SPEAKING TELEPHONES.
We have seen that the telephones just described can only transmit musical sounds, since they can merely repeat simple vibrations, in greater or less number, it is true, but not in simultaneous combinations like those which reproduce articulate sounds. Up to the time of Mr. Bell’s invention, the transmission of speech could only take place with the aid of acoustic tubes, or of the string telephones of which we have spoken. Although these instruments have no connection with the object of our study in this work, we have thought it necessary to say a few words about them, since they may sometimes be combined with electric telephones, and also represent the first stage of the invention.
String Telephones.—These instruments, which have flooded the cities of Europe for several years, since the date of the invention was 1867, are interesting in themselves, and we are surprised that they have not hitherto taken a place in the collections of physical science. They are made of two metal or cardboard tubes, in the form of a cylindrical cone: one end is closed by a tightly stretched membrane of parchment, in the centre of which the cord or string intended to connect the two cylinders is fastened by a knot. When two such tubes are connected in this way, and the cord is tightly stretched, as in fig. 6, it is only necessary to apply one tube to the ear, while another speaks into the opening of the other tube: the words spoken by the latter are instantly transmitted, and it is even possible to converse in quite an undertone. Under these conditions the vibrations of the membrane affected by the voice are mechanically transmitted to the other membrane by the string, which, as Robert Hooke declared in 1667, is a better transmitter of sound than the air. In this way it is possible to communicate at a distance of 170 yards, and the size and nature of the cord have some influence. The sellers of these instruments say that the best results are obtained from silken cords, and the worst from those made of hemp. Cords of plaited cotton are usually employed for the sake of cheapness.
In some patterns, the tubes are so arranged as to present, between the membrane and the mouth, a diaphragm pierced with a hole, and the instrument somewhat resembles a bell with its base bored and closed again a little above the parchment membrane; but I have not observed that this pattern is decidedly superior to the others.
It has also been asserted that horn-shaped tubes of nickel silver are to be preferred, of which I am equally doubtful. At any rate, these instruments have produced unexpected results; and although their practical use is very limited, they are interesting from a scientific point of view, and are instructive toys for children.
Mr. Millar, of Glasgow, declares that the effect produced by these telephones depends very much on the nature of the string, the way in which it is attached, and the way in which the membrane is fastened to the mouthpiece.
Improvements made in the String Telephone.—The amazing effects of the Bell telephones have lately brought the string telephones, which were only regarded as children’s toys, again into fashion. Since they have made it possible to transmit to several persons the words reproduced by an electric telephone, means have been sought for combining them usefully with the latter, and the best mode of making them speak on a string presenting several angles has been sought for: it has been shown that, under the usual conditions, these instruments only speak distinctly when the string is stretched in a right line. To solve this problem, it occurred to M. A. Bréguet to make use of a sort of tambourine for the supports, with the string passed through their centre; the sound conveyed by that part of the string which is in connection with the speaking-horn causes the membrane of the tambourine to vibrate, which again communicates the vibration to the next portion of string. In this way the angles may be multiplied at will, and the string may be supported throughout the length compatible with this kind of telephone, which does not exceed 112 yards.
M. A. Bréguet has also invented a system of relays to accomplish the same object. He makes the strings terminate in two membranes which close the two openings of a brass cylinder. The sounds reproduced on one of these membranes react upon the other, which vibrates under its influence, as if it were affected by the voice. The cylinder then acts as an ordinary acoustic tube, and its form may be varied at pleasure.
M. A. Badet, on February 1, 1878, succeeded in making string telephones in an analogous way, and he used parchment stretched upon frames which acted as resonant boards. The string was fixed in the centre of the membrane, and made with it the angle desired.
Several scientific men, among others Messrs. Wheatstone, Cornu, and Mercadier, have long been occupied about these ways of transmission by wire, and Messrs. Millar, Heaviside, and Nixon have lately made some interesting experiments, on which we must say a few words. Mr. Millar ascertained that by means of a telegraphic wire, stretched and connected by two copper wires with two vibrating disks, musical sounds might be conveyed to a distance exceeding 160 yards, and that by stretching these wires through a house, and connecting them with mouth-and-ear holes in different rooms, communication between them became perfectly easy.
