# Encyclopædia Britannica, Ninth Edition/Telephone

See also Telephone on Wikipedia; Telephone in the 11th Edition; and the disclaimer.
TELEPHONE
TELEPHONY is the art of reproducing sounds at a distance from their source. The term was first used by Philip Reis of Friedrichsdorf, in a lecture delivered before the Physical Society of Frankfort in 1861. But, although this lecture and Reis's subsequent work received considerable notice, little progress was made until the subject was taken up between 1874 and 1876 by Alexander Graham Bell, a native of Edinburgh, then resident in Boston, Mass. Bell, like Reis, employed electricity for the reproduction of sounds; but he attacked the problem in a totally different manner. This will be better understood if we consider shortly on what the chief characteristics of sound depend (compare Acoustics).
Characteristics of sound.The sensation of sound is produced by rapid fluctuations in the pressure of the atmosphere on the tympanum of the ear. If the fluctuations are irregular and non-periodic, the sound is called a noise; if they are cyclic and follow a regular and sufficiently rapid periodic law, the sound is musical. In connexion with the present subject it is important to notice the three characteristics of a musical sound, namely, pitch, loudness, and quality. The pitch of a musical sound depends on the number of cycles passed through by the fluctuations of the pressure per unit of time; the loudness depends on the amount or the amplitude of the fluctuation in each cycle; the quality depends on the form or the nature of the fluctuation in each cycle. The necessary condition for a successful system of telephony is the ability to reproduce these characteristics.
I. History.
In 1831 Wheatstone by his ”magic lyre“ experiment showed that, when the sounding-boards of two musical instruments are connected together by a rod of pine wood, a tune played on one will be faithfully reproduced by the other. This only answers, however, for telephoning musical sounds to short distances. Mechanical telephone.Another and somewhat similar example is furnished by what has been variously designated as the “string,” “toy,” “lovers,” and “mechanical” telephone. Two disks of thin metal, or two stretched membranes, each furnished with a mouthpiece, are connected together by a thin string or wire attached at each end to the centres of the membranes. A good example may be made with two cylindrical tin cups; the bottoms form the membranes and the cups the mouthpieces. When the connecting string is held taut and sounds, such as those of ordinary speech, are produced in front of one of the membranes, pulses corresponding to the fluctuations of the atmospheric pressure are transmitted along the string and communicated to the other membrane, which in its turn communicates them to the air, thus reproducing the sound. In both these examples all the three characteristics—pitch, relative intensity, and quality of sound—are reproduced.
Page's discovery.Let us now return to the development of the application of electricity to telephony. In July 1837 Dr C. G. Page of Salem, Mass., drew attention to the sound given out by an electromagnet at the instant when the electric circuit is closed or broken, and in October of the same year he discussed, in a short article entitled “Galvanic Music,” the musical note produced by rapidly revolving the armature of an electromagnet in front of the poles. Experiments bearing on this subject were subsequently made by a great number of investigators. Page's discovery is of considerable importance in connexion with the theory of action of various forms of telephone, and was a very important feature in the early attempts by Reis to transmit music and speech. On ​128 TELEPHONE Bour- 26th August 1854 there appeared in L Illustration (Paris) seul s an interesting article by Charles Bourseul on the electric transmission of speech. 1 The writer recommended the use of a flexible plate at the source of sound, which would vibrate in response to the varying pressure of the air, and thus open and close an electric circuit, and of a similar plate at the receiving station, which would be acted on electromagnetically and thus give out as many pulsations as there are breaks in the current. These suggestions were to some extent an anticipation of the work of Reis ; but the conditions to be fulfilled before the sounds given out at the receiving station can be similar in pitch, quality, and relative intensity to those produced at the transmitting station are not stated, and do not seem to have been appreciated. Reis s In Reis s lecture an apparatus was described which has given rise to much discussion as to priority in the invention P one. Q ^ telephone The instrument was described in over fifty publications 2 in various countries, and was well known to physicists previous to Bell s introduction of the electric telephone as a competitor with the electric telegraph. Reis caused a membrane to open and close an electric circuit at each vibration, thus transmitting as many electric pulses through the circuit as there were vibrations in the sound. These electric pulses were made to act on an electromagnet at the receiving station, which, in accord ance with Page s discovery, gave out a sound of a pitch corresponding to the number of times it was magnetized or demagnetized per second. Reis s object was to re produce at a distance not only music but also human speech ; but that he did not wholly succeed is clear from the following extract from his lecture : " Hitherto it has not been possible to reproduce human speech with sufficient distinctness. The consonants are for the most part repro duced pretty distinctly, but not the vowels as yet in an equal degree." Considering the time at which he wrote, Reis seems to have understood very well the nature of the vibrations he had to reproduce, but he failed to compre hend how they could be reproduced by electricity. His fundamental idea the interruption of the current was a fatal mistake, which was not at the time properly under stood. The suggestion of Bourseul and the experiments of Reis are founded on the idea that a succession of currents, corresponding in number to the successive undulations of the pressure on the membrane of the transmitting in strument, could reproduce at the receiving station sounds of the same character as those produced at the sending station. Neither of them seemed to recognize anything as important except pitch and amplitude, and Reis thought the amplitude was to some extent obtained by the varying length of contact in the transmitting instrument. This might possibly be to a small extent true ; but, considering the small capacity of the circuits he used and the nature of his receiving instrument, it is hardly probable that dura tion of contact sensibly influenced the result. The quality of the sounds was to some extent also reproduced ; but, judging from the results of recent telephone investigation, it is highly probable that this was due, not to the varying duration, but to the varying firmness of the contact. Since the effect of the degree of contact has, through the re searches of Bell, Berliner, Edison, Hughes, Elisha Gray, and others, become generally understood, it has become easy to make instruments very similar to those of Reis ; 1 See also Didaskalia : Blatter furGeist,Gemuth,u.Publicitat, Frank fort, No. 232, 28th September 1854 ; Du Moncel, Expose des Appli cations de I ElectricitS, Paris, vol. ii. p. 25, ed. 1854, vol. iii. p. 110, ed. 1856, and Comp. Rend., 26th November 1877. 2 The English reader may consult Journ. Soc. Tel. Eng., March 1883 ; British Assoc. Rep., 1863 ; Civ. Eng. and Arch. Journ., vol. xxvi. p. 307 ; R. M. Ferguson, Electricity, London, 1866, p. 257 ; S. P. Thompson, Philip Reis, the Inventor of the Telephone, London, 1883. and even his instruments, with slight modification, can be made to speak fairly well. The accidental transmission of words by Reis, the occasional recognition of the voice of a singer, and other instances of the transmission of quality were no doubt due to this element, the existence of or the necessity for which was never, so far as the present writer knows, hinted at by Reis. The next worker at the telephone, and the one to whom Bell s re- the present great commercial importance of the instrument searches, is due, was Bell. His aim was the production, by means of the undulations of pressure on a membrane caused by sound, of an electric current the strength of which should at every instant vary directly as the pressure varied. 