Tel′e-phone.
An instrument for conveying signals by sound.
It may consist of a steam-whistle, a fog-trumpet, or other audible alarm.
The term, until lately, has been particularly applied to a signal adapted for nautical or railroad use, in which a body of compressed air is released from a narrow orifice and divided upon a sharp edge, in the manner of a steam-whistle.
The term is now acquiring a different signification.
The subject of the number of vibrations per second, as constituting a given note, has been considered under music, pitch, pipe, etc.; and the mode of counting the vibrations under siren. The
vibroscope is another instrument invented by Duhamel for the same purpose.
See also Metronome; Tonometer.
Articulate sounds are accompanied by the expulsion of air from the mouth, which impulses vary in quantity, pressure, and in the degree of suddenness with which they commence and terminate.
An instrument which will record these impulses has been termed by its inventor,
Leon Scott, a
phonautograph, or
phonograph, and by
Mr. Barlow a
logograph; the pressure of the air in speaking is directed against a membrane which vibrates and carries with it a delicate marker, which traces a line on a traveling ribbon.
The excursions of the tracer are great or small from the base line, which represents the quiet membrane, according to the force of the impulse; and are prolonged according to the duration of the pressure, different articulate sounds varying greatly in their length as well as in intensity; farther, another great difference in them consists in the relative abruptness of the rising and falling inflections, which make curves of various shapes, of even or irregular shape.
The smoothness or ruggedness of a sound has thus its own graphic character, independent both of its actual intensity and its length.
Barlow's
logograph, described in the
London “Popular science Review,” Vol.
XIII. page 278,
et seq., consists of a small speaking-trumpet.
having an ordinary mouth-piece connected to a tube, the other end of which is widened out and covered with a thin membrane of gold-beater's skin or guttapercha.
A spring presses slightly against the membrane, and has a light arm of aluminium, which carries the marker.
consisting of a small sable brush inserted in a glass tube containing a colored liquid.
An endless strip of paper is caused to traverse beneath the pencil, and is marked with an irregular curved line, the elevations and depressions of which correspond to the force, duration, and other characteristics of the vocal impulse.
The lines thus traced exhibit remarkable uniformity when the same phrases are successively pronounced.
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Lines obtained by the Phonauto-graph. |
Fig. 6257 shows curves obtained by
Mr. Charles A. Morey by the interposition of a light lever between the membrane and the smoked glass, which is drawn along beneath the style, whose excursions are much magnified by the lever.
The curves show respectively the tongue trill or
German r prolonged, the mark produced by the sound of a trombone, and by the sound of
o o in mood.
See “American journal of science and Arts,” August, 1874, pages 130, 131.
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Logographic curves. |
Fig. 6258 shows a tracing from the utterance of the word “incomprehensibility,” with different degrees of effort.
It will be noticed that while a certain variation occurs due to the energy, each sound preserves a specific character.
Fig. 6259 shows in the upper portion the effect of words of quantity which require a large volume of air, and are maintained a relatively longer time than the more explosive or intense kind.
The lower diagram is what the tracer wrote when the familiar stanza from “
Hohenlinden” was repeated.
A much more delicate instrument for obtaining sonorous vibrations has been made by
Professor A. Graham Bell and
Clarence J. Blake, M. D., of
Boston, Mass. (June, 1874), by using the
membrana tympani of the human ear as a phonautograph.
Dr. Blake's mode of exposing the middle ear without injuring the ossiculae or the delicate tympanic membrane is described at length in the “Boston Medical and surgical journal,” February 4, 1875, pages 121-123.
The
stapes was removed, and a short style of hay substituted of about the same weight, so as to increase the amplitude of the vibrations and afford means of obtaining tracings upon smoked glass, as in the phonautograph experiments.
The membrane is kept moist by a mixture of glycerine and water, and the speci-
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men attached to a perpendicular bar sliding in an upright post, and moved by a ratchet-wheel.
To the upright is attached, horizontally, a metallic stage six inches in length, upon which slides a carriage with a glass plate, and having a regular movement given to it by wheel and cord.