For the vibrating disks he employed wood, metal, or gutta-percha, in the form of a drum, with wires fixed in the centre. The sound seems to become more intense in proportion to the thickness of the wire.
Messrs. Heaviside and Nixon, in their experiments at Newcastle-on-Tyne, have ascertained that the most effective wire was No. 4 of the English gauge. They employed wooden disks ⅛ inch in thickness, and these may be placed in any part of the length of the wire. When the wire was well stretched and motionless, it was possible to hear what was said at a distance of 230 yards, and it seems that Mr. Huntley, by using very thin iron diaphragms, and by insulating the line wire on glass supports, was able to transmit speech for 2,450 feet, in spite of the zigzags made by the line on its supports.
Mr. Graham Bell’s Electric Telephone.—Telephonic instruments were at this stage when Bell’s telephone was shown at the Philadelphia Exhibition of 1876. Sir William Thompson did not hesitate to call it ‘the wonder of wonders,’ and it instantly attracted universal attention, although there was at first much incredulity as to its genuineness. This telephone, in fact, reproduced articulate words, a result which surpassed all the conceptions of physicists. In this case it was no longer a conception, to be treated as visionary until there was proof to the contrary: the instrument spoke, and even spoke so loudly that it was not necessary to apply the ear. Sir William Thompson spoke to this effect on the subject at the meeting of the British Association at Glasgow in September 1876:—
‘In the department of telegraphs in the United States I saw and heard Mr. Elisha Gray’s electric telephone, of wonderful construction, which can repeat four despatches at the same time in the Morse code, and, with some improvements in detail, this instrument is evidently capable of a fourfold delivery. In the Canadian department I heard “To be or not to be? There’s the rub,” uttered through a telegraphic wire, and its pronunciation by electricity only made the rallying tone of the monosyllables more emphatic. The wire also repeated some extracts from New York papers. With my own ears I heard all this, distinctly articulated through the slender circular disk formed by the armature of an electro-magnet. It was my fellow-juryman, Professor Watson, who, at the other extremity of the line, uttered these words in a loud and distinct voice, while applying his mouth to a tightly stretched membrane provided with a small piece of soft iron, which executed movements corresponding to the sound vibrations of the air close to an electro-magnet introduced into the circuit. This discovery, the wonder of wonders in electric telegraphy, is due to a young fellow-countryman of our own, Mr. Graham Bell, a native of Edinburgh and now naturalised in New York.
‘It is impossible not to admire the daring invention by which we have been able to realise with these simple expedients the complex problem of reproducing by electricity the tones and delicate articulations of voice and speech; and it was necessary, in order to obtain this result, to find out the means of varying the intensity of the current in the same proportion as the inflections of the sound emitted by the voice.’
If we are to believe Mr. Graham Bell, the invention of the telephone was not due to a spontaneous and fortunate conception: it was the result of his long and patient studies in acoustic science, and of the labours of the physicists who preceded him.2 His father, Mr. Alexander Melville Bell, of Edinburgh, had studied this science deeply, and had even succeeded in representing with great ingenuity the adaptation of the vocal organs for the emission of sound. It was natural that he should instil a taste for his favourite studies into his son’s mind, and they made together numerous researches in order to discover the relations which exist between the different elements of speech in different languages, and the musical relations of vowels. It is true that several of these researches had been made by M. Helmholtz, and under more favourable conditions; but these studies were of great use to Mr. Bell when he was afterwards occupied with the telephone, and Helmholtz’s experiments, which he repeated with one of his friends, Mr. Hellis of London, concerning the artificial reproduction of vowels by means of electric tuning-forks, launched him into the study of the application of electricity to acoustic instruments. He first invented a system of an electric harmonica with a key-board, in which the different sounds of the scale were reproduced by electric diapasons of different forms, adapted to different notes, and which, when set in motion by the successive lowering of the keys, could reproduce sounds corresponding to the notes touched, just as in an ordinary piano.
