See also:art of reproducing sounds at a distance from their source, and a telephone is the instrument employed in sending or receiving such sounds . 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:Bell, a native of
See also:Edinburgh, then
See also:resident in Boston, Mass., U.S.A . Bell, like Reis, employed
See also:electricity for the
See also: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 . The sensation of sound is produced by rapid fluctuation in the pressure of the atmosphere on the tympanum of the charac- ear . If the fluctuations are irregular and non-periodic, terlstics the sound is called a
See also:noise; if they are cyclic and of sound. follow a
See also:regular and sufficiently rapid periodic
See also:law, the sound is musical . In connexion with the
See also: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
See also:time; the loudness depends on the amount or the
See also:amplitude of the fluctuation in each cycle; the quality depends on the
See also:form or the nature of the fluctuation in each cycle . The necessary
See also:condition for a successful
See also:system of telephony is the ability to reproduce these characteristics . 1 "
See also:Cher Telephonie durch den galvanischen Strom," in Jahresber. d. physikalischen Vereins zu Frankfurt am
See also:Main, 186o-61, p . 57 .
See also:Wheatstone by his " magic
See also:lyre" experiment showed' that, when the sounding-boards of two musical
See also:instruments are connected together by a
See also:rod of
See also:wood, a tune played on one will be faithfully reproduced by the other . This only answers, however, for telephoning musical sounds to
See also:short distances . Another and somewhat similar example is furnished by what has been variously designated as the "
See also:string," Meehan-" 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 mouthpiece, are connected together by a thin string or
See also:wire attached at each end to the centres of the membranes . A
See also: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 . In
See also:July 1837 Dr C . G . Page of
See also:Salem, Mass., drew
See also:attention to the sound given out by an electromagnet at the instant when the electric circuit is closed or broken, and in
See also:October page's of the same
See also:year he discussed, in a short article 3 dtsentitled " Galvanic
See also:Music," the musical note
See also:pro- covers. duced by rapidly revolving the armature of an electromagnet in front of the poles . 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 early attempts by Reis to transit music and speech . On the 26th of
See also:August 1854 there appeared in L'
See also:Illustration (
See also:Paris) an interesting article by
See also:Charles Bourseul on the electric transmission of Bourspeech.b The writer recommended the use of a sears flexible
See also:plate at the source of sound, which would 'wages-vibrate in response to the varying pressure of the a"' 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 .
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 electrictelegraph . 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, 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; 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: Bliitterfur 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 . Tel . Eng.,
See also:March 1883;
See also:British Assoc .
See also:Rep., 1863; Civ . Eng. and Arch . Jour., xxvi . 307; R . M .
See also:Ferguson, Electricity (
See also:London, 1866), p . 257; S . P .
See also:Thompson, Philipp Reis, the Inventor of the Telephone (London, 1883) .
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 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 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 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 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 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 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 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 motion could be controlled by the membrane . The instrument was joined in circuit with a 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:spring, about the
See also:size of a sixpence, glued to the centre of the 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 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.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 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 . In one form a
See also:drum, mounted on an axis and covered by a
See also:band of paper soaked in a solution of
See also:caustic potash, was turned under a spring the end of which was in contact through a platinum point with the paper . 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 . The current from the line was made to pass through the spring and paper to the cylinder . 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 . 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:Peirce of
See also:Brown University,
See also:Providence, R.I . Edison's instruments . phone Transmitter . currents, and sounds were given out corresponding in pitch, and also to some extent in quality, with the sounds produced at the transmitting station . A cylinder of
See also:chalk was used in some of Edison's later experiments with this receiver . The transmitter (fig . 4), in an early form, consisted of a
See also:cell of insulating material having at its bottom a
See also:flat-headed platinum
See also:screw G; on the top of G was a layer of
See also:carbon powder C, on the top of that a platinum disk D, and above that again, forming the cover of the cell, a disk of ivory B, held in position by a ring E .
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 place by the mouthpiece M . The varying pressure on A, when a sound was produced near it, caused corresponding variations in the pressure on the carbon powder, and this produced similar variations in its electric resistance . Experiments very similar to these of Edison were made by Elisha
See also:Gray of Boston, Mass., and described by him in papers Elisha communicated to the
See also:American Electrical Society in Gray's 1875 and 1878 . In these experiments the electric expert- current passed through the fingers of the operator's menu.
