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TELEPHONE (Gr. rjXs, far, and dxwvr7,...
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See also:TELEPHONE (Gr. rjXs, far, and dxwvr7, See also:voice)
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Telephony is the See also:art of reproducing sounds at a distance from their source, and a See also:telephone is the See also:instrument employed in sending or receiving such sounds
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The See also:term " telephony" was first used by Philipp Reis of See also:Friedrichsdorf, in a lecture delivered before the See also:Physical Society of See also:Frankfort in 18611 But, although this lecture and Reis's subsequent See also:work received considerable See also:notice, little progress was made until the subject was taken up between 1874 and 1876 by See also:
In 1831 See also:Wheatstone by his " magic See also:lyre" experiment showed' that, when the See also:sounding-boards of two musical See also:instruments are connected together by a See also:rod of See also:pine See also:wood, a tune played on one will be faithfully reproduced by the other
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This only answers, however, for telephoning musical sounds to See also:short distances
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Another and somewhat similar example is furnished by what has been variously designated as the " See also:string," Meehan-" See also:toy," " lovers," and " See also:mechanical " telephone. See also:mat tete-Two disks of thin See also:metal, or two stretched membranes, Phone-
each furnished with a See also:mouthpiece, are connected together by a thin string or See also:wire attached at each end to the centres of the membranes
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A See also:good example may be made with two cylindrical See also:tin cups; the bottoms form the membranes and the cups the mouthpieces
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When the connecting string is held taut and sounds, such as those of See also: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 See also:air, thus reproducing the sound
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In both these examples all the three characteristics—pitch, relative intensity, and quality—of sound are reproduced
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In See also:July 1837 Dr C
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G
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See also:Page of See also:Salem, Mass., See also:drew See also:attention to the sound given out by an electromagnet at the instant when the electric See also:circuit is closed or broken, and in See also:October page's of the same See also:year he discussed, in a short See also:article 3 dtsentitled " Galvanic See also:Music," the musical See also:note See also:pro- covers. duced by rapidly revolving the See also:armature of an electromagnet in front of the poles
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Experiments bearing on this subject were subsequently made by a See also:great number of investigators.' Page's See also:discovery is of considerable importance in connexion with the theory of See also:action of various forms of telephone, and was a very important feature in the See also:early attempts by Reis to transit music and speech
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On the 26th of See also:August 1854 there appeared in L'See also:Illustration (See also:Paris) an interesting article by See also: In Reis's lecture an apparatus was described which has given rise to much discussion as to priority in the invention of the telephone . The instrument was described in over Reis's fifty publicationss in various countries, and was well tee-known to physicists previous to Bell's introduction Phone. of the electric telephone as a competitor with the electric See also:telegraph . Reis caused a membrane to open and close an electric 2 See his Scientific Papers, p . 47 . ' See See also:Silliman's Jour., xxxii . 396, and xxxiii . 118 . ' M4rrian, Phil . Mag., 3rd See also:ser., vol. See also:xxv. p . 382; Beatson, See also:Arch. de l'Elect., v . 197; De la Rive, See also:Treatise on Electricity, i . 306, also Phil . Mag., 3rd ser., vol. See also:xxxv. p . 422, and Comp . Rend., xx . 1287, xxii . 432; See also:Matteucci, Arch. de l'Elect., v . 389; Guillemin, Comp . Rend., xxii . 264; Wertheim, Comp . Rend., xxii . 336, 544, See also:xxvi . 505, also See also:Ann. de Chim. et de Phys., See also:xxiii . 302, and Phil . Mag., 3rd ser., vol. xxxiii. p . 544; Jannair, Comp . Rend., xxiii . 319; See also:Joule, Phil . Mag., 3rd ser., vol. xxv. pp . 76, 225; Laborde, Comp . Rend., 1 . 692; See also:Poggendorff, Pogg . Ann., lxxxvii . 139, xcviii . 198; Du Moncel, Exp. de l'E°lect., ii . 125, iii . 83; and Delesence, Bibl . Univ . (1841), xvi . 406 . 6 See also Didaskalia: Bliitter See also:fur Geist, Gemiith, is . Publicitdt, Frankfort, No . 232, 28th See also:September 1854; Du Moncel, Expose See also:des Applications de l'.11ectricite (Paris), ii . 25, ed . 1854; Iii. no, ed . 1856, and Comp . Rend., 26th See also:November 1877 . ' The See also:English reader may consult—Jour .
