Edison’s two instruments, the receiver and sender, the upper part C C, corresponding to the vibrating disk, instead of being fixed by screws to the handle, is screwed on to the handle itself, which makes it much more easy to dismount the instrument.
Mr. Edison has varied the form of his instruments in many ways, and their cases have of late been made of metal with a funnel-shaped mouthpiece of ebonite.
When Mr. Edison had ascertained, as indeed Mr. Elisha Gray had done before him, that induced currents are more favourable to telephonic transmissions than voltaic currents, he transformed the currents from the battery which passed through his sender into induced currents by making them pass through the primary circuit of a carefully insulated induction coil; the line wire was then put into communication with the secondary wire of the coil. We shall afterwards describe some experiments which show the advantages of this combination: for the present we can only point out the fact, for it is now an integral quality of almost all the systems of battery telephones.
Edison’s Chemical Telephone.—The curious and really useful effects produced by Mr. Edison with his electro-motograph prompted, about the beginning of the year 1877, his idea of applying the principle of this instrument to the telephone for the reproduction of transmitted sounds; and he obtained such interesting results that the author of an article on telephones, published in the ‘Telegraphic Journal,’ August 15, 1877, put forward this invention as one of the finest of the nineteenth century. It certainly appears to have given birth to the phonograph, which has lately become famous, and has so much astonished men of science.
To understand the principle of this telephone, we must give some account of Mr. Edison’s electro-motograph, discovered in 1872. This instrument is based upon the principle that if a sheet of paper, prepared with a solution of hydrate of potash, is fastened on a metallic plate which is united to the positive pole of a battery, and if a point of lead or platinum connected with the negative pole is moved about the paper, the friction which this point encounters ceases after the passage of the current, and it is then able to slide as if upon a mirror until the current is interrupted. Now, as this reaction may be effected instantaneously under the influence of extremely weak currents, the mechanical effects produced by these alternations of arrest and motion may, by a suitable arrangement of the instrument, determine vibrations in correspondence with the interruptions of current produced by the transmitter.
In this system the telephonic receiver consists of a resonator and a drum mounted on an axis and turned by a winch. A paper band, wound upon a reel, passes over the drum, of which the surface is rough, and a point tipped with platinum, and fitted to the end of a spring which is fixed in the centre of the resonator, presses strongly on the paper. The current from the battery, first directed on the spring, passes by the platinum point through the chemical paper, and returns by the drum to the battery. On turning the winch, the paper moves forward, and the normal friction which is produced between the paper and the platinum point pushes the point forward, while producing by means of the spring a tension on one side of the resonator; but since the friction ceases at each passage of the current through the paper, the spring is no longer drawn out, and the resonator returns to its normal position. Since this double effect is produced by each vibration made in the sender, a series of vibrations takes place in the resonator, repeating those of the sender, and consequently the musical sounds which affected the sender are reproduced to a certain extent. According to the American journals, the results produced by this instrument are astonishing: the weakest currents, which would have no effect on an electro-magnet, become perfectly efficacious in this way. The instrument can even reproduce with great intensity the highest notes of the human voice, notes which can hardly be distinguished by the use of electro-magnets.
The sender nearly resembles the one we have previously described, except that, when it is used for musical sounds, a platinum point is employed instead of the disk of carbon, and it ought not to be in constant contact with the vibrating plate. According to the ‘Telegraphic Journal,’ it consists simply of a long tube, two inches in diameter, having one end covered with a diaphragm formed of a thin sheet of copper, and kept in its place by an elastic ring. A small platinum disk is riveted to the centre of the copper diaphragm, and a point of the same metal, fitted with a firm support, is adjusted before the disk. When the singer stands before the diaphragm, its vibration causes it to touch the platinum point, and produces the number of breaks in the current which corresponds to the vibration of the notes uttered.
