The receivers invented by Reis for the purpose of reconverting into audible mechanical vibrations the varying electric currents transmitted from the speaking end of the line were of two classes, viz.:
(1.) Those in which the magnetic expansion and contraction of a rod of steel or iron, under the influence of the varying current, set up mechanical vibrations and communicated them to a sound-board.
(2.) Those in which the current by passing round the coils of an electro-magnet caused the latter to vary the force with which it attracted its armature, and threw the latter into corresponding mechanical vibrations.
The first of these principles is embodied in the “knitting-needle” receiver described above and depicted in figures 22 & 23 on page 33. This receiver differs wholly from the later instruments of Bell, and others, and depended for its action upon the phenomenon of magnetic expansion discovered by Page and investigated by Joule. It was well known before Reis’s time that when a needle or bar of iron was magnetised it grew longer, and when demagnetised it grew shorter. Page detected the fact by the “tick” emitted by the bar during the act of magnetisation or demagnetisation. Joule measured the amount of expansion and contraction. To these discoveries Reis added two new facts; first, that if the degree of magnetisation be varied with rapid fluctuations corresponding to those of the sound waves impressed on the transmitter, the expansion and contraction of the rod followed these fluctuations faithfully, and therefore emitted at the receiving end sounds similar to those uttered at the transmitter. Secondly, by employing a needle of steel instead of the bar of iron used by Page, Reis obtained an instrument which once used could never become completely demagnetised on the cessation of the current; it was thenceforth a permanent magnet, and all that the fluctuating currents could do was to vary its degree of magnetisation. Reis carefully explained in his memoir “On Telephony,” how the frequency of such fluctuations in the magnetising current could act in reproducing the pitch, and further, how the amplitude of the fluctuations set up vibrations of corresponding amplitude in the rod: he added with significance, that the quality of the reproduced note depended upon a number of variations of amplitude occurring in a given time. His theory of these actions was that the atoms (or perhaps our modern word molecules would more correctly represent what Reis spoke of as atoms) of the rod or needle were pushed asunder from one another in the act of magnetisation, and that on the cessation of the magnetising influence of the current, these same atoms strove to return to their previous position of equilibrium, and thus the oscillations of the atoms led to the vibration of the needle as a whole. Whether all Reis’s speculations as to the behaviour of the atoms under varying degrees of magnetising force are justified in the present aspect of science or not, is, however, not of any great importance; the important point is, that, whether his theory be right or wrong, the instrument he devised will perform the function he assigned to it: it will reproduce speech, not loudly, but in reality far more articulately than many of the telephonic receivers in use under the names of Bell, Gower-Bell, &c.
One very curious point in connection with this “knitting-needle” receiver of Reis, is its extremely bad acoustical arrangements. It was laid horizontally upon a small sounding-box covered by a lid. If the end of the needle had been made to press on the resonant-board (as indeed appears to have been done at first with the violin, p. 29) the vibrations would have been much more directly reinforced. But when merely supported by two wooden bridges the direct communication was largely lost. The pressure of the lid downwards upon the spiral, as recommended by Reis, is no doubt an important matter acoustically. It is strange that a man who had grappled in so masterly a way with the acoustical problem of the transmitter, and had solved it by constructing that transmitter on the lines of the human ear, should not have followed out to the same extent those very same principles in the construction of his receiver. An extended surface he did employ, in the shape of a sounding-board; but it was not applied in the very best manner in this instrument.
The second principle applied by Reis in the construction of his telephone-receivers, was that of the electro-magnet. He arranged an electro-magnet so that the fluctuating currents passing round the coils set up corresponding variations in the degree of force with which it attracted its armature of iron, and so forced the latter to execute corresponding mechanical vibrations. This principle is common both to the receiver of Reis, and to the later receivers of Yeates, Bell, and Edison. Reis’s armature was an iron bar of oval section; Yeates’s an iron strip screwed to a sound-board, Bell’s was an iron plate, and Edison’s an iron plate also.