See also:Soc
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Tel
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Eng., See also: 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 See also:act on an electromagnet at the receiving station, which, in accordance 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 See also:object was to reproduce at a distance not only music but also human speech; but that he did not wholly succeed is clear from the following See also:extract from his lecture:—" Hitherto it has not been possible to reproduce human speech with sufficient distinctness . The consonants are for the most See also:part reproduced See also: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 comprehend how they could be reproduced by electricity . His fundamental See also:idea—the interruption of the current—was a fatal See also:mistake, which was not at the time properly understood . The See also:suggestion of Bourseul and the experiments of Reis are, founded on the idea that a See also:succession of currents, corresponding in number to the successive undulations of the pressure on the membrane of the transmitting instrument, could reproduce at the receiving station sounds of the same See also:character as those produced at the sending station . Neither of them seemed to recognize anything as import-See also:ant 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 true to a small extent; but; considering the small capacity of the circuits he used and the nature of his receiving instrument, it is hardly probable that duration of contact sensibly influenced the result . The quality of the sounds was to some extent also reproduced ; but, judging from the results of later telephone investigation, it is highly probable that this was due, not to the varying duration, but to the varying firmness of the contact . The next worker at the telephone, and the one to whom the present great commercial importance of the instrument is due; Berns re- was Bell . His aim was the See also:production, by means searches• 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) . 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 See also:parchment In the early trials the armature had the form of a hinged See also:lever of See also:iron carrying a See also:stud at one end, which pressed against the centre of a stretched membrane . Fig . 1 shows the arrangement . M was a membrane stretched by a See also:ring R over the end of a See also:tube T fixed at one See also:side of the See also:frame F . To the oppo- site side of the frame an electromagnet I was fixed with its See also:axis in See also:line with the tube T, and between the end of the electromagnet and the membrane a hinged armature A was arranged in such a way that its See also:motion could be controlled by the membrane . The instrument was joined in circuit with a See also:battery and another similar instru- ment placed at a distance; and a continuous current was made to flow through the circuit, keeping the electromagnets energized . The ex- as possible, he substituted for the comparatively heavy lever armature a small piece of See also:clock See also:spring, about the See also:size of a sixpence, glued to the centre of the See also:diaphragm . The magnet was mounted with its end carrying the coil opposite, and very close to, the centre of the piece of clock spring . This answered sufficiently well to prove the feasibility of the See also:plan, and subsequent experiments were directed to the discovery of the best form and arrangement of the parts . An increase in the size of the iron disk attached to the membrane augmented both the loudness and the distinctness of the sounds, and this finally led to the adopt on of a thin iron disk supported See also:round its edge, acting as both membrane and armature (fig . 2) . Again, the form of the opening or mouthpiece in front of the membrane exercised considerable ' See A . G . Bell, " Telephone Researches," in Joarn . Soc Tel . Eng., 31st October 1877.See also: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 See also: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 See also:pole in contact with E, electromagnet; C, diaphragm; t terminals . the end of the core of a short electromagnet, the coil of which was in circuit with the line, but which had no permanent current flowing through it, answered the purpose quite as well ? The apparatus thus acted as both a transmitter and a See also:receiver; indeed it is essentially the magneto-receiver which has come into universal use in See also:practical telephony, though for transmission it was soon superseded by forms of microphonic transmitters . One of the latest forms of S P• M' receiver, known as the See also:double pole, is shown in fig . 3 . M and M' are two permanent magnets; P and P' are soft iron pole-Pieces upon which are placed the electromagnet coils C and C ; D is the diaphragm; I is a soft iron distance piece placed between the magnets at the end remote from the diaphragm; B is the See also:brass See also:body of the instrument, over which is placed a thin ebonite See also:shell S . E is the ear-piece made of ebonite; F is a cap of the same material enclosing the receiver terminals, which are mounted upon the ebonite See also:block G, attached to the distance piece I . A telephone transmitter and a receiver on a novel plan were patented in July 1877 by See also:Edison, shortly after the introduction of Bell's instruments . 