3 His first idea seems to have been to employ the vibrations of the current in an electric circuit, produced by moving the armature of an electromagnet included in the circuit nearer to or farther from the poles of the magnet. He proposed to make the armature partake of the vibrations of the atmosphere either by converting it into a suitable vibrator or by controlling its vibrations by a stretched membrane of parchment. In the early trials the armature had the form of a hinged lever of iron carrying a stud at one end, which pressed against the centre of a stretched membrane. The experiments with this form were not successful, and, with the view of making the moving parts as light as possible, he substituted for the comparatively heavy lever armature a small piece of clock spring, about the size of a sixpence, glued to the centre of the diaphragm. The magnet was mounted with its end carrying the coil op posite, and very close to, the centre of the piece of clock spring. This answered sufficiently well to prove the feasibility of the plan, and subsequent experiments were directed to the discovery of the best form and arrange ment of the parts. An increase in the size of the iron disk attached to the membrane augmented both the loud- ness and the distinctness of the sounds, and this finally led to the adoption of the thin iron disk now in use, which is supported round its edge, and acts as both membrane and armature. Again, the form of the opening or mouth piece in front of the membrane exercised considerable influence on the efficiency of the instrument, and it was ultimately ascertained that a small central opening, with a thin air space extending across the face of the membrane, was best. It was also found that comparatively small magnets were sufficient, and that there was no particular virtue in the closed circuit and electromagnet, but that a small permanent magnet having one pole in contact with the end of the core of a short electromagnet, the coil of which was in circuit with the line, but which had no per manent current flowing through it, answered the purpose quite as well. 4 In fact the effect of keeping a permanent current flowing through the line and the coils of the electromagnet was to keep the core of the electro-magnet magnetized. This seems to have been almost simul taneously pointed out by Bell and others who were work ing in conjunction with him and by Professor Dolbear. Many experiments were made for ascertaining the best length of wire to use in the coil of the transmitting and the receiving instrument; but this is clearly a question dependent to a large extent on the nature of the line and the system of working adopted. After Bell s success a large number of experimenters entered the field, and an almost endless variety of modi fications have been described. But few possess any real merit, and almost none have any essentially new principle. 5 3 See A. G. Bell, "Telephone Researches," in Journ. Soc. Tel. Eng., 31st October 1877. 4 The extreme smallness of the magnets which might be successfully employed was first demonstrated by Professor Peirce of Brown Uni versity, Providence, R.I. 6 For a detailed description, jn a collected form, of a large number ​TELEPHONE 129 Ellison s trans mitter and fric tion re ceiver. Elisha Gray s experi ments. Dolbear s con denser tele phone. A telephone transmitter and a receiver on a novel plan were patented in July 1877 by Edison, shortly after the introduction of Bell s instruments. The receiver was based on the change of friction produced by the passage of an electric current through the point of contact of certain substances in relative motion. In one form a drum, mounted on an axis and covered by a band of paper soaked in a solution of caustic potash, is turned under a spring the end of which is in contact through a platinum point with the paper. The spring is attached to the centre of a diaphragm in such a way that, when the drum is turned, the friction between the point of the spring and the paper deflects the diaphragm. The current from the line is made to pass through the spring and paper to the cylinder. Now it had been previously shown by Edison that, when a current is made to pass through an arrange ment like that just described, the friction between the paper and the spring is greatly diminished. Hence, when the undulating telephonic currents are made to pass through the apparatus, the constant variation of the friction of the spring causes the deflexions of the diaphragm to vary in unison with the variation of the electric currents, and sounds are given out corresponding in pitch, and also to some extent in quality, with the sounds produced at the transmitting station. A cylinder of chalk was used in some of Edison s later experiments with this receiver. The transmitter is illustrated (see fig. 10) and described (p. 132) below. Experiments very similar to these of Edison were made by Elisha Gray of Boston, Mass., and described by him in papers communicated to the American Electrical Society in 1875 and 1878. In these experiments the electric current passed through the fingers of the operator s hand, which thus took the place of the spring in Edison s ap paratus. The diaphragm was itself used as the rubbing surface, and it was either mounted and rotated or the fingers were moved over it. When the current passed, the friction was felt to increase, and the effect of sending a rapidly undulating current through the arrangement was to produce a sound. The application of this apparatus to the transmission of music is described by Gray. 1 In another form of telephone, brought prominently forward by Professor Dolbear, 2 the effects are produced by electrostatic instead of electromagnetic forces, as in the Bell telephone. Sir W. Thomson observed in 1863 3 that when a condenser is charged or discharged a sharp click is heard, and a similar observation was made by Cromwell F. Varley, who proposed to make use of it in a telegraphic receiving instrument. 4 In Dolbear s instrument one plate of a condenser is a flexible diaphragm, connected with the telephone line in such a way that the varying electric potential produced by the action of the transmitting tele phone causes an increased or diminished charge in the condenser. This alteration of charge causes a correspond ing change in the mutual attraction of the plates of the condenser ; hence the flexible plate is made to copy the vibrations of the diaphragm of the transmitter. It is obvious that this apparatus may be used either as a transmitter or as a receiver, but that the effects must under ordinary circumstances be in either case extremely feeble. In the Reis instruments the transmitter and receiver are separate parts, which are not interchangeable. The Bell telephone can be used either as a transmitter or as a of these modifications, see Du Moncel, " Le Telephone," in Bibliotheque des Merveilles, Paris, 1882. 1 See George B. Prescott, The Speaking Telephone, London, 1879, pp. 151-205. 2 Scientific American, 18th June 1881. 3 Electrostatics and Magnetism, p. 236. 