He next, as he tells us, turned his attention to telegraphy, and thought of making the Morse telegraphs audible by causing the electro-magnetic organ to react on sounding contacts. It is true that this result had already been obtained in the sounders used in telegraphy, but he thought that by applying this system to his electric harmonica, and by employing such an intensifying instrument as Helmholtz’s resonator at the receiving station, it would be possible to obtain through a single wire simultaneous transmissions which should be due to the action of the voice. We shall see presently that this idea was realised almost at the same time by several inventors, among others by M. Paul Lacour, of Copenhagen, Mr. Elisha Gray, of Chicago, and Messrs. Edison and Varley.
Mr. Bell’s study of electric telephones really dates from this time, and he passed from complex to simple instruments, making a careful study of the different modes of vibration which arise from different modes of electric action. The following is an abstract, with the omission of more technical details, of the paper read by Mr. Bell to the Society of Telegraphic Engineers, London, October 31, 1877.
If the intensity of an electric current is represented by the ordinates of a curve, and the duration of breaks in the current by the abscissæ, the given curve may represent the waves of the positive or negative current respectively, above and below the line of X, and these waves will be more or less accentuated, just as the transmitted currents are more or less instantaneous.
If the currents which are interrupted to produce a sound are quite instantaneous in their manifestation, the curve represents a series of isolated indentations, as we see in fig. 7; and if the interruptions are so made as only to produce differences of intensity, the curve is presented under the form of fig. 8. Finally, if the emissions of current are so ordered that their intensity alternately increases and diminishes, the curve takes the form represented in fig. 9. In the first case, the currents are intermittent; in the second, pulsatory; in the third case, they are undulatory.
These currents are necessarily positive or negative, according to their position above or below the line x, and if they are alternately reversed, the curves present the form given in fig. 10, curves which essentially differ from the first, not merely in the different form of the indentations, but especially in the suppression of the extra current, which is always found in the pulsatory and undulatory currents.
The two former systems of currents have long been in use for the electric transmission of musical sounds, of which we have an interesting example in Reiss’s telephone already described. But Mr. Bell claims to have been the first to employ the undulatory currents, which made it possible to solve the problem of transmitting speech.3 In order to estimate the importance of this discovery, it will be enough to analyse the effects produced with these different systems of currents when several notes of varying pitch are to be combined.
Fig. 7 shows a combination in which the styles a, a′, of two sending instruments cause the interruption of the current from the same battery B, so that the given vibrations should be between them in the relation of a tierce major, that is in the relation of four to five. Under such conditions, the currents are intermittent, and four contacts of a are produced in the same space of time as the five contacts of a′, and the corresponding electric intensities will be represented by the indentations we see in A2 and in B2: the combination of these intensities A2 + B2 will produce the indentations at unequal intervals which may be observed on the third line. It is evident that although the current maintains a uniform intensity, there is less time for the breaks when the interrupting styles act together than when they act separately, so that when there are a number of contacts effected simultaneously by styles working at different degrees of velocity, the effects produced will have the effect of a continuous current. The maximum number of distinct effects which can be produced in this way will, however, greatly depend on the relation which exists between the durations of the make and break of the current. The shorter the contacts are, and the longer the breaks, the more numerous will be the effects transmitted without confusion, and vice versâ.
By the aid of pulsatory currents the transmission of musical sounds is effected in the way indicated in fig. 8, and it is seen that when they are produced simultaneously, the result A2 + B2 is analogous to that which would be produced by a continuous current of minimum intensity.
In the case of undulatory currents the result is different, but in order to produce them it is necessary to have recourse to inductive effects, and fig. 9 indicates the manner in which the experiment should be made. In this case, ‘the current from the battery B is thrown into waves by the inductive action of iron or steel reeds M, M, vibrated in front of electro-magnets e, e, placed in circuit with the battery: A2 and B2 represent the undulations caused in the current by the vibration of the magnetised bodies, and it will be seen that there are four undulations of B2 in the same time as five undulations of A2. The resultant effect upon the main line is expressed by the curve A2 + B2, which is the algebraical sum of the sinusoidal curves A2 and B2. A similar effect is produced when reversed undulatory currents are employed, as in fig. 10, where the current is produced by the vibration of permanent magnets united upon a circuit, without a voltaic battery.