A bell-shaped mouthpiece is inserted in the external auditory meatus and luted in position.
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Logographic curves. |
The vibrations of the membrane due to a musical tone sounded in the bell may be observed by means of a ray of light thrown upon small specula of foil attached to the
malleus, incus, or to different portions of the
membrana tympani, or may be recorded on smoked glass by a style fastened to the descending process of the
malleus or
incus by means of glue, in a line with the long axis of the process, and extending downward, so as to reach the plate of smoked glass, which is moved at a right angle to the excursion of the style; the latter then traces a wave-line corresponding to the character and pitch of the musical tone sounded into the ear.
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The Memberane of the tympanum used as a phonautograph. |
As the glass plates present plane surfaces, and as the point of the vibrating style sweeps through the segment of a circle, the curves obtained are apt to be discontinuous, especially when the amplitude is great.
To obviate this difficulty, a sheet of glass is employed having a
curved surface, the concavity being presented to the style.
The sheet of glass is a section of a cylinder whose semidiameter is equivalent to the length of the style.
In this way the point of the style never leaves the surface of the glass, and the curve resulting from its vibration is continuous.
The carbon film is preserved by pouring collodion upon it; as soon as this is dry, the film may be floated off with water and placed upon a plane sheet of glass, or upon paper, and varnished in the ordinary way.
See the following works: “Use of the
Membrana Tympani as a Phonautograph and Logograph, with Plates,”
Archive for Ophthalmology and Otology, 1876. “Use of the
Membrana Tympani as a Phonautograph and Logograph,”
Boston Med. and Surg.
Journal, Feb., 1875. “Mechanical Value of the Distribution of Weight in the Ossicula,” Trans.
Am. Otological Society, 1874.
Another step in the direction of the conveyance of sound consists in connecting a membrane in a mouth-trumpet by means of a fine cord with a similar membrane is a trumpet applied to the ear of a person at a considerable distance, say in another room.
The sounds are audible, not merely as to pitch, but are recognizable as articulate sounds.
The writer knew an officer who was with
Nelson at
Copenhagen, who was wounded so that his hearing was destroyed.
He was in the habit of placing a music-box against his teeth, or holding in his teeth a string whose other end was shut tightly between the lid and the box. He said he
heard very well.
The experiment of connecting distant sounding-boards, so that one is made to vibrate in unison with the other, is familiar; indeed, the synchronous vibration may be obtained even by the vibrations of the atmosphere, as when the sounds of a piano are repeated by those of a guitar which has been tuned in unison and stood up in the corner of the room.
This is even more palpable when the guitar is laid upon the piano top.
“There is an experment, first made by
Wheatstone, where the music of a piano is transferred from its sound-board, through a thin wooden rod, across several silent rooms in succession, and poured out at a distance from the instrument.
The strings of the piano vibrate, not singly, but ten at a time.
Every string subdivides, yielding not one note, but a dozen.
All these vibrations and subvibrations are crowded together into a bit of deal not more than a quarter of a square inch in cross section.
Yet no note is lost.
Each vibration asserts its individual rights; and all are, at last, shaken forth into the air by a second soundboard, against which the distant end of the rod presses.
Thought ends in amazement when it seeks to realize the motions of that rod as the music flows through it.” —
Prof. Tyndall, in “
Martineau and Materialism.”
From the mechanical transmission of sounds by the air, string, or wooden rod, we pass by a decided step to devices by which vibrations are made to produce rapid contacts and breaks of an electric circuit, and thus become means of transmitting tones; and, conversely, to means of making rapid contacts, so as to set matter vibrating, and causing it to generate musical tones.
The knowledge that the pitch of tone depended upon the rapidity of vibration was well known to
Pythagoras; the generation of sound by motion is familiar in the vibration of strings and wires in the Aeolian harp, and the sound of the wind in a tube.
The tuning-fork is so proportioned that it produces sounds of but one
pitch, while its
timbre depends on the quality of its material, and its
intensity upon the amplitude of its vibrations.