4 See Tel. Journ., 1st August 1877, p. 178 ; also Adams, Journ. Soc. Tel. Eny., 1877, p. 476. receiver. The Edison receiver and the Dolbear condenser were only intended to be used as receiving instruments. It was very early recognized and, indeed, is mentioned Liquid in the first patents of Bell, and in a caveat filed by Elisha trans- Gray in the United States patent office only some t hours after Bell s application for a patent that sounds and spoken words might be transmitted to a distance by Gray, causing the vibrations of a diaphragm to vary the re sistance in the circuit. Both Bell and Gray proposed to do this by introducing a column of liquid into the circuit, the length or the resistance of which could be varied by causing the vibrations of the diaphragm to vary the depth of immersion of a light rod fixed to it and dipping into the liquid (see figs. 8, 9 below). This idea has been per haps the most fruitful of any modification of telephonic apparatus introduced. On 4th April 1877 Mr Emile Berliner filed a caveat inBer- the United States patent office, in which he stated that, liner s on the principle of the variation with pressure of the resist- !^" ance at the contact of two conductors, he had made an trans- instrument which could be used as a telephone transmitter, mitter. and that, in consequence of the mutual forces between the two parts of the current on the two sides of the point of contact, the instrument was capable of acting as a receiver. The caveat was illustrated by a sketch showing a diaphragm with a metal .patch in the centre, against which a metal knob was lightly pressed by an adjusting screw. This seems to have been the first transmitter in which it was proposed to use the resistance at the contact of two conductors. Almost simultaneously with Berliner, Edison conceived Edison s the idea of using a variable resistance transmitter. 3 He micro- proposed to introduce into the circuit a cell containing P. carbon powder, the pressure on which could be varied by m itter. the vibrations of a diaphragm. He sometimes held the carbon powder against the diaphragm in a small shallow cell (from a quarter to half an inch in diameter and about an eighth of an inch deep), and sometimes he used what he describes as & fluff ^ that is, a little brush of silk fibre with plumbago rubbed into it. In another form the plum bago powder was worked into a button cemented together with syrup and other substances. In the specification of the patent applied for on 21st July 1877 he showed a sketch of an instrument which consisted of a diaphragm, with a small platinum patch in the centre for an electrode, against which a hard point, made of plumbago powder cemented together with india-rubber and vulcanized, was pressed by a long spring, the pressure of the carbon against the platinum disk being adjusted by a straining screw near the base of the spring. Subsequently he filed an application for a patent in which various forms of springs and weights assisted in maintaining the contacts and otherwise improved the instrument. In the early part of 1878 Professor Hughes, while en-Hughes s gaged in experiments upon a Bell telephone in an electric micro- circuit, discovered that a peculiar noise was produced when- P none&lt; ever two hard electrodes, such as two wires, were drawn across each other, or were made to touch each other with a variable degree of firmness. Acting upon this discovery, he constructed an instrument which he called a microphone, 6 and which consisted essentially (see fig. 11) of two hard carbon electrodes placed in contact, with a current passing through the point of contact and a telephone included in the same circuit. One of the electrodes was attached to a sounding board capable of being vibrated by sound waves, and the other was held either by springs or weights 5 See Journal of the Telegraph, New York, April 1877 ; Philadelphia Times, 9th July 1877 ; and Scientific American, August 1877. 6 This term -was used by Wheatstone in 1827 for an acoustic ap paratus intended to convert very feeble into audible sounds ; see his Scientific Papers, p. 32. XXIII. 17 ​130 TELEPHONE Radio phone. in delicate contact with it. When the sounding board was spoken to or subjected to sound-waves, the mechanical re sistance of the loose electrode, due to its weight, or the spring, or both, served to vary the pressure at the contact, and this gave to the current a form corresponding to the sound-waves, and it was therefore capable of being used as a speaking-telephone transmitter. 1 The best transmitters now in use are modifications of Hughes s apparatus. A microphonic apparatus very similar to it is described in the specification of a German patent taken out by Robert Lutdge on 12th January 1878. In this patent the action of the microphone is also described. 2 The next transmitter of note, introduced by Mr Francis Blake, U.S. (see fig. 13 below), although it does not, like the first microphones, embody anything intrinsically new, is one of the most perfect and convenient forms of micro phone. It is at present almost universally used in the United States. It appears to be pretty well established that carbon in one form or another is the best material for one or both of the contacts of a microphone transmitter. When both the contacts are of carbon and the surfaces have consider able area, say from a quarter to half an inch in diameter, the sounds are loud, but have a tendency to harshness. When, as in the Blake transmitter, one of the contacts is a piece of polished gas carbon and the other a small sphere of platinum about the twentieth of an inch in diameter, the articulation is clear, but less loud. For most purposes, however, the increased clearness more than compensates for the diminished loudness. Many transmitters in actual use as, for instance, the " Gower," largely employed in the United Kingdom have a number of contacts. Some of these when properly adjusted are both loud and clear in their action. Although the Blake instrument is most in vogue in America, in the United Kingdom and on the Continent multiple contact microphones have found more favour. Carbon powder instruments have been to some extent used, and in one or two cases as, for example, the Hunnings transmitter with considerable success. The fault in most of them is the tendency of the powder to " pack," which causes the instrument to rapidly lose sensi bility. In the Hunnings transmitter this difficulty is to a large extent overcome by the use of a coarse granular powder in a somewhat large cell (about an inch in diameter and from one-eighth to one-fourth of an inch deep). The front face of the cell is a piece of platinum foil, which serves both as an electrode and as a diaphragm. The cell is placed either on edge or in an inclined position when in use, the action being precisely similar to that in other transmitters. In addition to its freedom from packing, the carbon, in consequence of the inclined position of the cell, is also less liable to fall away from the electrode and break the circuit. Some packing of the powder, however, does occur, and several modifications have been proposed by Blake and others for making the sound vibrations stir the powder and keep it loose. Good results appear to have been got by placing the cell mouth downwards, the carbon powder lying on the platinum foil, and by forming the upper electrode either of wire gauze or of a perforated plate completely immersed in the powder. The sound vibrations are con veyed to the bottom of the cell by a bent tube communi cating with a mouthpiece. Instruments of this class are very loud-speaking, and therefore very serviceable for long or disturbed circuits. The radiophone is an instrument proposed by A. G. Bell 1 See Proc. Roy. Soc., vol. xxvii. p. 362 ; Proc. Phys. Soc.,vo. ii. p. 255 ; Phil. Mag. , 5th ser., vol. vi. p. 44 ; Preece, Journ. Soc. Tel. Eng. , vol. vii. p. 270. 2 Although this patent is dated prior to Hughes s publications, it does not follow that the descriptions were filed before these. and Sumner Tainter in 1880 for utilizing radiant energy, such as light or radiant heat, for the transmission of sound. The apparatus forms a telephone transmitter of a particu larly interesting kind. In the earlier papers describing it and the experiments which led to its invention it is called photophone, because at that time the effects were supposed to be wholly due to light. Afterwards, in order to avoid ambiguity, Bell changed the name to radiophone and sug gested that, to distinguish between instruments depending on the different kinds of radiation, the names photophone, thermophone, &c., should be employed. He also proposed the name spectrophone for an application of this instrument to spectrum investigation. 