The experiments lately made in America, in order to decide on the merits of various telephonic systems, show that Mr. Edison’s telephone gives the best results. The ‘Telegraphic Journal,’ May 1, 1878, states that on April 2 Mr. Edison’s carbon telephone was tested between New York and Philadelphia on one of the numerous lines of the West Union. The length of the line was 106 miles, and ran parallel to other wires almost throughout its length. The effects of induction caused by telegraphic transmissions through the adjacent wires were enough to make speech inaudible through the other telephones, but they had no influence on Edison’s telephone, which was worked with a battery of two cells and a small induction coil, and Messrs. Batchelor, Phelps, and Edison were able to converse with ease. Mr. Phelps’ magnetic telephone, which is considered to be the most powerful of its kind, did not afford such good results.
In the experiments made between the Paris Exhibition building and Versailles, the jury commission was able to ascertain that the results were equally favourable.
Telephones by Colonel Navez.—Colonel Navez of the Belgian Artillery, inventor of the well-known balistic chronograph, has endeavoured to improve the Edison telephone by employing several disks of carbon instead of one. He considers that the variations of electric resistance produced by carbon disks under the influence of unequal pressure depend chiefly on their surface of contact, and he consequently believes that the more these surfaces are multiplied, the greater the differences in question will be, just as it happens when light is polarised through ice. He adds that these disks act well by their surfaces of contact, since, if they are separated by copper disks, the speech reproduced ceases to be articulate.6
I am not surprised to learn that Colonel Navez has found a limit to the number of carbon disks, for the reproduction of speech in this system is due both to the greatness of the differences of resistance in the circuit, and to the intensity of the transmitted current. If therefore the instrument’s sensitiveness to articulate sounds is increased by increasing the number of imperfect contacts in the circuit, the intensity of the transmitted sounds is diminished, and thus sounds lose their power. There is consequently a limit to be observed in the number of carbon disks placed upon each other; and it depends on the nature of the imperfect contacts which are employed, and on the tension of the electric generator.
In order to stop the unpleasant musical vibrations which accompany telephonic transmissions, Colonel Navez employs for the vibrating plate of the sender a silver-plated copper disk, and for the vibrating plate of the receiver an iron disk lined with brass and soldered together. He also employs caoutchouc tubes with mouthpieces and ear-tubes for the transmission and reception of sound, and these instruments are placed level on a table. For this purpose the magnetised bar of the receiving telephone is replaced by two horizontal magnets, acting through a pole of the same nature on a little iron core which carries the coil, and which is placed vertically between the two magnets. He necessarily makes use of a small Ruhmkorff coil to transform the electricity of the battery into induced electricity.
Figs. 30 and 31 represent the two parts of this telephonic system. The carbon battery is in C (fig. 30), the vibrating disk in L L, and the mouthpiece E, fitted to a caoutchouc tube T E, corresponds at the lower end to the vibrating disk. The carbon battery is placed in metallic contact with the circuit by a platinum rod E C, and the vibrating disk also communicates with the circuit through a binding-screw. In the receiving telephone (fig. 31) the upper part is arranged much as in the ordinary telephones, except that, instead of a mouthpiece, the instrument is fitted with an ear-tube T O. The two horseshoe magnets, A, A, which communicate a uniform polarity to the iron core N, support the induction coil B. The two terminals of this receiver are connected with the supplementary wire of the induction coil, and the two terminals of the sender are connected with the two ends of the primary of this coil, and with the battery which is inserted in the circuit near this instrument.
The Pollard and Garnier Telephones.—The battery telephone made by MM. Pollard and Garnier differs from the foregoing in this particular: it simply employs two points of graphite, mounted in metallic porte-crayons, and these points are directly applied against the vibrating plate with a pressure which must be regulated. Fig. 32 represents the arrangement adopted, which, however, may be infinitely varied.
L L is the vibrating tin plate, above which is the mouthpiece E, and P, P′ are the two graphite points with their porte-crayons. There is a screw on the lower part of the porte-crayons which is fixed in a hole pierced in a metallic plate C C, and by this means the pressure of the pencils against the disk L L can be regulated. The metallic plate C C is made in two pieces, placed side by side, but insulated from each other, so that they may be placed in communication with a cylindrical commutator, and by its means the circuit can be arranged in different ways. Since the commutator consists of five sheets, the transition from one combination to another is instantaneous, and these combinations are as follows:
1. The current enters by the pencil P, passes into the plate, and so to line.
2. The current enters by the pencil P′, passes into the plate, and so to line.
3. The current comes simultaneously by the two pencils P and P′, goes into the plate, and thence to line.
4. The current comes by the pencil P, goes thence to the plate, then into the pencil P′, and so to line.
By this means there are two elements of combination, which may be employed separately, or by coupling them for tension or quantity.