Besides the tone generated by the agitation of the atmosphere, it was observed as long ago as 1785, by the Canon
Gottoin de Coma, that an iron wire of at least 10 yards in length, when stretched in the open air, spontaneously gives forth a sound under the influence of certain variations in the state of the atmosphere, the effect being due to the transmission of atmospheric electricity.
This transmission does not in fact occur in a continuous manner like that of a current, but rather by a series of discharges.
Mr. Beatson has proved that the discharge of a Leyden jar through an iron wire causes this wire to produce a sound if the discharge is first passed through a moist conductor, such as a wet string or animal tissue.
Professor Page in
America,
De la Rive in
France, Gassiot in
London, and Marrian in
Birmingham, discovered that rods of iron placed in the interior of a helix through which a current of electricity is passed give out decided sounds at the moment when the circuit is made or broken.
Reis applied this discovery in his telephone.
By causing the circuit to be made and broken very rapidly, a musical note was emitted by a rod placed in a helix traversed by the current.
De la Rive found that exactly similar sounds were produced where the intermittent current was passed through the iron rod itself; and
Professor Bell has produced similar effects by passing the current through retort carbon, plumbago, Germansilver wire, and platinum wire.
In fact, the intermittent current occasions a molecular vibra-
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tion in the conductor through which it is passed, whatever may be the material of which it is made, and this vibration may be rendered audible by coiling up the conducting-wire and applying it to the ear; as, for instance, an empty helix of copper wire, through which a discontinuous current is passed.
In the electro-magnetic telephone of Kirpath, the telephone sending-instrument consists of an open funnel in which the tone is sounded.
and a diaphragm of thin elastic membrane, which is set in vibration by the tone, and which by its vibrations continually makes and breaks the electric circuit.
The number of breaks in a second corresponds with the number of vibrations belonging to the tone sounded (see pipe, page 1708); and since the current passes at the other end of the line through a helix around a soft iron bar, this bar is magnetized and demagnetized, alternately, as many times in a second as there are vibrations in the tone sounded at the sendingin-strument At each demagnetization, a longitudinal shrinkage of the bar occurs, and a resulting sound; this is rendered more audible by placing the bar on a delicate sounding-board, which partakes of the vibrations synchronously with those of the original membrane, and reproduces the same sound.
Fig. 6261 shows one of
Reis's telephones, used by
Legat in his experiments upon
Telephonic. The discontinuous current, produced by the oscillating lever which vibrates in concert with the membrane on the end of the trumpet, is passed through an electro-magnet (shown beneath), so as to occasion vibration in its armature, which is attached to a sounding-board.
Fig. 6262 principally differs from the other in its receivinginstrument.
The intermittent current traverses a coil surrounding a rod of iron, occasioning a molecular vibration in the iron.
The resulting sound is rendered more intense by placing the rod and helix upon a sounding-board.
Another form of receiving-instrument is thus described by
De la Rive:—
“The most brilliant sound is that which is obtained by stretching upon a sounding-board well-annealed wires, 1/20 or 2/20 of an inch in diameter and from 3 to 6 feet in length.
They are placed in the axis of one or several bobbins, the wires of which are traversed by electric currents, and they produce an assemblage of sounds, the effect of which is surprising, and which greatly resembles that to which several church-bells give rise when vibrating harmonically in the distance.”
De la Rive mentions one or two
rheotomes or
circuit-breakers, forms of apparatus employed by him in his experiments upon the production of sound telegraphically, for the purpose of making and breaking the circuit very rapidly.
Fig. 6263 is simply a toothed wheel so arranged as to make and break contact with a spring resting against the edge.
By giving the wheel a movement upon its axis, we cause the plate to leap from one tooth to another; each leap produces a rupture in the circuit, which is closed again immediately afterward.
The musical tone given out by the plate, when we have no other means of measuring it, gives us exactly the number of times that the circuit has been opened and closed, that is to say, interrupted, in a second.