3 The apparatus is founded on the discovery, made by Mr May while carrying out experi ments on selenium for Mr Willoughby Smith, that when selenium is exposed to light its electrical resistance is very different from what it is in the dark. This discovery led to a great many interesting experiments by other investi gators. 4 In thinking over this discovery in 1878 Bell con ceived the idea that, if a beam of light proceeding from one station could be made to fall on a selenium plate at another station, and if its intensity could be varied by the voice of a speaker, then by connecting a telephone and a battery in circuit with the selenium plate the words spoken at the distant station would be heard in the telephone. This was found to be the case. At first, to vary the intensity of the beam, it was passed through a small opening, the width of which could be varied by the vibrations of a diaphragm against which the speech was directed. But better results were afterwards obtained when the diaphragm formed a mirror from which the beam of light was reflected. The spreading of the beam, due to the vibrations of the mirror diaphragm, served to vary its intensity (see fig. 18 below). Edison s phonograph (see fig. 19 below) is an instrument Edison s whose action somewhat resembles that of a telephone trans- phono- mitter and which has been much talked of in regard to its possible applications in telephony. It was invented shortly after the introduction of the telephone for the purpose of recording sounds, and was included in some of Edison s telephone patents as a means of working a telephone trans mitter, and thus telephoning sounds which had been pre viously recorded on the phonograph sheets. II. TELEPHONIC INSTRUMENTS. One of the best-known forms of the Reis telephone is shown in fig. 1. The transmitter consists of a box A, provided with a mouth piece M. In the top of the box a round hole is cut and across it a membrane S of hog s bladder is stretched. A thin strip of platinum p fixed to the box at one side of the hole and extend ing to the centre of the membrane, supports at that point one foot of a light metal tripod egf. One of the feet, e or /, rests in a cup containing mercury, which is in metallic connexion with the terminal b, while FIQ. 1. Reis s telephone. 3 On this subject see A. G. Bell, Phil. Mag., 5th ser., vol. xi. p. 510, and Journ. Soc. Tel. Eng., vol. ix. p. 404 ; Mercadier, Phil. Mag., 5th ser., vol. xi. p. 78 ; Tyndall, Proc. Roy. Soc., vol. xxxi. p. 307 ; Routgen, Phil. Mag. , 5th ser. , vol. xi. p. 308 ; Preece, Proc. Roy. Soc. , vol. xxxi. p. 506 ; Rayleigh, Nature, vol. xxiii. p. 274, and Proc. Roy. Soc., 1877; Bidwell, Phil. Mag., 5th ser., vol. xi. p. 302; S. P.Thomp son, Phil. Mag., 5th ser., vol. vi. p. 276. 4 See W. Smith, Journ. Soc. Tel. Eng., vol. v. p. 183, and vol. vi. p. 423 ; M. L. Sale, Proc. Roy. Soc., vol. xxi. p. 283, and Phil. Mag., 4th ser., vol. xlvii. p. 216 ; Draper and Moss, Proc. Roy. Irish Acad., vol. i. p. 529 ; Rosse, Phil. Mag., 4th ser., vol. xlvii. p. 161 ; W. G. Adams, Proc. Roy. Soc., vol. xxiii. p. 535 and vol. xxiv. p. 163 ; W. G. Adams and B. E. Day, ibid. , vol. xxv. p. 113 ; Werner Siemens, Monatsber. kiin. Preuss. Akad. der Wissensch. zu Berlin, 1875, p. 280, andPM. Mag., 4th ser., vol. L p. 416 ; Sabine, Phil. Mag., 5th ser., vol. v. p. 401. ​TELEPHONE 131 FIG. 2. Bell s first telephone ; one- fifth full size. the end of the stripy is similarly in connexion with the terminal a. The receiver consists of an electromagnet made up of a magnetiz ing coil H, with a stout knitting needle for a core. When in use these two instruments are joined in circuit with a battery B, so that under ordinary circumstances a continuous current is flowing through the line. Suppose a sound is then produced in front of the mouthpiece M, the successive variations in the pressure of the air are communicated to the inside of the box, and cause the mem brane to vibrate in unison with the sound. Reis s theory of the action of the instrument was that at each outward impulse of the membrane the point g would be thrown out of contact with the plate underneath it and would thus break the circuit. There would con sequently result as many breaks in the circuit as there were vibra tions in the sound, and, in conformity with Page s discovery, the electromagnetic receiver would give out a rapid succession of beats, which would together form a continuous sound of the same pitch as that to which the transmitter was subjected. Bell s Fig. 2 shows the first telephone made by Bell for transmitting first tele- speech. It consisted of a wooden frame F, to one side of which a phone, tube T was fixed ; over the end of the tube a membrane M was stretched taut by a stretching ring R. To the opposite side of the frame and with its axis in line with that of the tube T was fixed an electromagnet H, and between the membrane M. and the end of the electromagnet a hinged arma ture A was arranged in such a way that its motions would be con trolled by the membrane. The instrument was joined in circuit with a battery and another simi lar instrument placed at a dis tance. A continuous current was made to flow through the circuit, which kept the electromagnet magnetized. Bell reasoned thus : when words are spoken in front of the tube T the membrane will be set in vibration and with it the armature A, and the vibration of the armature in front of the electro magnet will induce variations in the line current ; their magnitude will be proportional to the amplitude, and their frequency to the frequency, of the vibrations of the armature ; in fact, the difference between the actual and the average current in the circuit will be at each instant proportional to the rate of motion of the armature. It follows from this that the armature and membrane of the distant instrument should have induced in them a motion precisely similar to that of the membrane of the transmitter. This telephone was made in June 1875, but was put aside after trial as unsatisfactory on account of the feebleness of the sounds it produced ; since then, however, a successful telephone has been made on precisely the same plan as that here indicated. The next form tried is shown in fig. 3. It is very similar except in constructive details to the first ; the hinged armature, however, is omitted, its place being taken by a small iron disk A fixed to the centre of the diaphragm D. The electro magnet H is, as before, placed so as to have the centre of the soft iron core C opposite to the centre of the disk, and the theory ac cording to which it Fl - 3. Bell s second telephone ; one-fifth full size, was expected to act is the same. The results obtained with this instrument were much more satisfactory ; indeed it was with one precisely like that shown in the figure that the remarkable results of the Philadelphia exhibition in 1876 were obtained. A perspec tive and a sectional view of the receiving instrument used along with that shown in fig. 3 are illustrated in figs. 4 and 5. It consisted of an iron cylindrical box B, through the axis of which a rod of soft iron C was passed to form the core of an electromagnet, having the magnetizing helix H wound on the upper half of its length. Fig. 4. Fig. 5. Across the top of the FIGS. 4, 5. Bell s iron box receiver (1876). box a thin disk D of Fi - 4 &gt; perspective view ; fig. 5, sectional view, soft iron was fixed, the core C being just clear of the disk when the strongest current is flowing through the helix. In the per spective view the disk is removed, showing the end of the core. These instruments are interesting, not only because they may be considered the first really successful speaking telephones, but be cause they are of the same form as those brought to Great Britain in 1876 by Sir W. Thomson, and exhibited before the British Asso ciation at Glasgow in that year. Fig. 6 shows one of the earliest forms brought into commercial use. On each pole of a somewhat large horse-shoe permanent magnet FIG. 6. Bell s multiple pole telephone (1877) ; one-fifth