Part 4

‘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 Gray[4] 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.’

[Illustration: FIG. 20.]

_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.

EXAMINATION INTO THE FUNDAMENTAL PRINCIPLES ON WHICH BELL’S TELEPHONE IS BASED.

Although the preceding account would suffice to make the principle of Bell’s telephone intelligible to persons acquainted with electric science, this would not be the case with the majority of our readers, and we therefore think it necessary to enter into some details as to the source of the electric currents which are employed in telephonic transmissions. These details seem to us the more necessary, since many persons still believe that Bell’s telephones are not electric, because they do not require a battery, and they are often confounded with string telephones, so that the difference of price between Bell’s instruments and those hawked in the streets seems astonishing.

Without defining what is meant by an electric current, which would be too elementary, we may say that electric currents can be produced by different causes, and that, in addition to those which are due to batteries, strong currents are also produced by the force exerted by magnets on a conducting circuit properly arranged. Such currents are called induction currents, and are used in Bell’s telephone. In order to understand how they are developed under these conditions, it will be enough to examine what takes place when the pole of a magnet is brought near to, and withdrawn from, a closed circuit. To do this, let us suppose a copper wire attached to a galvanometer in the form of a circle, and that one pole of a permanent magnet is directed towards the centre of the circle. Now observe what happens:

1. At the moment when the magnet approaches an electric current arises, causing the galvanometer to deviate to one side. This deviation will be great in proportion to the extent of the movement, and the tension of the current will be great in proportion to the abruptness of the movement. The current will however be only instantaneous.

2. At the moment when the magnet is withdrawn, a fresh current of the same nature will arise, but it will appear in an opposite direction from the former. It will be what is called a direct current, because it is in the same direction as the magnetic current of the magnet which produces it, while the other current is called _inverse_.

3. If, instead of advancing or withdrawing the magnet by means of a single movement, it is advanced in jerks, a succession of currents in the same direction is produced, of which the existence can be ascertained by the galvanometer when there is a sufficient interval between the movements, but when the intervals are very slight the currents are interfused; and since inverse effects take place when the magnet is moved in a contrary direction, the needle of the galvanometer follows the movements of the magnet, and to a certain extent stereotypes them.

4. If, instead of reacting on a simple closed circuit, the magnet exerts its force on a considerable number of circumvolutions of this circuit, that is, on a bobbin of coiled wire, the effects will be considerably increased, and they will be still greater if there be a magnetic core within the bobbin, since the inducing action will then be more effectually exerted throughout the bobbin. As the magnetic core, when it is magnetised and demagnetised under the influence of its approach to or withdrawal from the inducing magnet, is subject to the reaction from all the fluctuations which occur in the movements of the magnet, the induced currents which ensue are perfectly defined.

5. If, instead of a movable magnet, we suppose it to be fixed in the centre of the coil, the induced currents of which we have spoken may then be determined by modifying its force. In order to do so, it is enough that an iron armature should react upon its poles. When this armature is brought close to one of the poles, or to both at once, it acquires force, and produces an inverse current, that is, a current in the direction which would have corresponded to an approach of the magnet to the closed circuit. On its withdrawal the inverse effect is produced; but in both cases the induced currents correspond with the extent and direction of the movements accomplished by the armature, and consequently they may reproduce its movements by their effects. If this armature is an iron plate, which vibrates under the influence of any sound in this disposition of the electro-magnetic system, the alternate movements of the plate will be transformed into the induced currents, and these will be stronger or weaker, more or less definite, according to the range and complexity of the vibrations: they will, however, be undulatory, since they will always result from successive and continuous movements, and will consequently be in the conditions which, as we have seen, are required for the transmission of speech.