See also:hand, which thus took the place of the spring in Edison's apparatus . The diaphragm was itself used as the rubbing
See also:surface, and it was either mounted and rotated or the fingers were moved over it . When the current passed, the friction was
See also:felt to increase, and the effect of _Aiding a rapidly undulating current through the arrangement was to produce a sound . The application of this apparatus to the transmission of music was described by Gray.' In another form of telephone, brought prominently forward by Professor A . E . Dolbear,2 the effects were produced by Dolbear's electrostatic instead of electromagnetic forces, as in
See also:con- the Bell telephone .
See also:Sir W .
See also:Thomson (
See also:Lord Kelvin) denser observed in 18633 that when a
See also:condenser is charged tele- or discharged, a
See also:sharp click is heard, and a similar phone. observation was made by
See also:Cromwell F . Varley, who proposed to make use of it in a telegraphic receiving instrument.* In Dolbear's instrument one plate of a condenser was a flexible diaphragm, connected with the telephone line in such a way that the varying electric potential produced by the action of the transmitting telephone caused an increased or diminished
See also:charge in the condenser .
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 eithercase extremely feeble . It was very early recognized—and, indeed, is mentioned in the first
See also:patents of Bell, and in a 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 . Prescott, The Speaking Telephone (London, 1879), pp . 151-205 . 2 Scientific American, 18th
See also:June 1881 . '
See also:Electrostatics and Magnetism, p . 236 . * See Tel .
Journ., 1st August 1877, p . 178; also
See also:Adams, Journ . Soc . Tel . Eng., 1877, p . 476.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 the idea of using a variable resistance transmitter .5 He proposed to introduce into the circuit a cell containing carbon Edison's powder, the pressure on which could be varied by the micro- vibrations of a diaphragm . He sometimes held the phone carbon powder against the diaphragm in a small trans shallow cell (from a quarter to
See also:half an inch in diameter d1tteer. and about an eighth of an inch deep), and sometimes he used what he describes as a fluff, that is, a little
See also:brush of
See also:silk fibre with
See also:plumbago rubbed into it . In another form the plumbago powder was worked into a button cemented together with
See also:syrup and other substances . In the
See also:specification of the patent applied for on the 21st of 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
See also: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 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 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 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 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 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 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 series . In the Ader transmitter as many as twelve carbon pencils were employed, arranged in a series of two 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 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 kind was the Hunnings transmitter, patented i,n 1878 . This was constructed of a shallow box placed in a 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 . The vast number of micro-phonic contacts present give rise to very strong electrical undulations, and hence to a loud sound . 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 . The difficulty was first satisfactorily overcome in the long-distance transmitter, invented by A . C .
See also:White in the laboratory of the American Bell Telephone
See also:Company, and commonly known as the " solid back transmitter " (fig . 6) . The microphonic portion of the transmitter is contained in a thin cylindrical box or case of brass A, the inner curved surface of which is covered with an insulating layer of paper . The case is firmly fixed to a "
See also:bridge " B with its back or bottom in a vertical position . To the brass bottom of the case is attached a thin disk of polished hard carbon C, which is slightly less in diameter than the brass bottom, so that the carbon disk almost entirely covers this brass back, leaving only a slight
See also:annular space around its edge . The front or cover of the case is a similar button of hard polished carbon D, also slightly smaller in diameter than the cylindrical
See also:wall of the box . It is attached to a brass disk E, which is fastened to the centre of the diaphragm F by means of a
See also:rivet,and is capable of moving to and fro like a plunger when the diaphragm vibrates .
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 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 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 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 . These inter-area or long-distance lines, calledtrunk circuits in 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 . Circuit and Working Arrangements.—The method first employed for working a telephone line was extremely
See also:simple . 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 . 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 contrivance most generally adopted for calling attention was a
See also:rung either by a small magneto-electric machine (magneto-generator) or by a battery . The telephone was switched out of circuit when not in use and the bell put in its place, a
See also:key being used for throwing the battery into circuit to make the
See also:signal . This arrangement is still employed, a
See also:hook being attached to the switch lever so that the mere
See also:hanging up of the telephone puts the bell in circuit . In some cases when a magneto-generator is employed for calling purposes the coil of the machine is automatically cut out of circuit when it is not in action, and is brought into circuit when the handle is turned by the operation of a centrifugal or other arrangement . At first it was usual to join the microphone transmitter in the
See also:direct circuit .
TELEPATHY (Gr. rnXe, far, wan, feelings), or THOUGH...
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