full size. M a short coil E with a soft iron core was fixed. This is one of the early forms of permanent magnet telephones, of which there were at that time several, including a hand telephone very similar to that shown in fig. 7. In another form, introduced about the end of 1877, the small magnetizing coils and soft iron cores were fixed on the side and opposite the poles of the horse-shoe magnet, and the diaphragm was placed with its plane parallel to that of the magnet. The diaphragm in these telephones was of thin sheet iron and a little over 4 inches in diameter. The form of telephone now almost universally in use is shown in Bell s fig. 7. It was introduced in December 1877 and consists of a com- hand magnet M, fitted into the centre of a tele phone. pound permanent tube of vulcanite or " hard rubber " and carrying at one end a short electro magnet, the coil of which through its terminals . * is in- ESTCCGI r&gt; . ** . i j j Iv. Hi^ Fl - 7. Bells hand telephone, present form, eluded in the cir- cuit when the instrument is in use. In front of the electromagnet, with its plane normal to the axis of the magnet, is fixed a thin soft iron disk about If inches in diameter, which has its cover cut to a convenient shape to form a mouthpiece. This telephone acts well either as a transmitter or as a receiver ; but for the former purpose it is now seldom used on account of the great advances which have been made in " microphone " transmitters. It has been stated that Bell and Elisha Gray almost simultane- Bell s ously suggested the use of a column of liquid to vary the resistance liquid in the circuit. The form of instrument proposed by the former and said to have been exhibited at the Philadelphia exhibition is shown in fig. 8. It con sists of a speaking tube or mouthpiece M, across the lower end of which a membrane D is stretched. To the centre of the mem brane a light rod R, made of metal or of carbon, is fixed with its length at right angles to the plane of the membrane. Under the lower end of R a small metallic vessel C is sup ported on a threaded rod, working in a nut fixed to the sole F, so that its height may be readily adjusted. Fig.~8. Fig. 9. Suppose C to be filled with FIG. 8. Bell s liquid transmitter, water or any other con- FIG. 9. Elisha Gray s liquid transmitter, ducting liquid, and the rod R to be of metal. C is raised until the liquid just touches the point of the rod, when advantage is taken of the change of contact resistance with the greater or less immer sion of R during the vibration of D. Good results were obtained with mercury as the liquid and with a rod of carbon. The arrangement proposed by Elisha Gray is almost identical in E. Gray s form with Bell s. The only difference seems to be that Gray in- liquid tended the rod R (fig. 9) to reach near to the bottom of the vessel trans- B or to the end of another rod, a prolongation of b, projecting up mitter. from the bottom. The variation of the current was produced by the variation of the distance between the ends of the rod caused by the vibrations of the diaphragm. This plan was not tried until after the success of Bell s experiments was known, and when it was ​132 TELEPHONE Edison s micro phone trans mitter. tried the results did not prove encouraging. Indeed the variations of the resistance which can be produced in this way must be excess ively small, unless the liquid has a very high specific resistance, the distance between the ends is very small, and the sides of the rods are prevented by an insulating covering from interfering with the results. Neither of these transmitters has any great merit as such, but they show that both Bell and Gray clearly recognized the principle on which successful transmission of the different forms of sound, including speech, could be accomplished. The first successful microphone transmitter was Edison s. An early form of it (fig. 10) somewhat resembles Bell s hand tele- Hughes s micro phone. phone in ex- cell of insulating its bottom a flat- screw G ; on the layer of carbon top of that a thin D, and above cover of the cell, position by a centre of this rubber tubing, the diaphragm hand telephone, is held in M. The varying pressure duced near it, causes corre- pressure on the carbon similar variations in its elec- when the instrument is in- cuit through which a cur- tions in the pressure on the spending undulations in ternal form. A material has at headedplatinum top of G is a powder C, on the platinum disk that, forming the a disk of ivory B, held in ring E. Resting on the disk is a small piece of which is lightly pressed by A, and this, as in the position by the mouthpiece on A, when a sound is pro- sponding variations in the powder, and this produces trical resistance. Thus, eluded in an electric cir- rent is flowing, undula- diaphragm produce corre- the current. Perhaps the best known p IG 10 _ E j;. forms of the microphone are those introduced by son s micro- Prof. Hughes. One of the commonest is shown phone trans- i n fig. H. It consists of two rectangular pieces of wood, B and D, fixed to gether with their planes at right angles to each other. D forms the base, and to B two small blocks of carbon C, C are attached. A of the same small cups formed two electrodes e, e pose of inserting circuit. The ma- most suitable for was wood charcoal Between these a light rod material is supported on in C, C. To the blocks are connected for the pur- the instrument in an electric terial which Hughes found the carbon blocks and rod metallized by heating it to redness and plunging it while hot into mercury. If this microphone is joined in circuit with a telephone and a small battery, say one or two small Daniell cells, the vibration pro duced by a fly walking on Fl - n. Hughes s microphone, the base D can be distinctly heard in the telephone. The same apparatus will also act as a microphone transmitter, but the sounds are apt to be harsh. A better form for this purpose is shown in is pivoted at h and carbon c, c, the Blake trans mitter. fig. 12. In this a light pencil of carbon M has one end resting on two blocks lower one being fixed to the base. The pressure of M on the carbon block is regulated by a spring s. This ar- Flo 12 ._ M icrophone transmitter, rangement is en closed in a box of thin wood, against which the sound is directed. It is capable of acting well as a transmitter, and especially in a modified form used by Hughes as a microphone receiver. The lower block c is then attached to the centre of a vertical diaphragm and against it the sounds are directed. The Blake transmitter, which is perhaps most widely used of all, is a simple modification of the Hughes instrument last described. It consists (fig. 13) of a frame F, to which is attached a diaphragm D of thin sheet iron ; in front of this is a cover M, M provided with a suitable cavity for directing the sound-waves against the dia phragm. The microphonic arrangement consists of a spring S, about the hundredth of an inch thick and the eighth of an inch broad, fixed at one end to a lever L, and carrying at its free ex tremity a brass block W. In one side of W a small disk C of gas carbon is inserted, resting on the hemispherical end of a small platinum pin K, about the twentieth of an inch in diameter, held in position by a thin spring A. The pressure of the carbon on the platinum point can be adjusted by the screw N, which turns the lever about the flexible joint G. The electrical connexions of the instrument as arranged for actual use are also illustrated in the figure. The current circuit goes through S, W, C, K, A, and the primary circuit of the induction coil I to the battery B, and thence to S again. This forms a local circuit at the transmitting station. The line of circuit passes through the secondary of the induction la
