The applications of the telephone are much more numerous than might be supposed at the first glance. As far as the telegraphic service is concerned, its use must evidently be rather limited, since it cannot register the messages sent, and the speed of transmission is inferior to that of the improved system of telegraphs; yet in many cases it would be very valuable, even for a telegraphic system, since it is possible to work it without any special telegraphic training. The first comer may send and receive with the telephone, and this is certainly not the case even with the simplest forms of telegraphic instruments. This system is therefore already in use in public offices and factories, for communication in mines, for submarine works, for the navy, especially when several vessels manœuvre in the same waters, some towed by others; finally, for military purposes, either to transmit orders to different corps, or to communicate with schools of artillery and rifle practice. In America the municipal telegraphic service and that of telegraphs limited to the area of towns are conducted in this way, and it is probable that this system will soon be adopted in Europe. Indeed, a service of this kind was established in Germany last autumn at the telegraph offices of some towns, and the London Post Office is now thinking of establishing it in England.

But, besides its use for the purposes of correspondence, the telephone can be useful to the telegraphic service itself by affording one of the simplest means of obtaining a number of simultaneous transmissions through the same wire, and even of being combined in duplex with the Morse telegraphs. Its applications in the microphonic form are incalculable, and the proverb which declares that ‘walls have ears’ may in this way be literally true. It is alarming to think of the consequences of such an indiscreet organ. Diplomatists must certainly redouble their reserve, and tender confidences will no longer be made with the same frankness. On this point we cannot think that much will be gained, but on the other hand the physician will probably soon make use of this invention to ascertain more readily the processes going on within the human body.

APPLICATION OF THE TELEPHONE TO SIMULTANEOUS TELEGRAPHIC TRANSMISSIONS.

The simultaneous transmission of several messages through the same wire is one of the most curious and important applications of the telephone to telegraphic instruments which can be made, and we have seen that it was this application which led Messrs. Gray and Bell to the invention of speaking telephones. The admiration which these instruments have excited has thrown the original idea into the background, although it has perhaps a more practical importance. We will now consider these systems.

An articulating telephone is not necessary in order to obtain simultaneous transmission: the musical telephones devised by MM. Petrina, Gray, Froment, &c., are quite sufficient, and a brief explanation of their principle will make this intelligible. Suppose that there are seven electro-magnetic vibrators at the two corresponding stations, which are tuned with the same tuning-fork on the different notes of the scale, and suppose that a key-board, resembling the Morse telegraph key, is arranged so that, by lowering the keys, electric reaction takes place on each vibrator: it is easy to see that these vibrators may be made to react in the same way on the corresponding vibrators of the opposite station; but they must be tuned on the same note, and the sounds emitted will continue while the keys are lowered. By keeping them down for a shorter or longer time, the long or short sounds which constitute the elements of telegraphic language in the Morse system may be obtained, and consequently an audible transmission becomes possible. Let us now suppose that a telegraphist accustomed to this mode of transmission is placed before each of the vibrators, and that they transmit different messages at the same moment in this way: the telegraphic wire will be instantaneously traversed by seven currents, broken and massed upon each other, and they might be expected to produce a medley of confused sounds on the vibrators at the receiving station; but since they each harmonise with the corresponding vibrator, they have no sensible influence except on those for which they are intended. The dominant sound may be made still more distinct by applying a Helmholtz resonator to each vibrator,18 that is, an acoustic instrument which will only vibrate under the influence of the note to which it is tuned. In this way it is possible to select the transmitted sounds, and only to allow each employé to hear that which is intended for him. Consequently, however confused the sounds may be on the receiving vibrators, the person to whom do is assigned will only receive do sounds, the person to whom sol is assigned will only receive sol sounds, so that correspondence may be carried on as well as if they had each a special wire.