Fig. 6264 is another form of rheotome used by
De la Rive, consisting of four needles arranged upon a horizontal rod, so as to dip into mercury alternately when the axis is caused to revolve.
The two needles are inserted perpendicularly and parallel with each other, and so arranged that when they are immersed simultaneously in two capsules filled with mercury, and insulated from each other, the circuit is closed; and when they are not immersed, it is open.
A clock-work movement, or simply a winch moved by the hand, gives a rotatory movement to the axis; whence it follows that in a given time — a second, for example — the circuit may be closed or interrupted a great number of times.
Attention has long since been directed to the utilization for telegraphic purposes of the means afforded and effects elicited by the numerous observers and experimenters in this branch of acoustics.
It is well known that many distinct sounds pass through the air simultaneously without interference, and that the ear is capable of receiving several impressions at the same time, or is capable of directing its attention to one set alone, as when one listens to an alto alone of a quartette, oblivious at the time of the other voices, though not perhaps of the words of a friend seated by one's side.
Gray's electro-harmonic telegraph is founded upon the principle before stated, that an electro-magnet elongates under the action of the electric current, and contracts again when the current ceases.
Consequently, a succession of impulses or interruptions will cause the magnet to vibrate, and if these vibrations be of sufficient frequency a musical tone will be produced, the pitch of which will depend upon the rapidity of the vibrations.
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Gray's electro-harmonic telegraph. |
By interrupting an electric current at the transmitting end of a line with sufficient frequency to produce a musical tone by an instrument vibrated by said interruptions, and transmitting the impulses thus induced to an electro-magnet at the receiving end of the line, the latter will vibrate synchronously with the transmitting-instrument, and thus produce a musical tone or note of a corresponding pitch.
The instrument shown in
Fig. 6265 consists of the transmitting-apparatus mounted on a base board, and a receivingappa-ratus, shown in a position beneath the former.
The induction-coil
b′ has the usual primary and secondary circuits.
An ordinary automatic electrotome
c has a circuit-closing spring
c′ so adjusted as when in action to produce a given musical tone.
A common telegraph-key
d is placed in the primary circuit
a a to make or break the battery connection.
The key being depressed, and the electrotome consequently vibrated, the interruptions of the current will simultaneously produce in the
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secondary circuit
b b of the induction-coil a series of induced currents or impulses corresponding in number with the vibrations of the electrotome; and as the receiving electro-magnet
e is connected with this circuit, it will be caused to vibrate by successive elongations and contractions, thus producing a tone of corresponding pitch, the sound of which may be intensified by the use of a hollow cylinder
s of metal, placed on the poles of the magnet.
When a single electrotome
c is thrown into action, its corresponding tone will be reproduced on the sounder by the magnet.
When electrotomes
c c′ of different pitch are successively operated by their respective keys
d d′, their tones will be correspondingly reproduced by the receiver; and when two or more electrotomes are simultaneously sounded, the tone of each will still be reproduced without confusion on the sounder, so that by these means melodies or tunes may be transmitted.
Another system is founded upon the alternate making and breaking of a telegraphic circuit by means of the vibration of tuning-forks or musical reeds, as in
Helmholtz's apparatus for the production and transmission of vocal sounds.
If a given fork be made to interrupt an electric circuit by its vibrations, and the intermittent current thus produced be passed through a series of electro-magnets, each in connection with a fork of different pitch, and consequently different rate of vibration, only that fork will be thrown into vibration which is in unison with the first one.
Practically the time required to do this is a small fraction of a second.
The advantages of this method are numerous.
Not only may many receiving-instruments at one station be operated, each by its own key, through a single wire, but many different stations in the same circuit may be operated, that one alone receiving the message which has an instrument with the requisite pitch, so as to vibrate in synchronism.
Many signals may in this way be transmitted over the same wire at the same time, and many dispatches sent simultaneously to as many stations.
All this may be done, too, without affecting the line for its ordinary use.
Three inventors have been and are working at this form of apparatus, —
Elisha Gray and A. G
Bell of the
United States, and La Cour of
Denmark.
