Chapter M: J. Luvini, in an article inserted in ‘Les Mondes,’ March 7, (4)

_Its application to Telephonic Relays._--In February 1878, I first began to consider the mode of forming telephonic relays, but I was checked by the discovery that there was no vibration in the receiving telephone, and I made the following communication on the subject to the Académie des Sciences on February 25:--‘If the vibrations of the disk in the receiving telephone were the same as those of the sending telephone, it is easy to see that if a telephone with a local battery, acting both as sender and receiver, were substituted for the receiving telephone, it might, by the intervention of the induction coil, act as a relay, and might therefore not only amplify the sound, but also transmit it to any distance. It is, however, doubtful whether the vibrations of the two corresponding disks are of the same nature, and if the sound be due to molecular contractions and expansions, the solution of the problem becomes much more difficult. Here is therefore a field for experiments.’ These experiments have been successfully made by Mr. Hughes, who acquainted me with them early in June 1878, and they led to the discovery of a most interesting system of microphonic relays.

On a wooden board of moderate size, such as a drawing board, he placed a microphone with a carbon brought to a fine point at each end, and fixed in a vertical position. One or more telephones were placed in the circuit, with their membranes facing the board, and a continuous sound was heard, sometimes resembling a musical note, sometimes the singing of boiling water in an oven; and the sound, which could be heard at a distance, went on indefinitely, as long as the electric force was exerted. Mr. Hughes explains this phenomenon in the following way.

The slightest shock which affects the microphone has the effect of sending currents, more or less broken, through the telephones, which transform them into sound vibrations, and since these are mechanically transmitted by the board to the microphone, they maintain and even amplify its action, and produce fresh vibrations on the telephones. Thus a fresh action is exerted on the microphone, and so on indefinitely. Again, if a second microphone, in connection with another telephonic circuit, be placed upon the same board, we have an instrument which acts as a telephonic relay, that is, it transmits to a distance the sounds communicated to the board, and these sounds may serve either as a call, or as the elements of a message in the Morse code, if a Morse manipulator is placed in the circuit of the first microphone. Mr. Hughes adds that he has made several very successful experiments with this system of instruments, although he only employed a Daniell battery of six cells without any induction coil. By fastening a pasteboard tube, 40 centimètres in length, to the receiving telephone, he was able to hear in all parts of a large room the continuous sound of the relay, the ticking of a watch, and the scratching of a pen upon paper. He did not try to transmit speech, since it would not have been reproduced with sufficient distinctness under such conditions.

Since this first attempt, Mr. Hughes has arranged another and still more curious system of microphonic relays, for which two microphones with vertical carbons are required. He places two microphones of this description on a board, and connects one of them with a third microphone, which acts as a sender, while the second is in communication with a telephone and a second battery: in this way the words uttered before the sender are heard in the telephone, without employing any electro-magnetic organ for the telephonic relay.

In August 1878, Messrs. Houston and Thomson likewise arranged a system of telephonic relays which only differs from that of Mr. Hughes in the particular of having the microphone fixed on the diaphragm of the telephone, and not on the board beside it. The system consists of three vertical microphones, which can be combined for tension or quantity, according to the conditions for which they are required. The model of this instrument was represented in the ‘Telegraphic Journal’ of August 15, 1878, to which we must refer our readers, if they wish for further information on the subject.

_Its application to Medicine and Surgery._--The extreme sensitiveness of the microphone suggested its use for the observation of sounds produced within the human body, so that it might serve as a stethoscope for listening to the action of the lungs and heart. Dr. Richardson and Mr. Hughes are now busy in the attempt to carry out this idea, but so far the result is not very satisfactory, although they still hope to succeed. Meanwhile, M. Ducretet has made a very sensitive stethoscopic microphone, which we represent in fig. 51. It consists of a carbon microphone C P, with a simple contact, of which the lower carbon P is fitted to one of M. Marais’ tambourines with a vibrating membrane T. This tambourine is connected with another T′, by a caoutchouc tube, which is to be applied to the different parts of the body which demand auscultation, and which is therefore termed the _tambour explorateur_. The sensitiveness of the instrument is regulated by means of a counterpoise P O, which is screwed upon the arm of a bent lever, and to this the second carbon C is fixed. The extreme sensitiveness of M. Marais’ tambourines in transmitting vibrations is well known, and since their sensitiveness is further increased by the microphone, the instrument becomes almost too impressionable, since it reveals all sorts of sounds, which it is difficult to distinguish from each other. Such an instrument can only be of use when entrusted to experienced hands, and a special education of the organ of hearing is needful, in order to turn it to account.