When the pencils are properly regulated and give a regular transmission of equal intensity, the effects produced in the transition from one combination to another may be easily studied, and it has been ascertained: first, that in a short circuit there is no appreciable change, whatever be the combination employed; secondly, that when the circuit is long, or of great resistance, the tension arrangement is the best, and this in proportion to the length of the line.
This telephonic system, like the two preceding ones, requires an inducing machine to transform voltaic into induced currents: we shall presently speak of this important accessory of these instruments.
Besides this arrangement, MM. Pollard and Garnier have employed the one we have represented in fig. 5, which has given better results. We shall see presently that it can be used as the receiving organ of sounds. In each case the two carbons must be placed in contact, and subjected to a certain initial pressure, which should be regulated by the screw fitted to the support of the lower carbon.
As for the receiving telephone, the arrangement adopted by MM. Pollard and Garnier is the same as Bell’s, except that they employ tin plates and helices of greater resistance. This resistance ranges in fact from 100 to 125 miles. ‘We have always held,’ these gentlemen say, ‘that whatever may be the resistance of the outer circuit, there is an advantage in increasing the number of spirals, even when using wire No. 42, which is the one we prefer.’
M. Hellesen’s Reaction Telephone.—M. Hellesen believed that the vibrations produced by the voice on the carbon of a telephonic sender would be magnified if the moveable part of the rheotome were subjected to an electro-magnetic action resulting from the vibrations themselves, and he has contrived a sender, which is based on the principle shown in fig. 33, and which has the merit of forming in itself the inducing apparatus intended to transform the voltaic currents employed. This instrument is composed of a vertical iron tube, supported on a magnetic bar N S, and surrounded by a magnetising coil B B, above which is fixed an inducing helix of fine wire I I, communicating with the circuit. Within the tube there is a lead pencil C, held by a porte-crayon which can be raised or lowered by means of a screw V fixed below the magnetic bar. Finally, above this pencil, there is an iron vibrating plate L L, with a platinum point in communication with the battery in its centre; the local circuit communicates with the pencil by means of the magnetising helix B, and for this purpose one end is soldered to the iron tube.
From this arrangement it follows that the vibrations of the plate L L, at the moment when it comes nearest to the pencil, tend to become greater in consequence of the attractive force exerted on the plate, and as the pressure of the lead pencil is increased, it increases the differences of resistance which result from it, and consequently causes greater variations in the intensity of the transmitted currents.
Reaction Telephone of Messrs. Thomson and Houston.—The telephonic arrangement we have described has lately been adopted by Mr. Elihu Thomson and Mr. Edwin J. Houston, who, on June 21, 1878, two months after M. Hellesen explained his system to me,7 published an article in ‘The English Mechanic and World of Science’ about an instrument very similar to that of M. Hellesen.
In their instrument, the current, which passes through a body of moderately conducting capacity, acts on an electro-magnet provided with an induction coil, and this electro-magnet reacts on the diaphragm, in order to increase the range of its vibrations, and to create at the same moment two electric actions in the same direction: the only difference lies in the arrangement of the contact of this indifferent conductor with the vibrating plate. Instead of a simple contact effected by pressure between this plate and a carbon pencil, a fragment of the same substance with a sharpened point is fixed on the vibrating plate, and it dips into a drop of mercury which has been poured into the receptacle made for it at the upper end of the electro-magnet. In other respects, the arrangement of the instrument is that of an ordinary telephone, and the iron rod of the electro-magnet represents the magnetised bar of the Bell telephone. The inventors assert that this instrument can be used both as a sender and receiver, and it is in the following manner that it is worked in each case.