‘It will be understood from figs. 9 and 10 that the effect of transmitting musical signals of different pitches simultaneously along a single wire is not to obliterate the vibratory character of the current, as in the case of intermittent and pulsatory currents, but to change the shapes of the electrical undulations. In fact, the effect produced upon the current is precisely analogous to the effect produced in the air by the vibration of the inducing bodies M, M′. Hence it should be possible to transmit as many musical tones simultaneously through a telegraph wire as through the air.’
After applying these principles to the construction of a telegraphic system for multiple transmissions, Mr. Bell lost no time in making use of his researches to improve the vocal training of deaf mutes. ‘It is well known,’ he said, ‘that deaf mutes are dumb merely because they are deaf, and that there is no defect in their vocal organs to incapacitate them from utterance. Hence it was thought that my father’s system of pictorial symbols, popularly known as visible speech, might prove a means whereby we could teach the deaf and dumb to use their vocal organs and to speak. The great success of these experiments urged upon me the advisability of devising methods of exhibiting the vibrations of sound optically, for use in teaching the deaf and dumb. For some time I carried on experiments with the manometric capsule of Koenig, and with the phonautograph of Léon Scott. The scientific apparatus in the Institute of Technology in Boston was freely placed at my disposal for these experiments, and it happened that at that time a student of the Institute of Technology, Mr. Maurey, had invented an improvement upon the phonautograph. He had succeeded in vibrating by the voice a stylus of wood about a foot in length which was attached to the membrane of the phonautograph, and in this way he had been enabled to obtain enlarged tracings upon a plane surface of smoked glass. With this apparatus I succeeded in producing very beautiful tracings of the vibrations of the air for vowel sounds. Some of these tracings are shown in fig. 11. I was much struck with this improved form of apparatus, and it occurred to me that there was a remarkable likeness between the manner in which this piece of wood was vibrated by the membrane of the phonautograph and the manner in which the ossiculæ of the human ear were moved by the tympanic membrane. I determined therefore to construct a phonautograph modelled still more closely upon the mechanism of the human ear, and for this purpose I sought the assistance of a distinguished aurist in Boston, Dr. Clarence J. Blake. He suggested the use of the human ear itself as a phonautograph, instead of making an artificial imitation of it. The idea was novel, and struck me accordingly, and I requested my friend to prepare a specimen for me, which he did. The apparatus, as finally constructed, is shown in fig. 12. The stapes was removed, and a stylus of hay about an inch in length was attached to the end of the incus. Upon moistening the membrana tympani and the ossiculæ with a mixture of glycerine and water, the necessary mobility of the parts was obtained; and upon singing into the external artificial ear the stylus of hay was thrown into vibration, and tracings were obtained upon a plane surface of smoked glass passed rapidly underneath. While engaged in these experiments I was struck with the remarkable disproportion in weight between the membrane and the bones that were vibrated by it. It occurred to me that if a membrane as thin as tissue paper could control the vibration of bones that were, compared to it, of immense size and weight, why should not a larger and thicker membrane be able to vibrate a piece of iron in front of an electro-magnet, in which case the complication of steel rods shown in my first form of telephone, could be done away with, and a simple piece of iron attached to a membrane be placed at either end of the telegraphic circuit?
‘For this purpose I attached the reed A (fig. 13) loosely by one extremity to the uncovered pole h of the magnet, and fastened the other extremity to the centre of a stretched membrane of goldbeaters’ skin n. I presumed that upon speaking in the neighbourhood of the membrane n, it would be thrown into vibration and cause the steel reed A to move in a similar manner, occasioning undulations in the electrical current that would correspond to the changes in the density of the air during the production of the sound; and I further thought that the change of the intensity of the current at the receiving end would cause the magnet there to attract the reed A′ in such a manner that it should copy the motion of the reed A, in which case its movements would occasion a sound from the membrane n′ similar in timbre to that which had occasioned the original vibration.