In a work lately published by M. Giboux on the application of the microphone to medicine, this stethoscopic system is rather severely criticised, and not without reason if, as M. Giboux asserts, it is only sensitive to the movements which take place on the surface of the body, and those which are internal are either lost or altogether changed in character. But without pronouncing on the improvements which may ultimately be made in the instrument, M. Giboux thinks that its most important use in medical practice consists in its allowing a certain number of students to observe with the professor the different sounds of the body, to study them with him in their different phases, and thus to profit more readily by his teaching. A microphonic circuit might bifurcate between several telephones, so that each person might hear for himself what is heard by others.

The most important application of the instrument to surgical purposes has lately been made by Sir Henry Thompson, aided by Mr. Hughes, for the examination of the bladder in cases of stone. It enables him to ascertain the presence and precise position of calculi, however small they may be. For the purpose of research, he uses a sound, made of a Maillechort rod, a little bent at the end, and placed in communication with a sensitive carbon microphone. When the sound is moved about in the bladder, the rod comes in contact with stony particles, even if they are no larger than a pin’s head, and friction ensues, producing in the telephone vibrations which can be easily distinguished from those caused by the simple friction of the rod on the soft tissues of the sides of the bladder. The arrangement of the instrument is shown in fig. 52. The microphone is placed in the handle which contains the sound, and is the same as that given in fig. 42, but of smaller size, and the two conducting wires _e_ which lead to the telephone, issue from the handle by the end _a_ opposite to that _bb_ to which the sound _dd_ is screwed. As this instrument is not intended to reproduce speech, retort carbons instead of wood carbons may be used.

Some deaf people, whose sense of hearing is not completely destroyed, have been able to hear by an expedient based upon the principle of the microphone. For this purpose two telephones, connected by a metallic crown, which is placed on the temples, are applied to the ears of the deaf person, and the telephones are placed in communication with a battery microphone, which hangs to the end of a double conducting wire. The deaf man keeps the microphone in his pocket, and presents it as an acoustic tube to the person who wishes to converse with him. Mr. Hughes’s speaker, represented fig. 42, is the one used.

_Various Applications._--The microphone may be used in many other ways, some of which are suggested in the ‘English Mechanic’ of June 21, 1878. The article states that by means of this instrument, engineers will be able to estimate the effects of the vibrations caused on old and new buildings by the passage of heavy loads; a soldier will be able to discover the enemy’s approach when he is several miles off, and may even ascertain whether he has to do with artillery or cavalry; the approach of ships to the neighbourhood of torpedoes may be automatically heralded on the coast by this means, so that an explosion may be produced at the right moment.

It has also been proposed to use the microphone to give notice of an escape of gas in coal-mines. The gas, in escaping from between the seams of coal, makes a whistling noise, which might, with the aid of the microphone and telephone, be heard at the top of the shaft. Again, it has been suggested that the microphone might be used as a seismograph to reveal the subterranean noises which generally precede earthquakes and volcanic eruptions, and which would be much intensified by this instrument. It might even be of use to Signor Palmieri for his observations in the Vesuvius Observatory.

The microphone has also been used by Mr. Chandler Roberts to render the diffusion of gaseous molecules through a porous membrane sensible to the ear.

As might have been expected, the acclamation with which Mr. Hughes’s invention was received led to the assertion of other claims to priority, and in addition to that of Mr. Edison, on which we have already given our opinion, there are several others, showing that if some microphonic effects were discovered at different times before the date of Mr. Hughes’s discovery, they could not have been considered important, since they were not even announced. Among the number was that of Mr. Wentworth Lascelles Scott, specified in the ‘Electrician’ of May 25, 1878, and that of M. Weyher, presented to the Société de Physique, Paris, in June 1878. Another, made by M. Dutertre, is of somewhat greater importance, for his experiments were reported in the Rouen papers in February of the same year: yet there is no just ground for such claims, since the earliest date of his experiments is subsequent to the experiments first made by Mr. Hughes. These began early in December 1877, and in January 1878 they were exhibited to officials of the Submarine Telegraph Company, as Mr. Preece declared in a letter addressed to the several scientific men.

EXTERNAL INFLUENCE ON TELEPHONIC TRANSMISSIONS.

The obstacles which occur in telephonic transmissions proceed from three causes: 1. The intensity of sound is diminished by the loss of current in transmission--a loss which is much greater in the case of induced currents than in those received from a battery. 2. Confusion is caused by the influence of adjacent currents. 3. The induction from one wire to another. This last influence is much greater than is usually supposed. If two perfectly insulated wires are placed side by side, one in communication with the circuit of an electric bell, and the other with the circuit of a telephone, the latter will repeat the sounds of the bell with an intensity often great enough to act as a call without applying the instrument to the ear. MM. Pollard and Garnier, in their interesting experiments with the induced currents of the Ruhmkorff coil, have ascertained that in this way not merely sounds may be obtained which correspond with the induced currents resulting from the action of the primary current, but also those which result from the action of the secondary current on other helices, which are termed currents of the second order. These different reactions frequently cause the telephonic transmissions made on telegraphic lines to be disturbed by irregular sounds, arising from the electric transmissions on adjoining lines; but it does not appear that these influences altogether neutralise each other, so that conversation held in the ordinary way and a message sent in the Morse code may be heard simultaneously.