Fio. 13. Blake s transmitter. coil I to the line, from that to the telephone T at the receiving station, and then either to earth or back to the induction coil by a return line of wire. Telephonic Circuits. The lines used for telephone purposes are, generally speaking, Tele- so far as erection, mode of insulation, and so on ate concerned, much phone the same as those used for ordinary telegraphs. In towns where wires. a very large number of wires radiate from one centre or exchange, as it is called, where thick wires are unsightly, and where it is often necessary to provide for long spans, a comparatively thin wire of strong material is employed. For this reason various bronzes, such as silicon, aluminium, &c., have come to be extensively used for making wires for telephone lines. They are made from about the twentieth to the thirtieth of an inch in diameter, and are found to wear well in the somewhat mixed atmosphere of a town ; and owing to their lightness and considerable tensile strength it is com paratively easy to erect them and keep them in order. The main objection to them is the high electrical resistance they oppose to the current. The lines on a town exchange system are not, how ever, as a rule, so long as to make this objection of great import ance. But long lines, such as those extending between towns some miles apart, should be made of pure copper wire hard drawn. It has lately been found possible to draw copper so hard as to be almost equal to bronze in strength, and yet to retain about three times the electric conductivity of that substance. Copper and bronze wires possess great advantages for telephonic purposes over the iron wires employed in telegraph lines, in that they offer a much lower effective resistance to the rapidly undulating and intermittent currents pro duced by telephonic transmitters. The electric resistance opposed by a wire to the passage of such a current is always greater than that opposed to a steady current, and this difference is much more marked when the wire is of magnetic material like iron. This in creased resistance rises in proportion to the rapidity of the undula tions of the current ; consequently high notes are more resisted than low notes. Besides this variable resistance, telephony has to contend with "self-induction" (see ELECTRICITY, vol. viii. p. 76 sq.) of the current on itself, and this is by no means unimportant, especially on long circuits. 1 The marked difference between iron and copper for long circuits is plainly shown by the fact that Rysselberg and others have spoken clearly to a distance of over 1000 miles through a copper wire insulated on poles, whereas Preece could not work a similar line of iron wire between London and Manchester. The electrostatic capacity of the line (see TELEGRAPH, p. 115 Capacity, above) is also diminished by the use of thin wires of highly con ducting material. They should all if possible be erected on poles at a considerable height above the earth. It is not practicable to work an ordinary underground line through more than 20 miles, and cable telephony through distances of over 100 miles may in the present state of science be put down as an impossibility. Another element of great importance in connexion with telephone 1 Sec papers by Prof. Hughes, Proc. Soc. Tel. Eng., vol. xv. p. 6, and Proc. Roy. Soc., vol. xl. p. 4(58, with remarks on them by Prof. H. F. Weber, Tel. Journ., vol. xviii. p. 321 and vol. xix. p. 30 ; by Oliver Heaviside, Phil. Mag., vol. xxii. p. 118 ; by Rayleigh, Phil. Mag., vol. xxi. p. 381 and vol. xxii. p. 469. See also Prof. Chrystal on the " Differential Telephone," in Trans. Roy. Soc. Edinb., vol. xxxi. pp. 609-636. ​TELEPHONE 133 Indue- lines, which in most cases does not require to be attended to in tion. ordinary telegraph circuits, is the induction from one line to an other (see ELECTRICITY, vol. viii. p. 76 sq. ). When two lines having, as in ordinary telegraphy, an earth connexion at each end run for any great distance, say a mile or more, parallel to each other on the same supports, a conversation Avhich is being carried on through one of them can be overheard by means of the telephones on the other. This is due to the fact that, when a current is suddenly set up in one closed circuit, it induces an instantaneous current in any other closed circuit which is near to it. This induced current not only destroys the privacy of the circuit in question but also lowers its efficiency. The mischief is even greater when telegraph and telephone lines run along the same route supported on the same poles, because the strong intermittent currents sent through telegraph wires, and the irregular manner in which the intermit- tences follow each other, induce a series of such powerful secondary currents in the telephone lines that the noise heard in the tele phone is often sufficient, when the line is a mile or two long, to Methods drown all speech. In the case of parallel telephone lines the best, for over- if not the only, cure is to use return wires, and arrange them so that coming the currents induced in the outgoing wire shall be neutralized by the indue- corresponding current induced in the incoming wire. For mixed tion. telegraph and telephone circuits various methods have been pro posed ; but the most generally approved plan is to have return wires. For circuits worked wholly on the return principle the main thing to be attended to is the symmetrical arrangement of the wires, so that the outgoing and incoming wires may be subjected to the same influence. This is nearly provided for by running them in such a way that they may be all supposed to lie on the surface of a cylinder in lines parallel to its axis, the two wires at the opposite ends of a diameter being always used for the same circuit. When more than four wires form the group complete compensation is not obtained in this way, because the current is always stronger near the transmitting end of the line than near the receiving end, on account of the very sensible effect of the capacity and the leakage of the line. It is therefore best to arrange the wires in groups of four that is, in pairs of circuits and run them so as to form spiral lines round an axial line equidistant from each of the four wires. Any pair of wires forming a circuit which runs parallel to other wires can be arranged so as to be very nearly free from induction by interchanging their position relatively to the other wires at short distances along the line. Care must, however, be taken, when more than one group of four or when more than one pair are run, that the compensation produced by the twisted arrangement of one set, or of the interchanges of the wires in the different pairs, is not spoiled by the twisting or interchanging of another set or pair. Telephone lines running parallel to telegraph lines should be formed into one or more groups, each being run on the twist plan so as to eliminate as completely as possible the effect of the telegraph signals ; the smajl residual effect of the telephone signals is of comparatively little importance in such a case. A twisted cable of telephone wire may, when each circuit is formed by diametrically opposite wires, be placed in the same tube with similar cables employed for telegraph purposes. The central wire of the cable may be used either as a telegraph line or as a telephone line having an earth return. Another method is to use powerful telephone transmitters and insensitive receivers ; that is to say, make the telephone currents so powerful that the telegraphic induced currents will be small in comparison, and use receivers so insensitive as to suit such currents. One of the main obstacles in the way of this method at present is the difficulty of getting strong telephonic currents, for even the best transmitters are not yet sufficiently powerful, and there is, besides, a decided tendency towards a loss of quality in the sound when the transmitter is made powerful. A third method is to render the telegraphic current comparatively harmless by taking away the suddenness of the intermittences. This is quite possible because the number of currents sent per second, even on fast working circuits, is not such as to produce a high musical note. If, then, the currents be made in some way to rise slowly to their full strength and fall again slowly to zero the diaphragm of the receiving instrument, instead of showing the sudden rise and sudden