In the mode we have described, this telegraphic system only admits of audible transmissions, and consequently cannot register messages. To supply this defect, it has been suggested to make the receiving vibrators react on registers, so arranging the latter that their electric organ may present such magnetic inertia, that, when it is influenced by the vibrations of sound, its effect may be maintained throughout the time of vibration. Experiments show that a Morse receiver, worked by the current of a local battery, will be enough for this purpose; so that if the musical vibrator is made to react as a relay, that is, on a contact in connection with the local battery and the receiver, the dots and dashes may be obtained on it which are the constituent elements of the Morse code.

On these principles, and considering that the musical spaces separating the different notes of the scale are such as may be easily distinguished by the resonator, seven simultaneous transmissions may be obtained on the same wire; but experience shows that it is necessary to be content with a much smaller number. Yet this number may easily be doubled by applying the mode of transmission in an opposite direction to the system.

Mr. Bell states that the idea of applying the telephone to multiple electric transmissions occurred simultaneously to M. Paul Lacour of Copenhagen, to Mr. Elisha Gray of Chicago, to Mr. Varley of London, and to Mr. Edison of New York; but there is some confusion here, for we have already seen, from reference to the patents, that Mr. Varley’s system dates from 1870, that of M. Paul Lacour from September 1874, that of Mr. Elisha Gray from February 1875, and those of Messrs. Bell and Edison were still later. Yet it appears from Mr. Gray’s specification that he was the first to conceive and execute instruments of the kind. In fact, in a specification drawn up on August 6, 1874, he distinctly put forward the system we have described, and which is the basis of those of which we have still to speak. This specification was only an addition to two others made out in April and June 1874. Mr. Varley’s system has only an indirect relation to the one we have described. It appears from what Mr. Bell said on the subject in a paper addressed to the Society of Telegraphic Engineers in London, that he himself only attaches a secondary interest to this invention.

He said that he had been struck with the idea that the greater or less duration of a musical sound might represent the dot and dash of the telegraphic alphabet, and it occurred to him that simultaneous telegraphic transmissions, of which the number should only be limited by the delicacy of the sense of hearing, might be obtained by suitable combinations of long and short sounds, and that these should be effected by a keyboard of tuning-forks applied to one end of a telegraphic line, and so arranged as to react electrically on electro-magnetic instruments striking on the strings of a piano. For this purpose it would be necessary to assign an employé to each of the keys for the service of transmission, and to arrange that his correspondent should only distinguish his peculiar note among all those transmitted. It was this idea, Mr. Bell adds, which led to his researches in telephony.

For several years he sought for the best mode of reproducing musical sounds at a distance by means of vibrating rheotomes: the best results were given by a steel plate vibrating between two contacts, of which the vibrations were electrically produced and maintained by an electro-magnet and a local battery. In consequence of its vibration, the two contacts were touched alternately, and the two circuits were alternately broken; the local circuit which kept the plate in vibration, and the other which was connected with the line, and reacted on the distant receiver, so as to effect simultaneous vibrations in it. A Morse key was placed in the latter circuit near the sending instrument, and when it was lowered, vibrations were sent through the line; when it was raised, these vibrations ceased, and it is easy to see that, by lowering the key for a longer or shorter time, the short and long sounds necessary for the different combinations of telegraphic language could be obtained. Moreover, if the vibrating plate of the receiving instrument were so regulated as to vibrate in unison with the sending instrument in correspondence, it would vibrate better with this sender than with another whose plate was not so adjusted.

It is evident that different sounds might be simultaneously transmitted with several plates by this arrangement of contact breaker, and that at the receiving station the sounds might be distinguished by each employé, since the one which corresponds to the fundamental note of each vibrating plate is reproduced by that plate. Consequently, the sounds produced by the vibrating plate of do, for example, will only be audible at the receiving station on the plate tuned to do, and the same will be the case with the other plates; so that the sounds will reach their destination, if not without confusion, yet with sufficient clearness to be distinguished by the employés.

Mr. Bell sums up the defects still existing in his system as follows:—1st. The receiver of the messages must have a good musical ear, in order to distinguish the value of sounds. 2nd. Since the signals can only take place when the transmitted currents are in the same direction, two wires must be employed in order to exchange messages on each side.