At the Artillery School, Clermont, a telephonic communication has been established, for the sake of experiments, between the school and the butts, which are at a distance of about eight miles. Another communication of the same kind has been established between the Clermont Observatory and the one at Puy-de-Dôme, which is nearly nine miles from the former. These two lines are carried on the same posts for a course of six miles, together with an ordinary telegraphic wire, and for a distance of 330 yards there are seven other such wires. The two telephonic wires are separated from each other by a space of 85 centimètres. The following facts have been observed under these conditions.

1. The school telephone is perfectly able to read off from their sound the Morse messages which pass through the two adjacent telegraph wires, and the ticking of the instrument does not at all interfere with the vocal communication of the telephone, nor render it inaudible.

2. The two adjacent telegraphic lines, although not in contact, confuse their messages together, and it has sometimes been possible to hear messages from Puy-de-Dôme at the school through the wire which runs to the butts, although the distance between the two lines is nowhere less than 85 centimètres.

These inconveniences have been in some degree remedied by inserting strong resistances in the circuit, or by putting the current to earth at some distance from the telephonic stations.

M. Izarn, Professor of Physics at the Lycée, Clermont, holds that telephonic electric currents may readily be turned aside by the earth, especially if in the course of their passage they encounter metallic conductors, such as gas or water pipes. He writes as follows on the subject, in a paper addressed to the Académie des Sciences, on May 13, 1878:--‘I set up a telephone in the Clermont Lycée with a single wire, more than 50 yards in length, which crosses the court-yard of the Lycée, and goes from the laboratory, where it is suspended to a gas-burner, to a room near the porter’s lodge, where it is suspended to another gas-burner. When I applied my ear to the telephone, I could distinctly hear the telegraphic signals, Morse or otherwise, which came either from the telegraph office at Clermont, or from the telephone office which was at work between the School of Artillery and the butts below Puy-de-Dôme, a distance of eight miles. I could overhear words, and especially the military orders issued at the butts for the purpose of being heard at the school. Yet my wire is perfectly independent of those used for signalling, and is even very remote from them; but as the wires of the telegraph office and of the School of Artillery go to earth at a little distance from the gas-pipes, it is probable that this phenomenon is caused by a diversion of the current produced in my wire, by means of the earth and the network of metal pipes.’

Mr. Preece made the same remark in his notice of ‘some physical points connected with the telephone.’ Again, we read in the ‘Telegraphic Journal’ of June 15, 1878, that in a telephonic concert transmitted from Buffalo to New York, the singers at Buffalo were heard in an office placed outside the telegraphic circuit in which the transmission was effected. On enquiry, it was ascertained that the wire through which the telephonic transmission took place, was at one point in its course close to the one which directly transmitted the musical sounds, but the distance between the two wires was not less than ten feet.

When the circuits are altogether metallic, there is much less risk of confusion, and M. Zetzche declares that sounds proceeding from other wires are in this case little heard, and then only momentarily, so that it is much more easy to hear with this arrangement than with the one in ordinary use. ‘It is not,’ he says, ‘the resistances of the wire, but rather the diversions of the current near the posts, which interfere with telephonic correspondence on long lines above ground. This was proved by the following experiments:--I connected the telegraphic line from Dresden to Chemnitz with a line from Chemnitz to Leipzig (54 miles), which made a circuit of 103 miles, going to earth at its two extremities. There was no communication between Dresden and Leipzig, but Leipzig and Dresden could communicate with ease, in spite of the greater extent of line. I broke the connection with earth, first at Leipzig, then simultaneously at Leipzig and Dresden, and I observed the following effects. When insulation took place at Leipzig only, the telephone could be heard at the stations of Dresden, Riesa, and Wurzen; when the line was insulated at both ends, the communication was good between the two latter stations, but it was observed that at the intermediate station the words spoken at Wurzen were more distinctly heard than the words spoken at Riesa were heard at Wurzen. Since the distance from Wurzen to Leipzig is little more than half that from Riesa to Dresden, there are consequently nearly twice as many posts on the latter line, which carry the currents to earth, and hence I conclude that these diversions of current explain the possibility of conversing on an insulated line, and also why sounds are more distinctly heard at the Riesa station in consequence of the greater intensity of current still remaining on the line.’