The receiver was based on the See also:change of See also:friction produced by the passage of an electric current through the point of contact of certain substances in relative motion
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In one form a See also:drum, mounted on an axis and covered by a See also:band of See also:paper soaked in a See also:solution of See also:caustic potash, was turned under a spring the end of which was in contact through a See also:platinum point with the paper
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The spring was attached to the centre of a diaphragm in such a way that, when the drum was turned, the friction between the point of the spring and the paper deflected the diaphragm
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The current from the line was made to pass through the spring and paper to the See also:cylinder
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Now it had been previously shown by Edison that, when a current was made to pass through an arrangement like that just described, the friction between the paper and the spring was greatly diminished
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Hence, when
the undulating telephonic currents
were made to pass through the apparatus, the See also:constant variation
of the friction of the spring caused the deflexions of the diaphragm to vary in unison with the variation of the electric
The extreme smallness of the magnets which might be success-fully employed was first demonstrated by See also:Professor See also:Peirce of See also:
Resting on the centre of the ivory disk was a small piece of See also:rubber tubing, and this was lightly pressed by the diaphragm A, which was held in See also:place by the mouthpiece M
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The varying pressure on A, when a sound was produced near it, caused corresponding See also:variations in the pressure on the carbon powder, and this produced
similar variations in its electric resistance
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Experiments very similar to these of Edison were made by See also:Elisha See also: This alteration of charge caused a corresponding change in the mutual attraction of the plates of the condenser; hence the flexible plate was made to copy the vibrations of the diaphragm of the transmitter . It is obvious that this apparatus might be used either as a transmitter or as a receiver, but that the effects must under ordinary circumstances be in either See also:case extremely feeble . It was very early recognized—and, indeed, is mentioned in the first See also:patents of Bell, and in a See also:caveat filed by Elisha Gray in the See also:United States patent See also:office only some two Liquid trans- thours after Bell's application for a patent—that maters sounds and spoken words might be transmitted to a of Bell distance by causing the vibrations of a diaphragm to and E. vary the resistance in the circuit . Both Bell and Gray G` ' proposed to do this by introducing a See also: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 See also:depth of See also:immersion of a See also:light rod fixed to it and dipping into the liquid . On the 4th of See also:April 1877 Emile Berliner filed a caveat in the United States patent office, in which he stated that, on the principle of the variation with pressure of the resist- uner s ance at the contact of two conductors, he had made micro- an instrument which could be used as a telephone phone transmitter, and that, in consequence of the mutual trans- forces between the two parts of the current on the See also:mitten two sides of the point of contact, the instrument was capable of acting as a receiver . The caveat was illustrated by a See also:sketch showing a diaphragm with a metal patch in the ' See See also:George B . See also:Prescott, The Speaking Telephone (London, 1879), pp . 151-205 . 2 Scientific American, 18th See also:June 1881 . ' See also:Electrostatics and See also:Magnetism, p . 236 . * See Tel .
Journ., 1st August 1877, p
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178; also See also: In the early part of x878 Professor D . E . See also:Hughes, while en-gaged in experiments upon a Bell telephone in an electric circuit, discovered that a See also:peculiar noise was produced when- Hughes's ever. two hard electrodes, such as two wires, were micro- See also:drawn across each other, or were made to See also:touch each Phone. other with a variable degree of firmness . Acting upon this discovery, he constructed an instrument which he called a "microphone,"' and which consisted essentially 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 See also:board capable of being vibrated by sound-waves and the other was held either by springs or weights in delicate contact with it . When the sounding board was spoken to or subjected to sound-waves, the mechanical resistance of the loose electrode, due to its See also: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' The next transmitter of note was that introduced by See also:Francis See also:Blake, which came into wide use in the United States of See also:America and other countries . In it the electrodes were of platinum and carbon . To a frame F (fig . 5) was attached a diaphragm D of thin See also:sheet iron ; in front of Allis was a cover M, M provided with a suitable cavity for directing the sound-waves against the diaphragm . The microphonic arrangement consisted 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 See also:free extremity a brass block W . In one side of W a small disk C of See also:gas carbon was in, serted, resting on the hemispherical end of a small platinum See also: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 could be adjusted by the screw N, which turned the lever about the flexible See also:joint G . The electrical connexions of the instrument as arranged for actual use are also illustrated in the figure . The current circuit went through S, W, C, K, A, and the See also:primary circuit of the See also:induction coil I to the battery B, and thence to S again . This formed a See also:local circuit at the transmitting station . The line of circuit passed through the secondary of the induction coil I to the line, from that to the telephone T at the receiving station, See See also:Journal of the Telegraph, New See also:York, April 1877; See also:Philadelphia 'Times, 9th July 1877; and Scientific American, August 1877 . ' This term was used by Wheatstone in 1827 for an acoustic apparatus intended to convert very feeble into audible sounds; see his Scientific Papers, p . 32 . ' See Proc . See also:Roy . Soc., See also:xxvii . 362; Proc . Phys . Soc., ii . 255; Phil . Meg., 5th ser., vol. vi. p . 44; W . H . Preece, Journ . Soc . Tel . Eng., vii . 270 . and then either to See also:earth or back to the induction coil by a return line of wire . Another type of microphone which was used in See also:Europe much more than in the United States was the multiple-contact instrument . In this several microphonic See also:joints were employed . Thus, in the Crossley transmitter four hard carbon pencils were arranged in a See also:lozenge-shaped figure, the ends of each See also:pencil resting loosely in a small carbon block . These blocks were fastened to a diaphragm of wood . The circuit connexions weie such that two adjacent sides of the lozenge were in parallel and two in See also:series . In the Ader transmitter as many as twelve carbon pencils were employed, arranged in a series of two See also:groups with six pencils in parallel in each See also:group . These were supported at their ends in parallel carbon bars, which were carried by a nearly See also:horizontal wooden diaphragm . Such multiple-electrode transmitters give a loud although somewhat harsh sound, and will See also:bear being spoken to very strongly without breaking the circuit . A type of transmitter which has come to be invaluable in connexion with long-distance telephony, and which has practically superseded all other forms, is the granular carbon transmitter . The earliest instrument of this See also:kind was the Hunnings transmitter, patented i,n 1878 . This was constructed of a shallow See also:box placed in a See also:vertical position, with metallic front and back and insulating sides . The front face was of thin metal, and served as a diaphragm . The box was filled nearly, but not quite full, of granulated hard carbon . The current from the battery used passed from the diaphragm through the granulated carbon to the metallic back of the box .