When the instrument is transmitting, the morsel of carbon dips more or less into the mercury, and consequently differences are produced in the surfaces of contact, according to the range of vibrations made by the plate; the current varies in intensity in proportion to this range, and induced currents in the induction coil result from these variations; the induced currents react on the receiving telephone, as in Bell’s instrument, and are further strengthened by those which are produced electrically by the movement of the diaphragm before the induction coil, and the iron of the electro-magnet.
When the instrument is used as a receiver, the usual effects are displayed, for since the iron of the electro-magnet is magnetised by the current, its conditions are precisely those of the ordinary Bell telephone, and the induced currents reach it in the same manner, only with greater intensity. Messrs. Thomson and Houston assert that their system has produced excellent results, and that by it the sound of the voice is much less altered than in other telephones.
Telephones with batteries and liquid senders.—We have seen that in 1867 Mr. Gray conceived the idea of a telephonic system based on the differences of resistance effected in a circuit completed by a liquid, when the layer of liquid interposed between the electrodes varies in thickness under the influence of the vibrations of the telephonic plate which is in communication with one of these electrodes. This system has since been the subject of study by several inventors, among others by MM. Richemond and Salet; and I give some of the accounts which have been published respecting their researches.
Another telephone for the reproduction of articulate sounds, which M. Richemond terms the electro-hydro telephone, has been recently patented in the United States. It resembles that of Mr. Edison in some respects, but instead of making use of carbon disks to modify the resistance of the circuit, water is employed, and this water is placed in communication with the circuit and battery by means of two platinum points, one of which is fixed on the metallic diaphragm which vibrates under the influence of the voice. As the vibrations of the diaphragm transport the point which is attached to it to different parts of the interpolar layer of liquid, they diminish or increase the electric resistance of this layer, and cause corresponding variations in the intensity of the current traversing the circuit. The receiving telephone is of the usual kind. (See ‘Telegraphic Journal,’ September 15, 1877.)
M. Salet writes: ‘I thought it would be interesting to construct a telephone in which there should be absolute solidarity in the movements of the two membranes, and for this purpose I have availed myself of the great resistance of liquids. Mr. Bell had already obtained some results by attaching to the vibrating membrane a platinum wire communicating with a battery, and dipping more or less into a metallic vessel, itself connected by the line with the receiving telephone and containing some acidulated water. I have substituted for the platinum wire a small aluminium lever supporting a disk of platinum, and at a very slight distance from it there is a second disk in connection with the line. The vibrations of the membrane, tripled or quadrupled in their range, are not altered in form, thanks to the small size and light weight of the lever: they cause variations in the thickness of the liquid layer traversed by the current, and consequently in its intensity, and these variations cause corresponding differences in the attractive force of the receiving electro-magnet. Under its influence the receiving membrane executes movements which are identical with those of the sending membrane. The sound transmitted is very distinct, and its timbre is perfectly maintained, a result which might have been anticipated. The consonants, however, are not so clearly pronounced as those transmitted by Mr. Bell’s instrument. This inconvenience is most apparent when the lever is heavy, and might easily be obviated. The electrolysis also produces a continual murmur, but this does not interfere with the distinctness of the sound.
‘Since on this system the voice is not required to produce, but only to direct the electric current generated by a battery, the intensity of the sound received might in theory be increased at pleasure. I have in fact been able to make the receiver emit very powerful sounds, and I think that this advantage greatly counterbalances the necessity of employing a battery, and a somewhat delicate sending instrument. Unfortunately it can only be used for moderate distances. Assuming that any displacement of the transmitting membrane increases the resistance to a degree equivalent to five or six hundred yards of wire: if the line is five hundred yards long, the intensity of the current will be reduced by one half, and the receiving membrane will take up a fresh position, considerably differing from the first; but if the line is three hundred miles in length, the intensity of the current will only be modified by a thousandth part. An immense battery must therefore be employed in order that this variation may be translated by a sensible change in the position of the receiving membrane.’ (See ‘Comptes Rendus de l’Académie des Sciences,’ February 18, 1878.)
M. J. Luvini, in an article inserted in ‘Les Mondes,’ March 7, 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.