Some vibrations also result from the action of currents of air on telegraphic wires, which produce the humming sound so well known on some lines, and these may also react on the telephone; but they are in this case generally mechanically transmitted, and they may be distinguished from the others, if the sounds which ensue are heard after the telephone is excluded from the circuit by a break with a short circuit and after the communication to earth established behind the telephone has been broken.

The induced reactions caused by the line wires on each other are not the only ones which may be observed on a telephonic circuit: every manifestation of electricity near a telephone may produce sounds of greater or less force. Of this we have already given a proof in M. d’Arsonval’s experiments, and others by M. Demoget demonstrate the fact still more clearly. In fact, if a small bar magnet provided with a vibrator be placed before one of the telephones of a telephonic circuit, and the vibrating plate of the telephone be removed, in order to draw away the sound produced by the vibrator, its humming noise may be distinctly heard on the second telephone of the circuit; a noise which attains its maximum when the two extremities of the electro-magnet are at their nearest point to the telephone without a diaphragm, and it is at its minimum when this electro-magnet is presented to it along its neutral line. M. Demoget supposes that the action which is exerted in this instance is that of a magnet exerting two inducing actions which are opposite and symmetrical, with a field limited by a double paraboloid and with an axis, according to his experiments, which extended 55 centimètres beyond the magnetic core, and a vertical diameter of 60 centimètres. He believes that in this way it would be easy to telegraph on the Morse system, and that, in order to do so, it would only be necessary to apply a key to the inducing electro-magnet.

Mr. Preece points out three ways of overcoming the difficulty presented by the induced reactions caused by the wires on each other.

1. By increasing the intensity of the transmitted currents, so as to make them decidedly stronger than the induced currents, and to reduce the sensitiveness of the receiving telephone.

2. To place the telephonic wire beyond the range of induction.

3. To neutralise the effects of induction.

The first mode may be effected by Edison’s battery system, and we have seen that it is very successful.

In order to put the second mode in practice, Mr. Preece says that it would be necessary to study the two kinds of induction which are developed on telegraphic lines: electro-static induction, analogous to that produced on submarine cables, and electro-dynamic induction, resulting from electricity in motion. In the former case, Mr. Preece proposes to interpose between the telephone wire and the other wires a conducting body in communication with the earth, capable of becoming a screen to the induction by itself absorbing the electro-static effects. He says that this might be accomplished by surrounding the telegraphic wires adjacent to the telephonic wire with a metallic envelope, and then plunging them in water. He adds that the effects of static induction are not completely destroyed in this way, since the substance used is a bad conductor, but they are considerably reduced, as he has proved by experiments between Dublin, Holyhead, Manchester, and Liverpool. In the second case, Mr. Preece admits that an iron envelope might paralyse the electro-dynamic effects produced by absorbing them, so that if insulated wires were employed, covered with an iron case, and communicating with the earth, the two induced reactions would be annulled. We will not follow Mr. Preece in his theory as to these effects--a theory which seems to us open to question, but we content ourselves with pointing out his proposed mode of attenuation.

In order to carry out the third expedient, it might be thought that it would be enough to employ a return wire instead of going to earth, for under such conditions the currents induced on one of the wires would be neutralised by those resulting from the same induction on the second wire, which would then act in an opposite direction; but this mode would only be successful when there is a very small interval between the two telephone wires, and they are at a considerable distance from the other wires. When this is not the case, and they are all close together, as in submarine or subterranean cables, consisting of several wires, this mode is quite inefficient. A small cable, including two conductors, insulated with gutta-percha, may be successfully carried through the air.

The use of two conductors has the further advantage of avoiding the inconvenience of stray currents on the line and through the earth, which, when the communications to earth are imperfect, permit the line current to pass more or less easily into the telephonic line.

In addition to the disturbing causes in telephonic transmission we have just mentioned, there are others which are also very appreciable, and among them are the accidental currents which are continually produced on telegraphic lines. These currents may proceed from several causes, at one time from atmospheric electricity, at another from terrestrial magnetism, at another from thermo-electric effects produced upon the lines, at another from the hydro-electric reactions produced on the wires and disks in communication with the earth. These currents are always very unstable, and consequently they are likely, by reacting on the transmitted currents, to modify them so as to produce sounds upon the telephone. Mr. Preece asserts that the sound proceeding from earth currents somewhat resembles that of falling water. The discharges of atmospheric electricity, even when the storm is remote, produce a sound which varies with the nature of the discharge. When it is diffused and the clap takes place near at hand, Dr. Channing, of Providence, U.S., says that the sound resembles that produced by a drop of fused metal when it falls into water, or, still more, that of a rocket discharged at a distance: in this case it might seem that the sound would be heard before the appearance of the flash, which clearly shows that the electric discharges of the atmosphere only take place in consequence of an electric disturbance in the air. Mr. Preece adds that a wailing sound is sometimes heard, which has been compared to that of a young bird, and which must proceed from the induced currents which terrestrial magnetism produces in the metallic wires when placed in vibration by currents of air.

