Popular Science Monthly/Volume 3/August 1873/Electric Telegraphs
THE
POPULAR SCIENCE
MONTHLY.
AUGUST, 1873.
| ELECTRIC TELEGRAPHS.[1] |
By Prof. A. P. DESCHANEL.
THE discovery that electricity could be transmitted instantaneously to great distances at once suggested the idea of employing it for signalling. Bishop Watson performed several experiments of this kind in the neighborhood of London, the most remarkable being the transmission of the discharge of a Leyden jar through 10,600 feet of wire suspended between wooden poles at Shooter's Hill. This was in 1747. A plan for an alphabetical telegraph to be worked by electricity is minutely described in the Scot's Magazine for 1753, but appears to have been never experimentally realized. Lesage, in 1774, erected at Geneva a telegraph-line, consisting of twenty-four wires connected with the same number of pith-ball electroscopes, each representing a letter. Reusser, in Germany, proposed, in the same year, to replace the electroscopes by spangled panes exhibiting the letters themselves. The difficulty of managing frictional electricity was, however, sufficient to prevent these and other schemes founded on its employment from yielding any useful results. Volta's discoveries, by supplying electricity of a kind more easily retained on the conducting wires, afforded much greater facilities for transmitting signals to a distance. Several suggestions were made for receiving-apparatus to exhibit the effects of the currents transmitted from a voltaic battery. Sommering, of Munich, in 1811, proposed a telegraph, in which the signals were given by the decomposition of water in thirty-five vessels, each connected with a separate telegraph-wire. Ampere, in 1820, proposed to utilize Œrsted's discovery by employing twenty-four needles, to be deflected by currents sent through the same number of wires; and Baron Schilling exhibited, in Russia, in 1832, a telegraphic model, in which the signals appear to have been given by the deflections of a single needle. Sir Francis Ronalds, before 1823, sent intelligible messages through more than eight miles of wire insulated and suspended in the air. His elementary signal was the divergence of the pithballs of a Canton's electrometer produced by the communication of a statical charge to the wire. He used synchronous rotation of lettered dials at each end of the line, and charged the wire at the sending end whenever the letter to be indicated passed an opening provided in a cover; the electrometer at the far end then diverged, and thus informed the receiver of the message which letter was designated by the sender. The dials never stopped, and any slight want of synchronism was corrected by moving the cover.
Weber and Gauss carried out Schilling's plan in 1833 by leading two wires from the Observatory of Göttingen to the Physical Cabinet, a distance of about 9,000 feet. The signals consisted in small deflections of a bar-magnet, suspended horizontally with a mirror attached, on the plan since adopted in Thomson's mirror galvanometer.
At their request, the subject was earnestly taken up by Prof. Steinheil, of Munich, whose inventions contributed more perhaps than those of any other single individual to render electric telegraphs commercially practicable. He was the first to ascertain that earth-connections might be made to supersede the use of a return wire. He also invented a convenient telegraphic alphabet, in which, as in most of the codes since employed, the different letters of the alphabet are represented by different combinations of two elementary signals. Two needles were employed, one or the other of which was deflected according as a positive or a negative current was sent, the deflections being always to the same side. Sometimes the needles were merely observed by eye, sometimes they were made to strike two bells, and sometimes to produce dots, by means of capillary tubes charged with ink, on an advancing strip of paper, thus leaving the permanent record on the strip in the shape of two rows of dots. His currents were magneto-electric, like those of Weber and Gauss.
The attraction of an electro-magnet on a movable armature furnishes another means of signalling. This was the foundation of Morse's telegraphic system, and was employed by Wheatstone for ringing a bell to call attention before transmitting a message.
About the year 1837 electric telegraphs were first established as commercial speculations in three different countries. Steinheil's system was carried out at Munich, Morse's in America, and Wheatstone and Cooke's in England. The first telegraphs ever constructed for commercial use were laid down by Wheatstone and Cooke on the London & Birmingham and Great Western Railways. The wires, which were buried in the earth, were five in number, each acting on a separate needle; but the expensiveness of this plan soon led to its being given up. The single-needle and double-needle telegraphs of the same inventors have been much more extensively used, the former requiring only one wire, and the latter two.
All public telegraphs have now for many years been worked by voltaic currents; the magneto-electric system, which was tried on some lines, having been found to involve a needless expenditure of labor.
Fig. 1.
Insulators.
According to Mr. Culley, engineer-in-chief to the post-office, the battery which had been adopted by the authorities of that department is a modified Daniell's, consisting of a teak trough, divided into cells by plates of glass or slate, and well coated with marine glue, each cell being divided into two by a slab of porous porcelain. The zinc plates measure four inches by two, and the copper plates, which are very thin, are four inches square. The zinc hangs at the upper part of its cell, which is filled with dilute solution of sulphate of zinc. The copper cell is filled with a saturated solution of sulphate of copper, and crystals of this salt are placed at the bottom. The expenditure in sulphate of copper is about a pound and a half for each cell per annum.
The wires for land-telegraphs are commonly of what is called galvanized iron, that is, iron coated with zinc, supported on posts by means of glass or porcelain insulators, so contrived that some part of the porcelain surface is sheltered from rain, and insulates the wires from the posts, even in wet weather. Wires thus suspended are called air-lines.