‘The results, however, were unsatisfactory and discouraging. My friend Mr. Thomas A. Watson, who assisted me in this first experiment, declared that he heard a faint sound proceed from the telephone at his end of the circuit, but I was unable to verify his assertion. After many experiments attended by the same only partially successful results, I determined to reduce the size and weight of the spring as much as possible. For this purpose I fastened a piece of clock spring, about the size and shape of my thumbnail, firmly to the centre of the diaphragm, and had a similar instrument at the other end (fig. 14); we were then enabled to obtain distinctly audible effects. I remember an experiment made with this telephone, which at the time gave me great satisfaction and delight. One of the telephones was placed in my lecture-room in the Boston University, and the other in the basement of the adjoining building. One of my students repaired to the distant telephone to observe the effects of articulate speech, while I uttered the sentence, “Do you understand what I say?” into the telephone placed in the lecture-hall. To my delight an answer was returned through the instrument itself, articulate sounds proceeded from the steel spring attached to the membrane, and I heard the sentence, “Yes, I understand you perfectly.” It is a mistake, however, to suppose that the articulation was by any means perfect, and expectancy no doubt had a great deal to do with my recognition of the sentence; still, the articulation was there, and I recognised the fact that the indistinctness was entirely due to the imperfection of the instrument. I will not trouble you by detailing the various stages through which the apparatus passed, but shall merely say that after a time I produced the form of instrument shown in fig. 15, which served very well as a receiving telephone. In this condition my invention was exhibited at the Centennial Exhibition in Philadelphia. The telephone shown in fig. 14 was used as a transmitting instrument, and that in fig. 15 as a receiver, so that vocal communication was only established in one direction.
‘The articulation produced from the instrument shown in fig. 15 was remarkably distinct, but its great defect consisted in the fact that it could not be used as a transmitting instrument, and thus two telephones were required at each station, one for transmitting and one for receiving spoken messages.
‘It was determined to vary the construction of the telephone, and I sought by changing the size and tension of the membrane, the diameter and thickness of the steel spring, the size and power of the magnet, and the coils of insulated wire around their poles, to discover empirically the exact effect of each element of the combination, and thus to deduce a more perfect form of apparatus. It was found that a marked increase in the loudness of the sounds resulted from shortening the length of the coils of wire, and by enlarging the iron diaphragm which was glued to the membrane. In the latter case, also, the distinctness of the articulation was improved. Finally, the membrane of goldbeaters’ skin was discarded entirely, and a simple iron plate was used instead, and at once intelligible articulation was obtained. The new form of instrument is that shown in fig. 16, and, as had been long anticipated, it was proved that the only use of the battery was to magnetise the iron core of the magnet, for the effects were equally audible when the battery was omitted and a rod of magnetised steel substituted for the iron core of the magnet.
‘It was my original intention, and it was always claimed by me, that the final form of telephone would be operated by permanent magnets in place of batteries, and numerous experiments had been carried on by Mr. Watson and myself privately for the purpose of producing this effect.
‘At the time the instruments were first exhibited in public the results obtained with permanent magnets were not nearly so striking as when a voltaic battery was employed, wherefore we thought it best to exhibit only the latter form of instrument.
‘The interest excited by the first published accounts of the operation of the telephone led many persons to investigate the subject, and I doubt not that numbers of experimenters have independently discovered that permanent magnets might be employed instead of voltaic batteries. Indeed one gentleman, Professor Dolbear, of Tufts College, not only claims to have discovered the magneto-electric telephone, but I understand charges me with having obtained the idea from him through the medium of a mutual friend.