M. Gressier, in a communication made to the Académie des Sciences on May 6, 1878, has spoken of some of these sounds, but he is totally mistaken in the source to which he ascribes them.

‘In addition to the crackling sound caused by the working of telegraph instruments on the adjacent lines, a confused murmur takes place in the telephone, a friction so intense that it might sometimes be thought that the vibrating disk was splitting. This murmur is heard more by night than by day, and is sometimes intolerable, since it becomes impossible to understand the telephone, although nothing is going on in the office to disturb the sound. The same noise is heard when only one telephone is used. A good galvanometer inserted in the circuit reveals the presence of sensible currents, sometimes in one direction, sometimes in another.’

I studied these currents for a long time with the galvanometer, and made them the subject of four papers which were laid before the Académie des Sciences in 1872, and I am convinced that they have in general nothing to do with atmospheric electricity, but result either from thermo-electric or hydro-electric influence. They take place constantly and in all weathers on telegraph lines, whether these lines are insulated at one end, or in contact with the earth at both ends. In the first case, the polar electrodes of the couple are formed by the telegraph wire and the earth plate, generally of the same nature, and the intermediate conducting medium is represented by the posts which support the wire and the earth which completes the circuit. In the second case, the couple is formed in almost the same way, but the difference in the chemical composition of the ground at the two points where the earth plates are buried, and sometimes their different temperature, exert a strong influence. If only the first case be considered, it generally happens that on fine summer days the currents produced during the day are inverse to those which are produced by night, and vary with the surrounding temperature in one or the other direction. The presence or absence of the sun, the passage of clouds, the currents of air involve abrupt and strongly marked variations, which may be easily followed on the galvanometer, and which cause more or less distinct sounds in the telephone.

During the day, the currents are directed from the telegraph line to the earth plate, because the heat of the wire is greater than that of the plate, and these currents are then thermo-electric. During the night, on the other hand, the wire is cooled by the dew, which causes a greater oxidation on the wire than that which takes place on the plate, and the currents then become hydro-electric.

I say more about these currents because, in consequence of a mistaken belief as to their origin, it has been supposed that the telephone might serve for the study of the variations of the atmospheric electricity generally diffused through the air. Such an application of the telephone would, under these conditions, be not only useless, but also misleading, by inducing the study of very complex phenomena, which could lead to nothing more than I have already stated in my different papers on the subject.

Certain local influences will also produce sounds in the telephone. Thus the distension of the diaphragm by the moist heat of the breath, when the instrument is held before the mouth in speaking, causes a perceptible murmur.

From the electro-static reactions, so strongly produced on the submarine cables, in consequence of electric transmissions, it might be supposed that it would not be easy to hold telephonic correspondence through this kind of conductor, and, to ascertain the fact, an experiment was made on the cable between Guernsey and Dartmouth, a distance of sixty miles. Articulate speech, only a little indistinct, was, however, perfectly transmitted. Other experiments, made by Messrs. Preece and Wilmot, on an artificial submarine cable, placed in conditions analogous to those of the Atlantic cable, showed that a telephonic correspondence might be kept up at a distance of a hundred miles, although the effects of induction were apparent. At the distance of 150 miles, it was somewhat difficult to hear, and the sounds were very faint, as if some one were speaking through a thick partition. The sound diminished rapidly until the distance of 200 miles was reached, and after that it became perfectly indistinct, although singing could still be heard. It was even possible to hear through the whole length of the cable, that is, for 3,000 miles, but Mr. Preece believed this to be due to the induction of the condenser on itself: he holds, however, that singing may be heard at a much greater distance than speech, owing to the more regular succession of electric waves.

Mr. Preece also made experiments on the subterranean telegraphs between Manchester and Liverpool, a distance of 30 miles, and found no difficulty in exchanging correspondence; and it was the same with the cable from Dublin to Holyhead, a distance of 67 miles. This cable had seven conducting wires, and when the telephone was connected with one of them, the sound was repeated through all the others, but in a fainter degree. When the currents of the telegraphic instruments passed through the wires, the induction was apparent, but not so great as to prevent telephonic communication.

ESTABLISHMENT OF A TELEPHONIC STATION.

Although the telephonic system of telegraphy is very simple, yet certain accessory arrangements are indispensable for its use. Thus, for example, an alarum call is necessary, in order to know when the exchange of correspondence is to take place, and information that the call has been heard is likewise necessary. An electric bell is therefore an indispensable addition to the telephone, and since the same circuit may be employed for both systems, if a commutator is used, it was necessary to find a mode of making the commutator act automatically, so as to maintain the simple action of the system which constitutes its principal merit.