For the better comparison of Reis’s electro-magnetic receiver with those of more modern date, we here present in Fig. 46 a comparative view of a number of different forms of receiver in which Reis’s principle of causing an electro-magnet to set up vibrations in an armature is applied. In this set of figures, A and B are the suggested forms mentioned in the letter of Mr. Horkheimer, p. 119, and show an electro-magnet, opposite the poles of which is placed an armature (a bar) which must be of iron or other metal capable of having magnetism induced in it, and which, by reason of its attachment to an elastic spring, is capable of being made to oscillate to and fro when attracted with a varying force. Reis clearly recognised the necessity of further providing a sufficient resounding surface by means of which the surrounding air could be set in motion: for in the case of these two suggestions the electro-magnet and its elastically-mounted armature were placed within a cigar box. C is a plan of the receiving instrument previously described and figured in Plate II. and in figures 21 and 34 on pages 32 and 109. In this instrument the electro-magnet was horizontal, the armature, a bar of iron of oval section (which in the original drawing in plate II. appears to have been in reality a hollow bar or tube) attached to a thin lever described as a plank, pivoted like a pendulum to an upright support, but prevented by a set-screw and a controlling spring from vibrating in the manner of a pendulum. Such an arrangement, in fact, vibrates in perfect correspondence with any vibrations that may be forced upon it by the electro-magnet. The broad flat surface of the lever—he specially directed that it should be broad and light—transfers the vibrations to the air, and is aided by the surface of the sounding-board on which the apparatus stands. This apparatus has, therefore, all the elements of a successful receiver, except only that its shape renders it inconvenient for portability. But by reason, firstly of its armature of iron, secondly of the elastic mounting of that armature, thirdly of the extended surface presented, it is admirably adapted to serve as an instrument for reproducing speech.
Fig. 46 D represents the excellent electro-magnetic receiver devised in 1865 by Yeates (compare Fig. 42, p. 128) to work with the Reis transmitter, and is in many respects identical with the preceding form. The armature, a strip of iron, was attached at one end by a very stiff steel spring to a pine-wood sounding-board over a hollow box, from the base of which rose the metal pillar which supported the electro-magnet. This receiver also contains all the elements of a successful receiver, the armature being of a material capable of inductive action, and elastically supported; whilst the sound-box provided adequate surface to communicate the vibrations to the air.
We now come to the more modern instruments of Gray, Bell, and Edison. So far the receivers of Reis and of Yeates were intended for reproducing any sound; but now for the first time, ten years after the date of these early telephonic receivers, we meet with instruments devised with the express purpose of receiving only certain selected tones.
For the purposes of multiple acoustic telegraphy, that is to say for the purpose of signalling the “dots” and “dashes” of the Morse code in a number of different fixed musical notes, each of which is to be signalled out and repeated by a receiver adapted to vibrate in that note alone, it is clear that the instruments of Reis, adapted as they were to transmit and receive any sound that a human ear can hear, would not answer. Accordingly those experimenters, who from about the year 1873 to the year 1870, applied themselves to multiple telegraphy—foremost amongst them being Mr. Elisha Gray and Prof. Graham Bell—dropped the use of the tympanum in the transmitter and devised new transmitters and new receivers, in most of which the ruling idea was that of employing a vibrating tongue or reed, tuned up to one particular note. Now it is obvious that a receiver which, like those of Reis, is adapted to receive any tone, can also receive a musical note. But for the operation of “selective” reception, a receiver must be employed, not only tuned to one note, but tuned to the very note emitted by the particular transmitter with which it is to be in correspondence. Elisha Gray found this out very early in his researches. In the winter of 1873-4[49] he was transmitting musical tones by a sort of tuning-fork interruptor, and received them on an instrument shown in Fig. 46 E, which represents a form of electro-magnet mounted for the purpose. It was “a common electro-magnet, having a bar of iron rigidly fixed at one pole, which extends across the other pole, but does not touch it by about one sixty-fourth part of an inch. In the middle of this armature a short post is fastened, and the whole is mounted on a box made of thin pine, with openings for acoustic effects.” It was, in fact, very similar to Yeates’s receiver just described, and Gray found it capable of receiving not only simple musical tones but composite tones, and even harmonies and discords. In fact, like Reis’s and Yeates’s receivers, it could receive anything that the transmitter sent to it, even including speech. Now this did not suit Gray, who wished to have selective receivers, one to take up note A, another note C, &c. Accordingly in 1870 we find Gray taking out a fresh patent[50] for selective receivers, which he also called harmonic analysers, each of which consisted of “a tuned bar or reed suitably attached to an electro-magnet, and the whole mounted upon a resonant box.” Fig. 46 F is reproduced from Gray’s British patent. “A vibrating tongue reed, or bar” of steel “is united with one pole of the magnet. The free end of the reed passes close to, but does not touch the other pole of the magnet.” Gray further says that the reed is made with parallel sides and tuned by cutting it away at one point, as this mode prevents false nodal vibrations from occurring.