‘A still more powerful form of apparatus was constructed by using a powerful compound horseshoe magnet in place of the straight rod which had been previously used (see fig. 17). Indeed the sounds produced by means of this instrument were of sufficient loudness to be faintly audible to a large audience, and in this condition the instrument was exhibited in the Essex Institute, in Salem, Massachusetts, on February 12, 1877, on which occasion a short speech shouted into a similar telephone in Boston, sixteen miles away, was heard by the audience in Salem. The tones of the speaker’s voice were distinctly audible to an audience of 600 people, but the articulation was only distinct at a distance of about 6 feet. On the same occasion, also, a report of the lecture was transmitted by word of mouth from Salem to Boston, and published in the papers the next morning.
‘From the form of telephone shown in fig. 16 to the present form of the instrument (fig. 18) is but a step. It is in fact the arrangement of fig. 16 in a portable form, the magnet N S being placed inside the handle, and a more convenient form of mouthpiece provided.
‘And here I wish to express my indebtedness to several scientific friends in America for their co-operation and assistance. I would specially mention Professor Peirce and Professor Blake, of Brown University, Dr. Channing, Mr. Clarke, and Mr. Jones. It was always my belief that a certain ratio would be found between the several parts of a telephone, and that the size of the instrument was immaterial; but Professor Peirce was the first to demonstrate the extreme smallness of the magnets which might be employed. The convenient form of mouthpiece shown in fig. 17, now adopted by me, was invented solely by my friend Professor Peirce.’
Another form of transmitting telephone exhibited in Philadelphia, intended for use with the receiving telephone (fig. 15), is represented by fig. 19.
A platinum wire attached to a stretched membrane completed a voltaic circuit by dipping into water. Upon speaking to the membrane, articulate sounds proceeded from the telephone in the distant room. The sounds produced by the telephone became louder when dilute sulphuric acid, or a saturated solution of salt, was substituted for the water. Audible effects were also produced by the vibration of plumbago in mercury, in a solution of bichromate of potash, in salt and water, in dilute sulphuric acid, and in pure water.
Mr. Bell goes on to say:
‘I have found also that a musical tone proceeds from a piece of plumbago or retort carbon when an intermittent current of electricity is passed through it, and I have observed the most curious audible effects produced by the passage of reversed intermittent currents through the human body. A rheotome was placed in circuit with the primary wires of an induction coil, and the fine wires were connected with two strips of brass. One of these strips was held closely against the ear, and a loud sound proceeded from it whenever the other slip was touched with the other hand. The strips of brass were next held one in each hand. The induced currents occasioned a muscular tremor in the fingers. Upon placing my forefinger to my ear a loud crackling noise was audible, seemingly proceeding from the finger itself. A friend who was present placed my finger to his ear, but heard nothing. I requested him to hold the strips himself. He was then distinctly conscious of a noise (which I was unable to perceive) proceeding from his finger. In this case a portion of the induced currents passed through the head of the observer when he placed his ear against his own finger; and it is possible that the sound was occasioned by a vibration of the surfaces of the ear and finger in contact.
‘When two persons receive a shock from a Ruhmkorff’s coil by clasping hands, each taking hold of one wire of the coil with the free hand, a sound proceeds from the clasped hands. The effect is not produced when the hands are moist. When either of the two touches the body of the other, a loud sound comes from the parts in contact. When the arm of one is placed against the arm of the other, the noise produced can be heard at a distance of several feet. In all these cases a slight shock is experienced so long as the contact is preserved. The introduction of a piece of paper between the parts in contact does not materially interfere with the production of the sounds, but the unpleasant effects of the shock are avoided.
‘When an intermittent current from a Ruhmkorff’s coil is passed through the arms, a musical note can be perceived when the ear is closely applied to the arm of the person experimented upon. The sound seems to proceed from the muscles of the fore-arm and from the biceps muscle. Mr. Elisha Gray4 has also produced audible effects by the passage of electricity through the human body.
‘An extremely loud musical note is occasioned by the spark of a Ruhmkorff’s coil when the primary circuit is made and broken with sufficient rapidity; when two rheotomes of different pitch are caused simultaneously to open and close the primary circuit, a double tone proceeds from the spark.