_MM. Pollard and Garnier’s System._--With this object, MM. Pollard and Garnier devised a very successful arrangement last March, which employs the weight of the instrument to act upon the commutator.

For this purpose, they suspended the instrument to the end of a spring plate, fastened between the two contacts of the commutator. The circuit wire corresponds with this plate, and the two contacts correspond, the one with the telephone, the other with the bell. When the telephone hangs below the spring-support, that is, when it is not at work, its weight lowers the spring plate on the lower contact, and the communication of the line with the bell is established: when, on the other hand, the telephone is raised for use, the spring plate touches the higher contact, and communication is established between the line and telephone. In order to make the bell sound, it is only necessary to establish, on the wire which connects the line with the bell contact of the commutator, a breaker which can both join and break the current, and which communicates on one side with the contact of the bell, and on the other with its battery. The ordinary push of an electric bell will be sufficient, if it is supplied with a second contact, but MM. Pollard and Garnier wished to make this action also automatic, and consequently they devised the arrangement represented in fig. 53.

In this system, as well as in those which have since been devised, two telephones are employed, one of which is constantly applied to the ear, and the other to the mouth, so as to make it possible to speak while listening. The telephones are supported by three wires, two of which contain flexible conductors, while the third only acts as a support.

Two of the four wires of the two telephones are connected with each other, and the other two are connected with the two binding screws of the commutator _t_, _t′_: the wires without conductors are suspended to the extremities of the two flexible plates _l_, _l′_, which correspond with earth and line.

When at rest, the weight of the telephones presses the two plates _l_, _l′_, on the lower contacts S, S′, but when the instruments are taken up these plates press against the higher contacts.

The two bell wires terminate on the lower contacts, those of the telephones on the higher contacts, and one of the poles of the battery is connected with the lower contact on the left S′, the other with the higher contact on the right T.

When at rest, the system is applied to the electric bell, and the current sent from the opposite station will follow the circuit L _l_ S S′ S′ _l′_ T, so that the call will be made. On taking up the two telephones, the circuit of the bell system is broken, and that of the telephones is established, so that the current follows the course L _l_ T _t t′_ T′ _l′_ T. If only one telephone is held at a time, the current is sent into the bell system of the opposite station, and follows the route + P S _l_ L T _l′_ T′ _t_ P --. In this way the three actions necessary for calling, corresponding, and enabling the corresponding instrument to give a call, are almost involuntarily made.

_System by MM. Bréguet and Roosevelt._--In the system established by the Paris agents of the Bell company, the arrangement resembles the one just described, except that there is only one spring commutator, and the call is made with the push of an ordinary electric bell. A mahogany board is suspended from the wall, and on it are arranged, first, the ordinary electric bell system, with a sending push fixed below it; second, two forks supporting two telephones, one of which is fastened to the bar of a commutator, arranged as a Morse key. The two telephones are connected by two conducting wires, so arranged as to be capable of extension, and two of their four binding screws are in immediate connection with each other, and the other two with the earth, line, and battery, by means of the commutator, the sending push, and the bell system. The arrangement is shown in fig. 54.

The commutator A consists of a metallic bar _a c_, bearing the suspension fork of one of the telephones F′ below its point of articulation: it ends in two pins _a_ and _c_, below which the two contacts of the commutator are fixed, and a spring compresses the lower arm of the bar, so as to cause the other arm to rest constantly on the higher contact. For greater security a steel tongue _a b_ is fastened to the lower end of the bar, and rubs against the small shaft _b_, which is provided with two insulated contacts, corresponding to those of the board. The bar is in communication with the line wire by means of the call-push, and the upper of the two contacts we have just described corresponds with one of the telephone wires which is inserted in the same circuit, while the other corresponds with the bell system S, which is in communication with earth. It follows from this arrangement, that when the right telephone presses its whole weight on the support, the bar of the commutator is inclined on the lower contact, and consequently the line is in direct communication with the bell, so that the call can be made. When, on the other hand, the telephone is removed from its support, the bar rests on the higher contact, and the telephones are connected with the line.

Pressure on the sending push serves to call the corresponding station: the connection of the line with the telephones is then broken, and it is established with the battery of the sending station, which sends its current through the bell of the corresponding station. In order to obtain this double effect, the contact spring of the sending push generally rests upon a contact fastened to a piece of wood shaped like a joiner’s rule, which covers it in front, and below this spring there is a second contact, which communicates with the positive pole of the station battery. The other contact corresponds with the line wire, and a connection takes place between the earth wire and the negative pole of the station battery, so that the earth wire is common to three circuits:

1st. To the telephone circuit. 2nd. To that of the bell system. 3rd. To that of the local battery.

The second fork, which supports the telephone on the right, is fixed to the board, and is independent of any electric current.