fall as at present, would move so slowly backwards and forwards that the ear would not be disturbed by the sound. Perhaps the simplest way to accom plish this is to place an electromagnet in the circuit of the tele graph line at the sending station, for the self-induction of the magnet coil prevents the current assuming its strength suddenly. But on telegraph circuits where speed is of great importance this method cannot be followed owing to the retardation of the telegraph signals and the consequent loss of speed thereby occasioned. An ingenious application of the method of compensation just indicated has been made by Rysselberg, who has used not only wires carried on the same poles as the telegraph but even the telegraph lines themselves for telephone purposes. The arrangement of his system is shown in fig. 14, where L and Lj represent two telegraph lines. Between these, at each end, are inserted two condensers Cj, C 2 and a telephone T, together with transmitters, &c. , so that, supposing the telegraph instruments removed, the two wires would be an ordinary telephone circuit worked through condensers. The telegraph apparatus consists of an ordinary receiver R, sending battery B, and key K, together with a con denser C, inserted be tween the earth and the line terminal of the key, and two electromag netic inductors E, E . When the key is de- retarded by the electro- denser C, which has to fact additional electro- pressed the current is magnet E and the con- charged, giving in static capacity at the The current is still electromanet E sending end of the line. | u*h further retarded by the Fig. 14. hence the condenser C x becomes charged so gradually that very little disturbance is noticeable in the telephone T. The condensers Cj, C 2 prevent leakage from one line to the other, but have suffi cient capacity to allow the telephone to act as if it were in a metallic circuit. The Working of Telephone Circuits. The method first employed for working a telephone line was Early extremely simple. A single line of wire, like an ordinary telegraph methods, line, had a Bell telephone included in it at each end and the ends were put to earth. Words spoken to the telephone at one end could be heard by holding the telephone to the ear at the other. To obviate the inconvenience of placing the telephone to the mouth and the ear alternately, two telephones were commonly used at each end, joined either parallel to each other or in series. The con trivance most generally adopted for calling attention is the call bell, rung either by a small magneto-electric machine or by a battery. The telephone was switched out of circuit when not in use and the bell put in its place, an ordinary key being used for putting the battery in circuit to make the signal. This arrangement is still employed, a hook being attached to the switch lever so that the mere hanging up of the telephone puts the bell in circuit. In some cases, when the bell is rung by a magneto machine, the coil of the machine is automatically cut out of circuit when it is not in action, but the turning of the handle moves a centrifugal arrangement by which it is thrown in. At first it was usual to employ the same instrument both as trans- Working mitter and as receiver, and to join it in the direct circuit. But it with was soon found that the microphone transmitter could only be used micro- to advantage in this way when the total resistance of the circuit, phone, exclusive of the microphone, was small compared with the resistance of the microphone, that is, on very short lines worked with low resistance telephones. The transmitter on long and high resistance lines worked better by joining indirectly in a local circuit, in the manner shown in fig. 13, the microphone, a battery, and the primary of an induction coil, and putting the line in circuit with the second ary of the induction coil, which acted as the transmitter. The resistance of the microphone can thus be made a large fraction of the total resistance of the circuit in which it is placed ; hence, by using considerable currents, small variations in its resistance can be made to induce somewhat powerful currents in the line wire. The requisite energy is derived from the battery. If there are other resistances in the circuit it is, in some cases, better to join it as a shunt to the primary circuit of the induction coil. It may even prove advantageous to insert resistances in the circuit, increase the battery power, and join the microphone as here indicated, because in this way powerful currents can be obtained in the line without the harshness which is apt to be produced by the variations of a strong current passing through the microphone. Translation from one line to another, or from one section to Transla- another of the same line, is effected by putting the primary of an tion. induction coil in the place of the receiving telephone, the secondary being in circuit with the second line or section. This plan is use ful where the same message is to be sent to different places at once (distributed), and is sometimes used for translating from a double wire to a single wire system. Probably a better plan is to work a microphone by the membrane of the receiving telephone, and re transmit the message, taking new energy from a second battery. 1 When the induction coil arrangement is used for translating from a double to a single wire circuit, or vice versa, it is necessary to make the induction coil suit the circuits, so that either coil may be used as primary, according to the end from which the message is sent. Everything else being similar, the resistances of the coils should be in nearly the same ratio as the resistances of the lines in which they are placed. In a large town it is neither practicable nor desirable to connect Ex- each subscriber directly with all the other subscribers, hence a changes, system of " exchanges " has been adopted. An exchange is a central station to which wires are brought from the different subscribers, any two of whom can be put in telephonic communication with each othei when the proper pairs of wires are joined together in the ex- 1 See Thomson and Houston, Tel. Journ., 15tli August 1878. ​134 T ELEPHONE change. The arrangement is illustrated in fig. 15, where C represents an exchange from which wires radiate to the points a, b, c, d, . . . Suppose a wishes to speak to d ; he communicates his wish to an attendant at C, who first calls d, and then con nects 6 to 1, making the circuit continuous from a to d. The ar rangements at the ex- change for facilitating connexions vary con siderably, but are simi lar in principle to the ~ a. switch boards used in telegraphy. Each of the Fro. 15. -Telephone exchange. wires is first brought to an indicator and then to a set of terminals arranged in an orderly manner on a board, the number of the terminal for any one wire being the same as the number under the shutter of the indicator in that wire circuit. In many cases the terminals take the form of spring clips, which connect the line to earth, and under which a thin piece of metal, covered with insulat ing material on one side and called a "jack," can be readily inserted for connecting that circuit with any other. A piece of flexible wire cord, carrying a jack at each end, forms a ready and common medium of connexion ; but in many cases the switch board is arranged with cross strips of metal so that by inserting a jack into the terminals of the two wires they can be both connected to the same strip of metal and therefore together. In large exchanges one switch board of moderate size is not sufficient, and so a number are fitted, being connected together by several conductors, in order that no interrup tion may ensue in consequence of these being all occupied. A line on one board is connected with one on another board by joining the terminal of the first to one of the conductors connecting the two boards by a jack -cord, and then by another jack-cord connecting that conductor to the terminal of the other line. Thus different switch boards may be looked upon as separate exchanges, connected together