Selective receivers for multiple telegraphy were also invented by Graham Bell. The form shown in Fig. 46 I is transcribed from Fig. 15 of Bell’s Specification to his British Patent, No. 4765, of the year 1876 (dated 9th December), which the inventor thus describes: “It is preferable to employ for this purpose an electro-magnet E, Fig. 15, having a coil upon only one of its legs. A steel spring armature A is firmly clamped by one extremity to the uncovered leg h of the magnet, and its free end is allowed to project above the pole of the covered leg.” In fact the arrangement was almost identical with, but not quite as good mechanically as that patented seven months previously by Gray. The inventor further said that a number of these instruments might be placed on one circuit, and that if one of them were set in vibration, only those would respond which were in unison with its note; and further that “the duration of the sound may be used to indicate the dot or dash of the Morse alphabet, and thus a telegraphic despatch may be indicated by alternately interrupting and renewing the sound.”
Anything more totally different from Reis’s telephone than these selective harmonic telegraphs with their tuned tongues can hardly be imagined. Reis was not aiming at selective harmonic telegraphy; he wanted his one instrument to transmit every sound that a human ear could hear. He did not dream of using a tuned bar or reed; his typical structure was the tympanum of the ear. In fact, as we have seen above, the tuned reed or tongue was introduced into telegraphy for the purpose of transmitting single selected notes to the exclusion of all others.
Strange though it may seem, a tongue receiver like those of Graham Bell and of Gray just described can be used for receiving speech! It is true, as Gray remarks, that a thick bar of steel, cut away as described, is best adapted for its own tone only. But Bell’s thin steel tongue, though it has its own fundamental note (and so has every tympanum, for that matter) when left free to vibrate in its own time, will reproduce any other note or sound that may be forced upon it by the varying attraction of the electro-magnet. There is, indeed, the whole difference between “free” and “forced” vibrations. One of the strangest delusions that has somehow grown up in recent telephonic discussions is the almost incredible proposition that a tongue cannot talk because it is a tongue. It would be equally veracious to affirm that an ear (i.e. a tympanum) cannot hear because it is an ear.
But leaving harmonic telegraphy and its “tuned bars,” both Gray and Bell applied themselves to the old problem of transmitting human speech. What was their very first step? They threw away their “tuned bars” and “steel springs,” and returned to the tympanum! Elisha Gray devised the receiver shown in Fig. 46, G, taken from his caveat of date February 14, 1876.[51] In that document Gray says: “My present belief is that the most effective method of providing an apparatus capable of responding to the various tones of the human voice, is a tympanum, drum, or diaphragm,” stretched across one end of a chamber. He adds that in the receiver there is (see Fig. 46, G) an electro-magnet, acting upon a diaphragm to which is attached a piece of soft iron, and which diaphragm is stretched across a vocalising chamber.