It is clear that this arrangement may be varied in a thousand ways, but the model we have just described is the most practical.

_Edison’s System._--The problem becomes more complex in the case of battery telephones, since the battery must be common to both systems, and the induction coil must be inserted in two distinct circuits. Fig. 55 represents the model adopted in Mr. Edison’s telephone.

In this arrangement, there is a small stand C on the mahogany board on which the bases of the two telephones rest. The bell system S is worked by an electro-magnetic speaker P, which serves, when a Morse key is added to the system, for exchange of correspondence in the Morse code, if there should be any defect in the telephones, or to put them in working order. Above the speaker there is a commutator with a stopper D to adapt the line for sending or receiving, with or without the bell; and below the stand C the induction coil, destined to transform the voltaic currents into induced currents, is arranged in a small closed box E.

When the commutator is at reception, the line is in immediate correspondence either with the speaker or with the receiving telephone, according to the hole in which the stopper is inserted; when, on the other hand, it is at sending, the line corresponds to the secondary circuit of the induction coil. Under these conditions the action is no longer automatic; but since this kind of telephone can only be usefully employed for telegraphy, in which case those who work it are acquainted with electric apparatus, there is no inconvenience in this complication.

CALL-BELLS AND ALARUMS.

The call-bells applied to telegraphic service have been arranged in different ways. When the vibrating bells are in use, like those of which we have just spoken, it is necessary to use a battery, and the advantages offered by telephones with induced currents are thus sensibly diminished. In order to dispense with the battery, the use of the electro-magnetic bell has been suggested.

In this case there are usually two bells, with a hammer oscillating between them, and a support formed of the polarised armature of an electro-magnet. The electro-magnetic instrument is placed below this system; it is turned by a winch, and sends the currents, alternately reversed, which are necessary to communicate the vibratory movement to the hammer, and this movement is enough to make the two bells tinkle. Below the winch of this electro-magnetic instrument there is a commutator with two contacts, which adapts the instrument for sending or receiving.

M. Mandroux has simplified this system, and has reduced it to small dimensions by the following arrangement. He fixes two magnetic cores, furnished with coils, on each of the two poles of a horseshoe magnet, composed of two bars connected by an iron coupler, and between the poles expanded by these four cores he inserts an armature, within which there is a steel spring fastened to one of these poles. In this way the armature is polarised, and oscillates under the influence of the reversed currents transmitted by an instrument of the same kind provided with an induction system. These oscillations may have the effect of producing the sound of a call-bell, and the induction system may consist of a manipulating key, fastened to a duplex system of armature, regularly applied to the magnetic cores, taken in pairs. On communicating a series of movements to this manipulator, a series of induced currents in an inverse direction are produced, which cause the armature of the corresponding station to act as we have already seen, and which may even, when necessary, furnish a series of Morse signals for a suitable manipulation. On account of the small size of this system, it might be applied to the telephonic service of the army.

The Bell Telephone Company in Paris has arranged another little call-system which is quite satisfactory and has the advantage of acting as a telephone at the same time. The model resembles the one we have termed a snuff-box telephone, and it has a button commutator by means of which the instrument is placed in communication with the electro-magnetic system of the instrument, or with a battery which is able to make the telephone vibrate with some force. To make a call, the button must be pressed, and the battery current is communicated to the corresponding instrument, which begins to vibrate when the call is made; and when notice is given of the receipt of the signal, the pressure on the button is removed, and it becomes possible to speak and receive as in ordinary telephones.

_M. de Weinhold’s System._--M. Zetzche speaks highly of an alarum devised by Professor A. de Weinhold, which resembles that by M. Lorenz, represented in fig. 56. Its organ of sound consists of a steel bell T, from 13 to 14 centimètres in diameter, and toned to give about 420 double vibrations in a second. ‘Its diameter and tone,’ he says, ‘are important, and any great departure from the rule laid down diminishes the effect. The opening of the bell is below, and it is fixed on a stand by its centre. A slightly curved bar magnet, provided at its two ends with iron appendices enclosed in a coil, traverses the stand. The bar magnet of the telephone also terminates in an iron appendix enclosed in a coil. In both cases the changes produced in the magnetic condition appear to be more intense than they are in magnets without appendices. The bar magnet is placed within the bell in the direction of one of its diameters, so that the appendices almost touch its sides.

‘When the bell is struck on a spot about 90° from this diameter with a wooden clapper M, which acts with a spring, and is withdrawn by stretching the spring and then letting it go, as in a bell for the dinner-table, the vibrations imparted to it send currents into the coils, and these currents produce identical vibrations on the iron disk of the telephone, which are intensified by a conical resonator fitted to the telephone, so as to be easily heard some paces off. For ordinary use, the bell coil is broken into a short circuit by means of a metallic spring R, and consequently, when the bell is struck, the spring must be opened so as not to break the circuit. An instrument of the same kind has also been devised by Herr W. E. Fein at Stuttgardt.’