As for the action produced upon the receiver, that is, on the instrument for reproducing speech, it is somewhat complex, and we shall have occasion to speak of it presently; but we can get a general impression of it, if we consider that the effects produced by the induced currents of variable intensity, which traverse the coil of the electro-magnetic system, must determine, by the magnetisations and demagnetisations which ensue, the vibrations of the armature disk; these vibrations, more or less amplified and defined, exactly represent those of the disk before which the speaker stands, and can only be obtained from them. The effects are, however, in reality more complex, although they are produced under analogous conditions, and we shall have more to say about them when we come to speak of the experiments made with the telephone. It must meanwhile be observed that, for the reproduction of speech, it is not necessary that the magnetic core should be of soft iron, since the vibratory effects may follow from differential as well as from direct magnetisation.

ORDINARY ARRANGEMENT OF THE BELL TELEPHONE.

The arrangement most generally adopted for the telephone is the one represented in fig. 21. It consists of a kind of circular wooden box, fitted to the extremity of a handle M, which is also of wood, and contains the magnetic bar N S. This bar is fixed by means of a screw _t_, and is so arranged as to be moved forward and backward by tightening or loosening the screw, a condition necessary in order to regulate the instrument. At the free extremity of the bar the magnetic coil B is fixed; this must, according to MM. Pollard and Garnier, be made of wire No. 42, so as to present a considerable number of spirals. The ends of this coil generally terminate at the lower end of the handle in two copper rods _f_, _f_, which traverse its length, and are fastened to two binding-screws I, I′, where the line wires C, C are fixed. In the instruments made by M. Bréguet there are, however, no binding-screws, but a little twist, made of two flexible wires covered with gutta-percha and silk, is fastened to the two rods. A wooden cap is screwed to the end of the handle, and the twist passes through a hole made in this cap, so that there is no inconvenience in working the instrument. By laying hold of the ends of the wire twist with pliers it is possible to join them to the circuit. This instrument is represented in fig. 22.

[Illustration: FIG. 21.]

By another arrangement, the wires of the coil end immediately in the binding-screws which are placed below the wooden box, but this arrangement is inconvenient.

Above the pole of the magnetic bar is placed the iron vibrating plate L L, which is coated either with black or yellow varnish, with tin or blue oxide, but which must always be very thin. This plate is in the form of a disk, and by its rim, resting on a caoutchouc ring, it is firmly fixed to the circular edges of the wooden box, which is for this purpose made in two pieces. These pieces are adjusted to each other, either by screws or by spirals cut in half the thickness of the wood. This disk ought to be as near as possible to the polar end of the magnet, yet not so near as to produce contact between the two by the vibrations of the voice. Finally, the mouthpiece R R′ (fig. 21), which is in form of a wide funnel, terminates the upper part of the box, and should be so arranged as to leave a certain space between the disk and the edges of the hole V, which is open in its centre. The size of the box should be so calculated as to permit of its acting as a sounding-box, without however provoking echoes and a confusion of sounds.

[Illustration: FIG. 22.]

When the instrument is properly made, it will produce very marked effects; and M. Pollard, one of the first Frenchmen to take up the study of telephones, has written as follows on the subject:

‘The instrument which I have prepared gives results which are truly astonishing. In the first place, when considering the resistance, the introduction into the circuit of five or six persons does not sensibly diminish the intensity of sounds. On putting an instrument to each ear, the sensation is precisely the same as if the correspondent were speaking some yards behind. The intensity, the clearness, the purity of tone are irreproachable.

‘I can speak to my colleague in quite an undertone, scarcely breathing as I may say, and persons placed within two yards of me will be unable to catch a single word of our conversation.

‘On the part of the receiver, if anyone raises his voice to call me, I hear the call in all parts of my office, at least when silence prevails there; at any rate, when I am seated at my table with the instrument some yards off, I can always hear the call. In order to increase the intensity of sound, I fitted the mouthpiece with a copper horn of conical shape, and under these conditions words spoken in my bureau two or three yards from the mouthpiece can be heard at the other end of the line; from my station, a little more than a yard from the tube, I can hear and speak to my colleague without effort.’