He surmounted the first difficulty by providing the receiver with an instrument which he called the vibrating contact breaker, and which registered automatically the sounds produced. This contact breaker was placed in the circuit of a local battery, which could work a Morse instrument under certain conditions. When the sounds emitted by the instrument did not correspond with those for which it had been tuned, the contact breaker had no effect on the telegraphic instrument: it only acted when the sounds were those which were to be interpreted, and its action necessarily corresponded to the length of the sounds.

Mr. Bell adds that he applied the system to electro-chemical telegraphs; but we need not dwell on this part of the invention, since, as we have said, it is no longer his special study.

System of M. Lacour of Copenhagen.—M. Lacour’s system was patented on the 2nd September, 1874, but his experiments were commenced on the 5th June of the same year. Since M. Lacour believed that the vibrations would be imperceptible on long lines, his first attempts were made on a somewhat short line; but in November 1874 fresh experiments were made between Fredericia and Copenhagen on a line 225 miles in length, and it was ascertained that vibratory effects could be easily transmitted, even under the influence of a rather weak battery.

In M. Lacour’s system, the sending instrument is a simple tuning-fork, placed in a horizontal position, and one of its arms reacts on a contact breaker, which can produce precisely the same number of discharges of currents as there are vibrations of the tuning-fork. If a Morse manipulator is inserted in the circuit, it is evident that if it is worked so as to produce the dots and dashes of the Morse alphabet, the same signals will be reproduced at the opposite station, and the signals will be manifested by long and short sounds, if an electro-magnetic receiver is connected with the circuit. This sender is shown fig. 59.

Fig. 60 represents M. Lacour’s receiver. It consists of a tuning-fork F made of soft iron, not of steel like the sending tuning-fork, and each of its branches is inserted in the bobbin of an electro-magnetic coil C C; two distinct electro-magnets M M react close to the extremities of the fork, in such a way that the polarities developed on the two branches of the fork under the influence of the coils C C should be of contrary signs to those of the electro-magnets M M.

If this double electro-magnetic system is inserted in a line circuit, it follows that, for each discharge of the transmitted current, a corresponding attraction of the branches of the tuning-fork will take place, and consequently there will be a vibration, producing a sound, if the discharges are numerous. This sound will naturally be short or long in proportion to the duration of the sender’s action, and it will be the same as that of the tuning-fork in that instrument. Again, if one branch of the tuning-fork reacts on a contact P inserted in the circuit of the local battery communicating with a Morse receiver, traces will be produced on this receiver of length varying with the duration of the sounds, for the Morse electro-magnet will be so quickly affected by the successive breaks in the current that its armature will remain stationary throughout each vibration. ‘I have not yet been able,’ said M. Lacour in an address delivered before the Danish Academy of Science in 1875, ‘to calculate the time necessary for the production of definite vibrations in the tuning-fork. Different factors have to be considered, but experiment has shown that the time which elapses before the local circuit is broken is such a small fraction of a second as to be almost inappreciable, even when the current is very weak.

‘Since intermittent currents only affect a tuning-fork on condition that it vibrates in unison with the one which produces them, it follows that if a series of sending tuning-forks, tuned to the different notes of the scale, is placed at one end of a circuit, and if a similar series of electro-magnetic tuning-forks, in exact accordance with the first, is placed at the other end of the circuit, the intermittent currents transmitted by the sending tuning-forks will be added to each other without becoming confused, and each of the receiving tuning-forks will only be affected by the currents emitted by the tuning-fork in unison with it. In this way the combinations of elementary signals representing a word may be telegraphed simultaneously.’