See English patents Nos. 1,740 and 2,646 of 1874; 947 and 974 of 1874.
In Cour, 2,999 of 1874.
Gray's
United States patents, 166,094, 156,095, 166,096, of 1875; 175,971 of 1876.
Bell's patents, 161,739 of 1875; 174,465 of 1876.
In
Gray's electric telegraph for transmitting musical tones, the transmitting apparatus consists of a keyboard having a number of electro-magnets corresponding with the number of keys on the board, to which are attached vibrating tongues or reeds, tuned to a musical scale.
Any one of these tongues can be separately set in motion by depressing the key corresponding to it.
The closing of either of the keys completes the primary circuit from the battery through the electrotome connected with the key depressed, and the circuit-closing spring of said electrotome will immediately be thrown into rapid vibration, and a musical tone of a certain pitch will be given forth, while at each vibration the current in the primary circuit of the induction apparatus will be interrupted.
By the usual intermediate means secondary induced currents are transmitted to the receiving station.
Thus, for example, if the circuit-breaking spring of the electrotome vibrates one hundred and twentyeight times per second, the tone given forth is that known as the
fundamental C, while one hundred and twenty-eight terminal secondary currents will be induced in the secondary circuit of the induction apparatus, and transmitted through the animal tissue, — which may be the human body, — forming part of said circuit, to the resonant receiver, and will, from some cause not understood or explicable in the present stage of the art, vibrate the same synchronously with the transmitting electrotome, and thus give forth a musical tone of the same pitch.
This animal membrane must possess the specific characteristics of being a conductor of electricity; of being yielding and elastic; and of having a surface of greater electric resisting capacity than its interior.
These characteristics are found in the skin of the human body, as shown in
Fig. 6268.
The operation of the key and its corresponding electrotome may be more readily understood by referring to
Fig. 6266. In this,
a is a steel reed tuned to vibrate at a definite rate corresponding to its position in the scale.
One end is rigidly fixed to the post
b, while the other end is left free, and is actuated by a local battery.
The magnets
e and
f are arranged in the same local circuit, magnet
f having a resistance of about thirty
ohms, and magnet
e about four
ohms. When the reed
a is not in vibration, the point
c is in electrical contact with it, which throws a shunt-wire entirely around the magnet
f; thus, practically, the whole of the local current passes through magnet
e at the instant of closing the key
c. It is well known that when two electro-magnets are placed in the same circuit, the one which has the higher resistance (other things being equal) will develop the stronger magnetism, and that if the magnet of higher resistance be taken out of the circuit, the force of the other will be increased.
When the key
c, being depressed, closes the local circuit at
d, the operation of the reed is as follows: The whole of the current from battery
l passes through the magnet
e, which attracts the reed say with a power of four.
When the reed has moved toward
i far enough to leave the point
g, the shunt circuit is broken, and the current flows through both the magnets.
Immediately, the power in
f rises from zero to five, and that of
e drops from four to one, and the reed is attracted toward
f with an effective force of four, until contact is again established with the point
g. The operation is repeated at a rate determined by the size and length of the reed which corresponds with the fundamental of the note it represents.
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Diagram of Gray's telephonic transmitter and circuits. |
The main battery is connected as follows: One pole is connected to the ground; the other runs to the instrument, and, entering at binding-screw 4, runs to point
h of key
c; from key
c to point
i, which makes contact with the reed
a; from reed
a to binding-screw 1, and thence to line.
It will be seen that when the key is at rest the batteries are open at the points
d and
h.
The diagram,
Fig. 6266, shows but one key and connections, but all the keys in the instrument, whether one or more octaves, have corresponding reeds and actuating-magnets, the only difference being in the tuning of the reeds.
There is but one main and one local battery used, and the connections to each key are run in branch circuits from the binding-screws, as shown in Fig 6266.
But since all these branches are open at the key-points, neither of the batteries is closed unless a key is depressed.