_MM. Dutertre and Gouault’s System._--One of the most ingenious solutions of the problem of making the telephone call has recently been proposed by MM. Dutertre and Gouault. Figs. 57 and 58 represent the opposite faces of the instrument. It consists of a kind of snuff-box telephone, like the one shown in fig. 26, and it is so arranged as to send or receive the call, according to the way in which it is placed on its stand, which is only an ordinary bracket fastened to the wall. When it is placed on the bracket so as to have the telephone mouthpiece on the outside, it is adapted for receiving, and can then give the call. When, on the other hand, its position on the bracket is reversed, it permits the other station to make the call, by producing vibrations on a vibrator under the influence of a battery, and these vibrations reverberate in the corresponding instrument with sufficient force to produce the call. If the instrument is taken up, and the finger is placed on a small spring button, it may then be used as an ordinary telephone.

In this instrument, the magnet N S (fig. 57) is snail-shaped, like others we have mentioned, but the core of soft iron S, to which the coil E is fastened, can produce two different effects on its two extremities. On the one side, it reacts on a small armature which is fastened to the end of a vibrating disk C, fig. 58; the armature is placed against a contact fastened to the bridge B, and constitutes an electro-magnetic vibrator. For this purpose the bridge is in metallic communication with the coil wire, of which the other end corresponds with the line wire, and the spring C is mounted on an upright A, which also supports another spring D G acting on two contacts, one placed at G, and corresponding to the earth wire, the other at H, and connected with the positive pole of the battery. A small moveable button, which passes through a hole in the lid of the box, and projects beyond it, is fixed at G, and all this part of the instrument faces the bottom of the box. The upper part consists of the vibrating disk and the mouthpiece, so that the mechanism we have described is all mounted on an inner partition forming a false bottom to the box.

When the box rests upon its base, on the side shown in fig. 58, the button at G presses on the spring D G, and raises it so as to break the connection with the battery; the coil of the instrument is then united to the circuit, and consequently receives the transmitted currents, which follow this route: line wire, coil E, bridge B, spring C, spring D G, earth contact. If these currents are transmitted by a vibrator, they are strong enough to produce a noise which can be heard in all parts of a room, and consequently the call may be given in this way. If the currents are due to telephonic transmission, the instrument is applied to the ear, care being taken to put the finger on the button G, and the exchange of correspondence takes place as in ordinary instruments; but it is simpler and more manageable to insert a second telephone in the circuit for this purpose. When the box is inverted on its mouthpiece, and the button G ceases to press on the spring D G, the battery current reacts on the vibrator of the instrument, and sends the call to the corresponding station, following this route: I D A C B E, line, earth and battery; and the call goes on until the correspondent breaks the current by taking up his instrument, thus warning the other that he is ready to listen.

_System of M. Puluj._--There is yet another call system, devised by M. Puluj. It consists of two telephones without mouthpieces, connected together, and with coils placed opposite the branches of two tuning-forks, tuned as nearly as possible to the same tone. A small metal bell is fixed between the opposite faces of the tuning-forks, and a wire stretched near them is provided with a small ball in contact with their branches. When the tuning-fork at the sending station is put in vibration by striking it with an iron hammer covered with skin, the tuning fork at the other station vibrates also, and its ball strikes upon the bell. As soon as the signal is returned by the second station, mouthpieces with iron diaphragms are fastened to the telephones, and the correspondence begins. It seems that, by the use of a resonator, the sound which reaches the receiving station may be so intensified as to become audible in a large hall, and the bell signal may be heard in an adjoining room, even through a closed door.

_Mr. Alfred Chiddey’s System._--This arrangement consists of a slender copper tube, eight inches long, and with an orifice of 1/30 of an inch, of which the lower end is soldered to the diaphragm of a telephone. A branch joint, to which an india-rubber tube is fitted, connects it with a gas jet, which is lighted and surrounded with a lamp shade, in such a way as to make it produce, under given conditions, sounds resembling those of the singing flames. A perfectly similar system is arranged at the other end of the line, in such a way that the sounds emitted in each case shall be precisely in unison. If the two systems are so regulated as not to emit sounds in their normal condition, they can be made to sing by causing a tuning-fork in the vicinity of one or the other to vibrate the same note, and then the corresponding flame will begin to sing, producing a vibration in the diaphragm of the telephone with which it is in correspondence, and hence will follow the vibration of the diaphragm of the other telephone, and consequently the vibration of the flame of the calling instrument. In this way the call signal may be made without the intervention of any battery.

APPLICATIONS OF THE TELEPHONE.

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.