‘A curious discovery, which may be of interest to you, has been made by Professor Blake. He constructed a telephone in which a rod of soft iron, about six feet in length, was used instead of a permanent magnet. A friend sang a continuous musical tone into the mouthpiece of a telephone, like that shown in fig. 17, which was connected with the soft iron instrument alluded to above. It was found that the loudness of the sound produced in this telephone varied with the direction in which the iron rod was held, and that the maximum effect was produced when the rod was in the position of the dipping-needle. This curious discovery of Professor Blake has been verified by myself.
‘When a telephone is placed in circuit with a telegraph line, the telephone is found seemingly to emit sounds on its own account. The most extraordinary noises are often produced, the causes of which are at present very obscure. One class of sounds is produced by the inductive influence of neighbouring wires and by leakage from them, the signals of the Morse alphabet passing over neighbouring wires being audible in the telephone, and another class can be traced to earth currents upon the wire, a curious modification of this sound revealing the presence of defective joints in the wire.
‘Professor Blake informs me that he has been able to use the railroad track for conversational purposes in place of a telegraph-wire, and he further states that when only one telephone was connected with the track the sounds of Morse operating were distinctly audible in the telephone, although the nearest telegraph-wires were at least forty feet distant; and Professor Peirce has observed the most curious sounds produced from a telephone in connection with a telegraph-wire during the aurora borealis.’
Mr. Bell went on to describe instances in which airs sung or played upon a musical instrument are transmitted by a telephone, when it is not known whence they come; but the strongest proof of the extraordinary sensibility of this instrument consists in its becoming possible by its means to transmit speech through bodies which might be supposed to be non-conductors. Thus communication with the earth through the human body can be made in spite of the intervention of shoes and stockings; and it may even be effected if, instead of standing on the ground, the person stands on a brick wall. Only hewn stone and wood are a sufficient hindrance to communication, and if the foot touches the adjoining ground, or even a blade of grass, it is enough to produce electric manifestations.
Mr. Bell says in conclusion:
‘The question will naturally arise, Through what length of wire can the telephone be used? In reply to this, I may say that the maximum amount of resistance through which the undulatory current will pass, and yet retain sufficient force to produce an audible sound at the distant end, has yet to be determined; no difficulty has, however, been experienced in laboratory experiments in conversing through a resistance of 60,000 ohms, which has been the maximum at my disposal. On one occasion, not having a rheostat at hand, I may mention having passed the current through the bodies of sixteen persons, who stood hand in hand. The longest length of real telegraph line through which I have attempted to converse has been about 250 miles. On this occasion no difficulty was experienced so long as parallel lines were not in operation. Sunday was chosen as the day on which it was probable other circuits would be at rest. Conversation was carried on between myself in New York, and Mr. Thomas A. Watson in Boston, until the opening of business upon the other wires. When this happened the vocal sounds were very much diminished, but still audible. It seemed, indeed, like talking through a storm. Conversation, though possible, could be carried on with difficulty, owing to the distracting nature of the interfering currents.
‘I am informed by my friend Mr. Preece that conversation has been successfully carried on through a submarine cable, sixty miles in length, extending from Dartmouth to the Island of Guernsey, by means of hand telephones.’
Mr. Elisha Gray’s Share in the Invention of the Telephone.—We have seen that if Mr. Bell was the first to construct the speaking telephone in a practical form, Mr. Gray had at the same time conceived the idea of an instrument also capable of reproducing speech, and the description given of it in his caveat was so precise that if it had been made from his design, it would have acted perfectly. This was, in fact, afterwards proved by him. In order that our readers may judge from their own knowledge of the share which should be ascribed to Mr. Elisha Gray in the invention of the telephone, we reproduce in fig. 20 the drawing which accompanied the caveat in question.
The sender, as we see, is composed of a sort of tube, closed at its lower end by a membrane to which a platinum wire is fixed; this wire dips into a liquid of moderate conducting power, and an electrode made of platinum, in communication with a battery, is fixed at the bottom of the vessel containing the liquid. The receiver is composed of an electro-magnet, of which the armature is fixed to the centre of a membrane, stretched on a kind of resonator or ear-trumpet which is held to the ear, and the two instruments are united by the line wire as we see in the plate.