by a number of trunk wires after the manner described below. In a large system it is much more convenient and economical to have exchanges in the various districts, and connect these with a central exchange by a sufficient number of trunk lines. A sub scriber in one district wishing to speak to a subscriber in another calls the exchange in his own district and is put in communication by the attendant stationed there with the central exchange. The attendant at the central exchange puts the subscriber in communi cation with the district he requires, and the attendant there calls the other subscriber and joins the two subscribers lines together. In some cases neighbouring district exchanges have, besides a com mon means of communication through the central exchange, an independent connexion. These arrangements are diagrammatically Fio. 16. Telephone district exchanges. illustrated in fig. 16, where 1, 2, 3, 4, 5, 6 represent district ex changes and C the central exchange ; districts 3 and 4 and 4 and 5 are supposed to have independent connexions. Sinclair s An arrangement was proposed about two years ago by Mr D. automatic Sinclair of the Glasgow telephone exchange for allowing small dis- exchange. trict exchanges to be worked by the attendants at the central ex change. 1 The two exchanges are connected by a trunk line and from the district exchange wires are led to the different subscribers. These wires are in the normal state of matters connected with con tact plates, over which an arm joined to the trunk wire can be made to travel. Suppose the central exchange wishes to speak to any one of the subscribers, the arm is made to travel round, by currents sent from the exchange through an electromagnetic step by step arrangement, until it comes in contact with the proper plate, after which the subscriber is called in the ordinary way. When one subscriber belonging to the district exchange wishes to speak to another in the same district, he rings the bell in the ordi nary way, and this operation disconnects all other subscribers and puts him in connexion through the trunk line with the central ex- See Proc. Phil. Soc. of Glasgow, vol. xvii. p. 39. Fio. 17. Indicator or annunciator. change. The attendant there ascertains to whom it is that he wishes to speak, and by moving round the contact arm puts the two subscribers lines in contact. The indicator, or annunciator as it is sometimes called, is shown Indicator in fig. 17. It consists of an electromagnet M, which on a current or annun- being sent through it pulls down tin: E:^;y^""-v;~ :;-;;: ;-:;^-,::-:=;y;--T^ ciator. armature a, relieves the catch c, and allows the shutter d to fall down, exposing a plate p, on the front of which the number of the subscriber is printed. When the exchange is called, the shutter d is dropped, the attendant connects the line leading to the exchange table with the ter minal corresponding to the indicator, and finds who is wanted ; then he calls that subscriber, makes the through connexion, and puts up the shutter. When the subscribers have finished, both call the exchange or, as it is commonly put, "ring off"; this drops both shutters and serves as the signal that they have finished speaking. The principle of transmitting sound by the radiophone will be Radio- understood from fig. 18. M represents a mirror, from which a phone, beam of light is reflected through the lens I to a second mirror m, and m forms a diaphragm against the back of which the sound vibrations sent through the tube t are made to impinge. The beam of light, after being reflected from m, passes through the lower lens I, and thence as a nearly parallel beam to the parabolic reflector R. A photophonic receiver P, sup posed in this case to be a spiral of selenium wire wound on the surface of a cylinder, Fio. 18. Bell s radiophone. is placed at the focus of the reflector so that the beam of light from m is concentrated on it. In circuit with the receiver P a battery B and a telephone T are included and through the circuit a feeble electric current flows continuously. The photophonic receiver should be placed so as to receive as little light as possible from any other source than the mirror m. Words spoken through the tube t make the mirror m vibrate, so that the beam of light reflected from it becomes more or less spread. The lens I is then unable to bring the beam into parallelism, and the intensity of the reflexions from R to P is varied, therefore also the current through the coil of the telephone, which in consequence gives out a sound. The amount of spreading of the beam being proportional to the intensity of the vibrations of m, and this again proportional to the intensity of the sounds, the sounds heard in the telephone are similar to those pro duced at the end of t. Theoretically the receiver may be at any distance from the transmitter, but considerable difficulty arises if the distance is great. One of the simplest forms of the phonograph is shown in fig. 19. Phono- It consists of a rigid spindle S screwed for about one-third of its graph. length, and fitted to work smoothly but tightly in the frame /, /, which is se curely attached to a sole plate P. On the spindle a drum D is fixed, the axis of which coincides ac curately with that of the spindle. On the surface of the drum a screw is cut of precisely the F IO. 19. Edison s phonograph, same pitch as that on the spindle. A fly-wheel W is fixed to one end of the spindle, and is provided with a handle H, by which the spindle and drum can be conveniently turned. One of the bearings has either a screw thread cut along it, or is fitted with one or more studs which work easily, but without shake, in the screw thread. When the spindle is turned, it receives a trans verse motion, and a point fixed relatively to the sole plate P and touching the drum traces out a spiral on its surface, exactly coin ciding with the screw thread cut on it. A mouthpiece M, like that of a telephone transmitter, provided with a diaphragm of parch ment or similar substance, is mounted on a lever, which is pivoted at k and provided with a set screw 5. A blunt needle point is either fixed to the centre of the diaphragm or carried by a light spring in such a way as to press on the centre of the diaphragm with the needle point projecting outwards. To use the instrument, the drum D is covered with a sheet of somewhat stiff tinfoil, and the mouthpiece is adjusted as shown in the figure, with the needle point over the hollow part of the tinfoil, and fixed by the set screw to make a slight indentation in it. The drum is then turned and words spoken in a somewhat loud and clear tone in front of the ​mouthpiece. The vibrations of the diaphragm cause the needle point to make indentations more or less deep, according to the intensity of the sound, in the surface of the tinfoil. If the mouth- piece is then raised, the drum turned back to its original position, the mouthpiece lowered so that the point rests on the groove which it previously made, and the drum again turned, the diaphragm, acted on by the needle point passing over the indentation, will give out the same words which were spoken to it. (T. GR.)
* ↑ “Ueber Telephonie durch den galvanischen Strom,” in Jahresber. d. physikalischen Vereins zu Frankfurt am Main, 1860–61, p. 57.
* ↑ See his Scientific Papers, p. 47.
* ↑ See Silliman's Journ., xxxii. p. 396 and xxxiii. p. 118.
* ↑ Marrian, Phil. Mag., 3d ser., xxv. p. 382; Beatson, Arch. de l' Élect., v. p. 197; De la Rive, Treatise on Electricity, vol. i. p. 306, also Phil. Mag., 3d ser., vol. xxxv. p. 422, and Comp. Rend., xx. p. 1287, xxii. p. 432; Matteucci, Arch. de l'Élect., v. 389; Guillemin, Comp. Rend., xxii. p. 264; Wertheim, Comp. Rend., xxii. pp. 336, 544, xxvi. p. 505, also Ann. de Chim. et de Phys., xxiii. p. 302, and Phil. Mag., 3d ser., xxviii. p. 544; Jannair, Comp. Rend., xxiii. p. 319; Joule, Phil. Mag., 3d ser., xxv. pp. 76, 225; Laborde, Comp. Rend., 1. p. 692; Poggendorff, Pogg. Ann., Ixxxvii. p. 139, xcviii. p. 198; Du Moncel, Exp. de l'Élect., vol. ii. p. 125, iii. p. 83; and Delesenne, Bibl. Univ., 1841, xvi. p. 406.