Graham Bell’s receiver (the American specification of which was filed the same day as Gray’s caveat) is shown (in the form patented in Great Britain, Dec. 9, 1876) in Fig. 46 H, which is taken from Fig. 19 of Bell’s British patent. “The armature,” says the inventor, “is fastened loosely by one extremity to the uncovered leg, h, of the electro-magnet E, and its other extremity is attached to the centre of a stretched membrane.” The armature, in fact, was capable of vibrating like a pendulum on its pivot, but was elastically restrained by its attachment to the tympanum; the armature would therefore vibrate in perfect correspondence with any vibrations forced upon it by the electro-magnet. This instrument as also that of Gray, was admirably adapted to receive speech, for it embodied the three essential points which Reis had already discovered: viz., firstly, that the armature must be of iron, or capable of being acted upon by magnetic induction; secondly, that it must be elastically mounted; thirdly, that it should present an extended surface. Bell’s form of receiver had the advantage over Reis’s (compare p. 158), that its extended surface was a true tympanum of membrane, and not a mere broad thin plank. Being a tympanum, it therefore realised Reis’s fundamental notion of imitating the human ear more fully than even Reis’s own receiver did.
Figures 46, J, K, and L represent the more recent types of receiver of Bell and Edison. Fig. 46 J is reproduced from Fig. 20 of Bell’s British Patent, and shows the substitution of a thin steel plate, attached to a frame, in front of the electro-magnet, for the membrane and iron armature. This form of instrument also embodies Reis’s three principles—but with this improvement, the armature capable of inductive action, the elastic mounting, and the extended surface, are here all united in one organ, the thin flexible tympanum of steel. Apart from this unification of parts there is absolutely nothing in this form of Bell’s receiver, that Reis did not invent fourteen years before. Bell’s great and most signal improvement was not this beautiful mechanical modification of the Reis receiver, but lay in the entirely new suggestion to use such a receiver as a transmitter to work by magneto-electric induction. Two of Reis’s receivers (Fig. 21) if coupled up with a battery will talk together as transmitter and receiver: but Reis did not know and never suggested this. Two of Yeates’s receivers (Fig. 42) if coupled up with a battery will talk together as transmitter and receiver; but Yeates did not know and never suggested this. Bell did discover this, and thereby invented a transmitter which, though now abandoned as a transmitter, for want of loudness, was more reliable than the anterior transmitters of Reis had been. He made another discovery, presently to be alluded to—that of putting a permanent magnet into the transmitter, to enable him to dispense with the battery; but beyond this and the other mechanical simplifications previously mentioned, all that he discovered may be summed up by saying that he found out that a receiver constructed on Reis’s principles could work as a transmitter also. That was Bell’s really great and important discovery which took all the world by storm at the Centennial Exhibition of 1876.
Bell subsequently added to his claims the substitution of a permanent magnet with an iron pole-piece, in place of the simple electro-magnet, thus enabling him to transmit his fluctuating currents without the trouble of using a battery, and the Bell transmitter, thus modified, is used to this day as a receiver, Reis had in his “knitting-needle” telephone, employed a permanent magnet of steel to serve as a receiver, he had not, however, applied it as Bell did to attract a plate of thin steel.
Fig. 46, K, exhibits a form of electro-magnetic receiver described in Edison’s British Specification, No. 2909, 1877, Fig. 24. This instrument, though patented seven months after Bell’s instrument, differs from it in no point of importance. Its armature was a thin plate of iron, elastic, and having an extended surface; being, in fact, a tympanum.
No one can examine the set of receiving instruments collected in Fig. 46 without being struck with the extraordinary similarity of design which pervades the entire series. In every one of the set there is an electro-magnet, the function of which is to set an armature[52] into vibration by attracting it with a variable force. In every one the armature is of a material capable of magnetic induction; that is to say, iron, steel, or equivalent material. In every one of them the armature is either elastically mounted, or is in itself elastic. In every one of them (save only the two quite recent forms, F and I, which were intended not to speak, but to emit only one fixed musical note) there is an extended surface (either a sound-board or a tympanum) to communicate the vibrations to the air. Lastly, every one of these forms, when connected with the line through which the telephonic currents are being transmitted, is perfectly capable of reproducing articulate speech. But the inventor who had the genius to discover all these essential points, and to combine them in an instrument, and to use it to reproduce articulate speech, is surely the true inventor of the system. The inventor of the system embodying these essential points was Philipp Reis.