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Gray's telephonic transmitter, for Composite tones. |
If now the keys are manipulated, a tune may be played which is audible to the player.
When any key is depressed, the local battery sets in vibration its corresponding reed, which sounds its own fundamental note according to the law of acoustics.
So far, the instrument is an electrical organ with steel tongues, the motive power being electricity instead of air. The main battery has had no part whatever in its operation.
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Gray's telephonic receiver. |
If, however, the main circuit is closed by connecting the distant end to ground, and the point
i is properly adjusted so that it makes and breaks contact with the reed at each vibration, a series of electrical impulses, or waves, will be sent through the line, corresponding in number per second to the fundamental of the reed, and corresponding synchronous electrical waves can be converted into audible vibrations at the distant end of the line, the note produced being of the same pitch as that of the sending reed.
There are various ways by which these electrical waves may be converted into audible material vibrations.
One of them, shown in
Fig. 6268, is a thin cylindrical sounding-box, made of wood, the face of which is covered with a cap made of thin metal, electrically connected to the metal stand by means of a wire.
If the operator connects the cap, through the stand, to the ground, and, taking hold of the end of the line with one hand, presses his fingers against the cap, which he revolves by means
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of the crank with the other hand, the tune that is being played at the other end of the line becomes distinctly audible, the intensity of the sound being determined by the rate of revolution of the plate.
When the motion stops, the sound entirely ceases.
The animal tissue plays a prominent part, as previously stated.
If, instead of the revolving plate and the animal tissue, an electro-magnet be placed in the circuit, or a number of them, and a tune be played at the transmitting end, the tune will be heard from all these receiving electro-magnets.
These may be on the principle of that shown in
Fig. 6265, consisting of a common electro-magnet having a bar of iron rigidly fixed at one pole, which extends across the other pole, but does not touch it by about 1/64 of an inch.
In the middle of this armature a short post is fastened, and the whole mounted on a box made of thin pine, with openings for acoustic effects.
So far, consideration has been given to simple tones, but, as in the case of the string and rod referred to previously, a single wire has been found adequate to convey composite tones; a considerable number of tones of different pitch, either in harmony or discord, and the metallic plate or battery of magnets is adequate to translate them.
In adapting this principle to a rapid telegraphic system, it becomes necessary to analyze the tones at the receiving end, so that the various messages in the different tones from as many transmitters worked by distinct operators, and all sent to line without any regard to each other, may be sorted at the receiving end by a corresponding number of receiving instruments, each of which shall sound its appropriate message in its own tone, ignoring all the others.
The messages, say eight in number, and of as many varying tones, having been simultaneously sent to line, may be said — for the convenience of statement — to be carried as a composite tone to the receiving end, where they pass through a series of receivers, each one of which is in harmony with one of the transmitters.
Each receiver sorts out its own message, and among them they may be said to absorb the whole of the vast number of complicated vibrations which have been sent pellmell through the wire in the form of so many successive magnetizations and demagnetizations.
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Gray's analyzer, or receiving-instrument. |
Fig. 6269 is a perspective view of one form of a receiving instrument called an
analyzer. It consists of an electro magnet adapted to the resistance of the circuit where it is intended to be used, and of a steel ribbon strung in front of this magnet in a solid metal frame, and provided with a tuning-screw at one end, so as to readily give it the proper tension.
The length and size of the ribbon depend upon the tone required.
If this ribbon be tuned so that it will give a certain note when made to vibrate mechanically, and the note which corresponds to its fundamental is then transmitted through its magnet, it will respond and vibrate in unison with its transmitted note; but if another note be sent which varies at all from its fundamental, it will not respond.
If a composite tone is sent, the ribbon will respond when its own note is being sent as a part of the composite tone, but as soon as its own tone is left out it will immediately stop.
Thus it
analyzes the tones which are passing over the line, and selects its own, allowing the others to pass to other instruments with which they are in accord.
A successful experiment with this system was made at
Milwaukee, Wis., on January 8, 1876.