M. Lacour enumerates the ways in which this system may be applied as follows: ‘If the keys in connection with the sending tuning-forks are placed side by side, and are lowered in succession, or two or three together, it will be enough to play on the keys as on a musical instrument, in order that the air may be heard at the receiving station, or the signals transmitted simultaneously may each belong to a different message. This system will therefore allow the furthest station on a line to communicate with one or several intermediate stations, and vice versa, without disturbing the communication at other stations. In this way two stations can exchange signals, unperceived by the rest. The power of sending many signals at once affords a good means of improving the autographic telegraph. In the instruments now in use, such as those of Caselli and D’Arlincourt, there is only one tracing stylus, and this stylus must pass over the whole surface of the telegram in order to obtain a copy of it, but with the telephone a certain number of styli may be placed side by side in the form of a comb, and this comb need only be drawn in a certain direction to pass over the surface of the telegram. In this way a more faithful copy will be obtained in a shorter time.’

M. Lacour also observes that his system possesses a merit already pointed out by Mr. Varley, namely, that the instruments permit the passage of ordinary currents without revealing their presence, whence it follows that the accidental currents which often disturb telegraphic transmissions will have no effect on these systems.

M. Lacour began without applying an electro-magnetic system to his instrument in order to maintain the movement of the tuning-fork, but he soon saw that this accessory was indispensable, and he made the tuning-forks themselves electro-magnetic. It also occurred to him to convert the transmitted currents into pulsatory currents by inserting an induction coil in the circuit, which was also done by Mr. Elisha Gray. Finally, in order to obtain the immediate action of the tuning-forks and the immediate cessation of their action, he constructed them so as to reduce their inertia as much as possible. This was effected by inserting the two branches of the tuning-fork in the same coil and by lengthening its handle, and turning it back so that it might pass through a second coil, dividing into two branches and embracing the two vibrating branches, but without touching them. When a current traverses both coils, it produces, in the kind of horseshoe magnet formed by the two systems, opposite polarities which provoke a double reaction in the vibrating branches—a reaction by repulsion exerted by the two branches in virtue of the same polarity, and a reaction by attraction by the other two branches in virtue of their opposite polarities; and this double action is repeated by the movements of a contact breaker applied to one of the vibrating branches of the tuning-fork.

Mr. Elisha Gray’s System.—According to the system originally patented, each sender, represented fig. 61, consists of an electro-magnet M M resting below a small copper tablet B S, in such a way that its poles pass through this tablet and are on a level with its upper surface. A steel plate A S is fixed above these poles; its tension can be regulated by means of a screw S; and another screw c is placed on the plate, and is in electric communication with a local battery R′ by means of a Morse key. Below the plate A S there is a contact d connected with the line wire L; this contact is met by the plate at the moment of its attraction by the electro-magnet, and breaks the current of a line battery P, which acts on the receiver of the opposite station. Finally, the electric communication established between the local battery R′ and the electro-magnet, as may be seen in the figure, produces vibrations in the steel plate A S at each lowering of the key, as in the case of ordinary vibrations—vibrations which, with a suitable tension of the plate and a given intensity of the battery R′, can produce a definite musical note. Moreover, since at each vibration the plate A S meets the contact, discharges of the line current take place through the line L, and react on the receiving instrument, causing it to reproduce exactly the same vibrations as those of the sending instrument.

The receiving instrument represented fig. 62 exactly resembles the one we have just described, except that there is no contact d below the vibrating plate A S, and the contact c, instead of communicating with the line wire, is in electric connection with a register E and a local battery P. It follows from this arrangement that when the plate A S vibrates under the influence of the broken currents passing through the electro-magnet M M, similar vibrations are sent through the register; but if the electro-magnetic organ of this register is properly regulated, these vibrations can only produce the effect of a continuous current, and hence the length of the traces left on the instrument will vary with the duration of the sounds produced. In this way the registration of the dashes and dots which constitute the signs of the Morse vocabulary will be effected.