When spoken to the diaphragm vibrated, and thus set the carbon granules into vigorous vibration
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The vast number of micro-phonic contacts present give rise to very strong electrical undulations, and hence to a loud sound
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The chief difficulty with this transmitter, and with various others of later date based upon it, has been the frequent packing of the carbon granules, which renders the instrument in-operative
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The difficulty was first satisfactorily overcome in the long-distance transmitter, invented by A
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C
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See also: A washer of thin flexible See also:mica G concentric with the carbon button is carried by the brass disk, and projecting over the edge of this is held firmly against the rim of the cylindrical wall of the case by an annular brass See also:collar H, which is screwed upon the See also:outer curved surface of this wall . The box is thus entirely closed at the front,, while the front carbon disk, which constitutes an electrode, is perfectly free to follow the motions of the diaphragm . The space enclosed between the front and See also:rear faces of the box is filled about three-quarters full of finely granulated hard carbon, which therefore lies in contact with the front and rear carbon disks of the apparatus, and also fills up the space lying between the See also:lower edge of these disks and the curved surface of the case . The, current from the battery passes from one of the carbon disks to the other through the particles of granulated carbon which fill the space between them . The disks and granules constitute a very powerful microphone . The motions impressed upon the carbon granules are very vigorous, and this together with the particular arrangement of the parts of the instrument is effectual in obviating the difficulty from packing which attended the use of earlier forms of granulated carbon transmitters . This instrument has almost entirely displaced all other forms of transmitter . Subscribers' Organization.—The employment of the telephone as one of the great means of communication requires a definite organization of the subscribers . It is not practicable to connect each subscriber directly to all the others, hence a system of exchanges has been adopted . The territory in which a telephone See also:administration operates is usually divided into a number of local areas, in each of which one or more exchanges are placed . An See also:exchange is a central station to which wires are brought from the various subscribers in its neighbourhood, any two of whom can be put in telephonic communication with each other when the proper pairs of wires are joined together in the exchange . When the subscribers in a local See also:area exceed a certain number, or when for some other See also:reason it is not convenient or economical to connect all the subscribers in the area to one exchange, it is usual to See also:divide the area into a number of districts in each of which an exchange is placed, and to connect these See also:district exchanges together by means of " junction circuits." In some cases the exchanges are connected together directly; but when the See also:volume of See also:traffic is not sufficient to See also:warrant the See also:adoption of such a course connexions between two exchanges are made through junction centres to which both are connected .
A system of wires, similar to that which connects the district exchanges in an area, links together the various local areas in the territory, and sometimes the territory of one administration with that of another
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These inter-area or long-distance lines, called See also:trunk circuits in See also:England, terminate at one exchange in each local area, and between that exchange and the various
district exchanges junction circuits are provided for the purpose of connecting subscribers to the trunk lines
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Circuit and Working Arrangements.—The method first employed for working a telephone line was extremely See also:simple
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A single line of wire, like an ordinary telegraph line, had a Bell telephone included in it at each end, and the ends were put to earth
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Words spoken to the telephone at one end could be heard by holding the telephone to the ear at the other
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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
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The contrivance most generally adopted for calling attention was a See also:call-bell See also:rung either by a small magneto-electric See also:machine (magneto-generator) or by a battery
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The telephone was switched out of circuit when not in use and the bell put in its place, a See also: |