A loop was arranged by way of
Horicon to
Portage and back by way of
Watertown, a distance of 200 miles. Over this single wire eight operators sent messages and signals in the same direction at the same time, without any one interfering with another.
See for fuller description “Journal of the
American electrical Society,” Vol.
I. 1875.
See also
Gray's patent, No. 175,971, dated April 11, 1876.
The same principle has been applied to the transmitting and receiving apparatus of sounding, recording, and printing telegraphs.
Bell's patent, No. 161,739, April 6, 1875, comprises a transmitter consisting of a local circuit containing an electromag-net, with a steel spring armature tuned to a pitch corresponding with a similar armature
b in the receiver
a, and vibrating between two platinum points, one communicating with the main line, and the other with the local.
The impulses are produced by a key
k, and are transmitted through the main line to the receiving magnet
a, the spring armature
b of which tilts a light, pivoted, non-conducting lever, having at its other end a platinum bow
g that dips into two cups of mercury and closes the circuit.
The wooden lever
f is balanced so as to vibrate much more slowly than the spring armature by which it is operated, and does not resume its normal position in contact with the latter until this has ceased to vibrate; that is, until the key
k has been raised.
An autographic telegraph on this principle is composed of a number of these transmitters, the armature of each being connected by a wire with a metallic or conducting bristle mounted in a non-conducting base placed over a metallic plate
p, with which the bristles are in contact.
The plate is in communication with the main battery
c. The bristles are insulated from each other, and are placed as near together as possible.
From the platinum point with which each transmitter comes in contact in making the circuit, wires
q q lead to the line wire
r. At the other end of the line is a series of receiving-instruments
s s corresponding in number to and having armatures vibrating in consonance with those of the transmitters.
Branch wires connect the receivers with the main line wire
r. In connection with each receiver is a local circuit comprising an electro-magnet
t and a vibratory lever
u, one arm of which constitutes the armature of the magnet, and the other arm terminates in a stylus which normally rests upon an inkribbon
v over the bed of the receiving-table
w. The stylus-armed ends of the levers converge, so as to be in the same relative position, and about as near together as the metallic bristles at the transmitting end of the line.
The message to be copied at the receiving end of the line is written or impressed with non-metallic ink on a sheet of metallic foil, and this sheet is placed on the metal plate under the bristles at the transmitting end.
The sheet of paper or other material on which the message is to be delivered is placed on the receiving-table
u, under the stylus-armed levers, and both sheets are simultaneously drawn over their respective tables.
So long as the bristles have metallic contact, intermittent electrical impulses from their respective transmitters pass along the line wire and operate the receivers, whose armatures
s s are attracted, and dip into the mercurial cups, which are in a circuit worked by the battery
d, lifting the recording ends of the levers away from the inkribbon.
When, however, the writing comes under the bristles, each, as the metallic contact is interrupted by the interposition of the non-metallic ink, ceases to transmit the electrical impulses, and the circuit-
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breaker of the corresponding receiver is raised out of contact with the mercury, the stylus end of the lever comes in contact with the ink-ribbon and causes a mark to be impressed upon the paper underneath.
Springs may be employed to give this downward impulse to the levers when the circuit is thus broken.
The effect of the alternate elevation and depression of the levers acting upon the ink-ribbon is to reproduce on the receivingsheet a fac-simile of the writing or object drawn upon the metallic foil at the transmitting end.
Bell's patent, No 174,465, March 7, 1876, describes a method of operating telephonic apparatus by means of undulatory currents of electricity.
Instead of making and breaking the circuit, he throws it into waves.
He distinguishes three kinds of vibratory currents by which telephonic effects may be produced.
1.
Intermittent: by making and breaking the circuit.
2.
Pulsatory; by occasioning
sudden changes in the intensity of a continuous current.
3.
Undulatory; by causing
gradual changes in the intensity of a continuous current.
The specification describes the effect upon the current of transmitting a number of musical notes simultaneously by intermittent or pulsatory impulses, and by undulatory currents respectively, and also instances several modes of throwing the current into waves either with or without a battery.