If it is remembered that the plate A S vibrates under the influence of electro-magnetic attractions more readily in proportion to their approximation in number to the vibrations corresponding to the fundamental sound it can emit, it becomes clear that if this plate is tuned to the same note as that of the corresponding instrument, it will be rendered peculiarly sensitive to the vibrations transmitted by the sender, and the other vibrations which may affect it will only act faintly. Moreover, a resonator placed above the plate will greatly increase this predisposition; so that if several systems of this kind, tuned to different notes, produce simultaneous transmissions, the sounds corresponding to the different vibrations will be in a certain sense selected and distributed, in spite of their combination, into the receivers for which they are specially adapted, and each of them may retain the traces of the sounds emitted by adding the register, which may be so arranged as to act as an ordinary Morse receiver. Mr. Gray states that the number of sending instruments and independent local circuits may be equal to that of the tones and semitones of two or more octaves, provided that each vibrating plate be tuned to a different note of the scale. The instruments may be placed side by side, and their respective local keys, arranged like the keys of a piano, will make it easy to play an air combining notes and chords; there may also be an interval between the instruments, which may be sufficiently far from each other to allow the employés to work without being distracted by sounds not intended for them.

In a new arrangement, exhibited at the Paris Exhibition, 1878, Mr. Gray considerably modified the way of working the various electro-magnetic organs which we have just described. In this case, the plates consist of tuning-forks with one branch kept in continual vibration at both stations, and the signals only become perceptible by intensifying the sounds produced. This arrangement follows from the necessity of keeping the line circuit always closed for multiple transmissions of this nature, so as to react with pulsatory currents, which are alone able, as we have already seen, to retain the individual character of several sounds simultaneously transmitted.

Under these conditions, the sender consists, as we see (fig. 63), of a bar tuning-fork, a, which is grooved for the passage of a runner, heavy enough to tune the fork to the desired note, and it oscillates between two electro-magnets e and f and two contacts I and G. The difference of resistance in the electro-magnets is very great: in the one f the resistance is equal to 2¾ miles of telegraphic wire, in the other it does not exceed 440 yards. When electric communication is established as we see in the figure, the following effect takes place. Since the current of the local battery through the two electro-magnets is broken by the rest-contact of the Morse key H, the plate a is subject to two contrary actions; but since the electro-magnet f has more turns than the electro-magnet e, its action is preponderant, and the plate is attracted towards f, and produces a contact with the spring G, which opens a way of less resistance for the current. Since the current then passes almost wholly through G, b, 1, 2, B, the electro-magnet is now able to act; the plate a is then attracted towards e, and, by producing a contact on the spring I, it sends the current of the line B P through the telegraphic line, if the key H is at the same time lowered on the sending contact: if not, there will be no effect in this direction, but since the plate a has left the spring G, the first effect of attraction by the electro-magnet f will be repeated, and this tends to draw the plate again towards f. This state of things is repeated indefinitely so as to maintain the vibration of the plate, and to send out signals corresponding with these vibrations whenever the key H is lowered. The elastic nature of the plate makes these vibrations more easy, and it ought also to be put in mechanical vibration at the outset.

The receiver, represented fig. 64, consists of an electro-magnet M, mounted on a sounding-box C, and having an armature formed by a tuning-fork L L firmly buttressed on the box by a cross bar T. There is a runner P on the armature, sliding in a groove, which makes it possible to tune the vibrations of the tuning-fork to the fundamental note of the sounding-box C, which is so arranged as to vibrate in unison with it. Under these conditions, the box as well as the tuning-fork will act as an analyser of the vibrations transmitted by the currents, and may set the register at work by itself reacting on a breaker of the local current. To obtain this result, a membrane of gold-beater’s skin or parchment must be stretched before the opening of the box, and a platinum contact must be applied to it, so arranged as to meet a metallic spring connected with any kind of register or a Morse instrument, when the membrane vibrates. As, however, in America the messages are generally received by sound, this addition to the system is not in use.

The instrument is not only regulated by the runner P, but also by a regulating screw V which allows the electro-magnet M to be properly adjusted. The regulating system is made more exact by the small screw V, and the instrument is connected with the line by the binding screw B. Of course this double arrangement is necessary for each of the sending systems.