Under these conditions, the undulatory currents necessary for the reproduction of speech were obtained in a mode analogous to that pointed out by Mr. Bell in his specification, that is, by the variations of resistance in the liquid layer interposed between the platinum wires of the transmitter; and their action, exerted on an electro-magnet, of which the armature was fixed on the diaphragm of the resonator, was produced under more favourable conditions than in Mr. Bell’s specification (see fig. 13), since that gentleman regards this arrangement (represented in fig. 14) as an important improvement on his first conception.
The whole importance of the invention rests on the intervention of undulatory currents, which, as we have seen, are indispensable for the reproduction of speech, and it concerns us to know whether it was Mr. Bell or Mr. Gray who first declared their importance; for in both the specifications deposited on February 14, 1876, the use of undulatory currents was declared to be indispensable. Mr. Gray asserts that he had recognised their importance for the transmission of combined sounds as early as 1874; but Mr. Bell believes that the undulatory currents mentioned by Mr. Gray at that time were only currents analogous to those he had designated under the name of pulsatory currents, which we have represented in fig. 8. We have seen that since these currents only represent the abrupt elevations and depressions of intensity, they are unfit for the reproduction of articulate sounds, which, on the contrary, demand that the variations of intensity should result from successive efforts, in exact correspondence with all the inflections of the sonorous vibrations effected by the voice. Mr. Bell’s claim to priority on this question has been recognised by the American Patent Office, since he has been placed in possession of the patent. However this may be, Mr. Gray’s telephonic system was complete, and we see in it, as we have already said, the origin of the battery telephones, which have recently produced such important results. Let us now consider the relation which this system bears to Mr. Bell’s.
The Bell system, as we have seen, although making use of a battery in the first instance, only obtained the diminution and increase of electric force necessary for the articulation of words by means of induction currents produced by the movements of an armature of soft iron, currents of which the intensity was consequently due to the range and inflections of these movements. The battery only intervened in order to communicate magnetic force to the inducer. This use of induced currents in telephonic transmissions was already of great importance, since various experiments subsequently made have proved their superiority to voltaic currents for this purpose. But experience soon convinced Mr. Bell that a powerful inductive apparatus worked by a battery was not only unnecessary for the action of this apparatus, but that a permanent magnet, very small and weak, would provide sufficient currents. This discovery, in which, as we have seen, Mr. Peirce had some share, was of great importance, since it became possible to reduce the size of the instrument considerably, so as to make it portable and adapted for sending and receiving; and it was shown that the telephone was the most sensitive of all instruments in revealing the action of currents. If, therefore, Mr. Bell was not the first to employ the successful mode of transmitting articulate words, it must be said that he sought, like Mr. Gray, to solve the problem by means of undulatory currents, and that he obtained these currents by the effect of induction, a system which, as soon as it was perfected, led to the important results with which we are all acquainted. If he had only given to the astonished world an instrument capable of reproducing speech telegraphically, his fame would be great; for this problem had hitherto been regarded as insoluble.
Mr. Gray’s claims to the invention of the telephone are given in the following summary from a very interesting work, entitled ‘Experimental Researches on Electro-harmonic Telegraphy and Telephony:’
‘1. I was the first to discover the means of transmitting compound sounds and variable inflections through a closed circuit by means of two or more electric waves.
‘2. I assert that I was the first to discover and utilise the mode of reproducing vibrations by the use of a magnet receiver constantly supplied with electric action.
‘3. I also assert that I was the first to construct an instrument consisting of a magnet with a circular diaphragm of magnetic substance, supported by its edge at a little distance from the poles of a magnet, and capable of being applied to the transmission and reception of articulate sounds.’
It is a curious fact, worth recording here, that Mr. Yates, of Dublin, in 1865, when trying to improve Reiss’s telephone, realised to a certain extent Mr. Gray’s conception of the liquid transmitter; for he introduced into the platinum contacts of Mr. Reiss’s instrument a drop of water which adapted it for the reproduction of articulate sounds. However, no notice was then taken of this result.