As I have already said, seven different messages might theoretically be sent at once in this way, but Mr. Gray has only adapted his instrument for four; he has, however, made use of the duplex system, which allows him to double the number of transmissions, so that eight messages may be sent at the same time, four in one direction, and four in another.

Mr. Hoskins asserts that this system has been worked with complete success on the lines of the Western Union Telegraph Company, from Boston to New York, and from Chicago to Milwaukee. Since these experiments were made, fresh improvements have rendered it possible to send a much larger number of messages.

Mr. Gray has also, aided by Mr. Hoskins, devised a system by which telephonic messages may be sent on a wire previously used for Morse instruments. Mr. Varley had already solved this problem, but Mr. Gray’s system seems to have produced important results, and has therefore a claim to our attention. We do not, however, describe it here, since it is not within the lines marked out for us, and those who are interested in the subject will find all the necessary details in a paper inserted in the ‘Journal of the Society of Telegraphic Engineers, London,’ vol. vi.

Mr. Varley’s System.—This system is evidently the earliest in date, since it was patented in 1870, and the patent describes the principle of most of the arrangements which have since been adopted by Messrs. Lacour, Gray, and Bell. It is based upon the use of his own musical telephone, which we have already described, but with some variations in its arrangement, which make it somewhat like the Reiss system.

It was Mr. Varley’s aim to make his telephone work in conjunction with instruments with ordinary currents, by the addition of rapid electric waves, incapable of making any practical change in the mechanical or chemical capacity of the currents which serve for the ordinary signals, yet able to make distinct signals, perceptible to the ear and even to the eye. He says: ‘An electro-magnet offers at first a great resistance to the passage of an electric current, and may consequently be regarded as a partially opaque body with respect to the transmission of very rapid inverse currents or of electric waves. Therefore, if a tuning-fork, or an instrument with a vibrating plate, tuned to a given note, be placed at the sending station, and so arranged as to be kept in constant vibration by magnetic influence, the current which acts upon it must be passed into two helices placed one above the other so as to constitute the primary helix of an induction coil: in this way it will be possible to obtain in two distinct circuits two series of rapidly broken currents, which will correspond to the two directions of the vibrations of the tuning-fork, and we shall also have the induced currents produced in the secondary helix by these currents, which may act on a third circuit. This third circuit may be placed in connection with a telegraphic line previously used by an ordinary telegraphic system, if a condenser is applied to it, and in this way two different transmissions may be obtained simultaneously.’

Fig. 65 represents the arrangement of this system. D is the vibrating plate of the tuning-fork designed to produce the electric contacts necessary to maintain it in motion. These contacts are at S and S′, and the electro-magnets which affect it are at M and M′. The induction coil is at I′, and the three helices of which it is composed are indicated by the circular lines which surround it. There is a Morse manipulator at A, another at A′, and the two batteries which work the system are at P and P′. The condenser is at C, and the telephone is at the end of the line L.

When the vibration of the plate D tends to the right, and the electric contact takes place at S′, the current of the battery P′, after traversing the primary helix, reaches the electro-magnets M M′, which give it an impulse in the contrary direction. When, on the other hand, it tends to the left, the current is sent through the second primary circuit, which will be balanced by the first. Consequently there will be a series of reversed currents in the induced circuit corresponding to the key A′, which will alternately charge and discharge the condenser C, thus sending into the line a corresponding series of electric undulations which will react on the telephone placed at the end of the line; and as the duration of the transmitted currents will vary with the time that the key A′ is lowered, a correspondence in the Morse code may be obtained in the telephone, while another correspondence is exchanged with the key A and the ordinary Morse receivers.

In order to render the vibratory signals visible, Mr. Varley proposes to use a fine steel wire, stretched through a helix and facing a narrow slit, to reproduce the vibrations. A light, which is intercepted by the wire, is placed behind the slit. As soon as a current passes, the wire vibrates and the light appears. A lens is placed so as to magnify the image of the luminous slit, and project it on a white screen while the wire is in vibration.