Underground wires are, however, sometimes employed. They are insulated by a coating of gutta-percha, and are usually laid in pipes, an arrangement which admits of their being repaired or renewed without opening the ground except at the drawing-in boxes. There is less leakage of electricity from subterranean than from air lines, but their cost is greater, and they are less suited for rapid signalling on account of the retardation caused by the inductive action between the wire and the conducting earth, which is similar to that between the two coatings of a Leyden jar.
The early inventors of electric telegraphs supposed that a current could not be sent from one station to another without a return-wire to complete the circuit. Steinheil, while conducting experiments on a railway, with the view of ascertaining whether the rails could be employed as lines of telegraph, made the discovery that the earth would serve instead of a return-wire, and with the advantage of diminished resistance; the earth, in fact, behaving like a return-wire of infinitely great cross-section, and therefore of no resistance.
We are not, however, to suppose that the current really returns from the receiving to the transmitting station through the earth. The duty actually performed by the earth consists in draining off the opposite
| Fig. 2. | Fig. 3. |
 | |
| Single needle Instrument. | Internal Arrangements. |
electricities which would otherwise accumulate in the terminals. It keeps the two terminals at the same potential; and, as long as this condition is fulfilled, the current will have the same strength as if the terminals were in actual contact.
One of the best-known telegraphs in England, though little or not at all employed elsewhere, is the single-needle instrument of Wheatstone and Cooke, represented in Figs. 2 and 3, the former showing its external appearance, and the latter its internal arrangements as seen from behind. The needle, which is visible in front, is one of an astatic pair, its fellow being in the centre of the coil (C C). When the handle (H) hangs straight down, the instrument is in the position for receiving signals from another station. The current from the line-wire enters at L, and, after traversing the coil and deflecting the needle, escapes through the earth-wire (E), having taken in its course the two tall contact-springs (t t'). To send a current to another station, the handle (H) is moved to one side, and the current sent will be positive or negative according to the side to which the handle is moved. The handle turns the cylindrical arbor (a b), which is divided electrically into two parts by an insulator in the middle of its length. Each of these parts has a pin projecting from it, one pin being above, and the other below. These are vertical when the handle is vertical, and are then doing no duty; but, when the handle is put to one side, the upper pin (which is attached to b) makes contact with one of the tall springs (t t'), at the same time pushing it away from the metallic rest (k), and thus putting it out of connection with the other tall spring; while the lower pin (which is attached to a) makes contact with one of two short springs (T T), only one of which is shown in the figure. There is permanent connection between a and the negative pole of the battery through the spring s, and between b and the positive pole through the spring s' . In the position represented in the figure, a serves to connect the negative pole of the battery with the earth, and b serves to connect the positive pole with the spring t', down which the current passes from the point of contact of the pin, and then through the coil to the line-wire at L. The needle of the sending station is thus deflected to the same side as that of the receiving station.
Fig. 4.
Breguet's Indicator.
If the handle were moved to the other side, b would serve to connect the positive pole with the earth, and a would establish connection between the negative pole and the coil, which is itself connected with the line-wire.
Since the English telegraphs came into the hands of the post-office, the alphabet devised by Wheatstone and Cooke has been given up, and the Morse alphabet, which we give in a later section, adopted in its place. In the Morse alphabet, which is now the telegraphic alphabet of all nations, the shortest signs are allotted to those letters which occur most frequently. This was not the case with the old needle-alphabet, which was rather planned with the view of assisting the memory; and experience has shown that such assistance is quite unnecessary. The needle instrument is also, to a great extent, being superseded by Morse's instrument.
Telegraphs in which the ordinary letters of the alphabet are ranged round the circumference of a dial, and are pointed at by a revolving hand, are specially convenient for those who are not professional telegraphists. They are constructed on the principle of step-by-step motion, the hand being advanced by successive steps, each representing one current sent or stopped.
One of the simplest instruments of this class is Breguet's, which is extensively used on the French railways. Fig. 4 represents the exterior of the receiving instrument. The dial is inscribed with the 25 letters of the French alphabet and a cross, making 26 signals in all. The hand (as in other step-by-step telegraphs) advances only in one direction, which is the same as that of the hands of a clock, stopping before each letter which is to be indicated, and pointing to the cross at the end of each word. Fig. 5 shows the mechanism by which the
Fig. 5.
Mechanism of Indicator.
motion is produced. A is the armature of an electro-magnet, the magnet itself being removed in the figure to allow the other parts to be better seen. The two dotted circles traced on the armature represent vertical sections of the two coils, which rest on the bottom of the box, and have their axes horizontal. If introduced, they would nearly conceal the armature from view. The armature turns about an horizontal axis (V V'), and is attached to an opposing spring which draws it back from the magnet. The tension of this spring can be regulated 
Fig. 6.—Escapement by means of a lever acted on by a key outside the box. When a current is sent, the armature is attracted to the magnet; when the current ceases, the spring draws it back; and it thus moves continually to and fro during the transmission of a message. An upright arm (l) is attached to the armature, and carries an horizontal arm (c), which lies between the two prongs of a fork (d), represented on a larger scale in Fig. 6. This fork vibrates about an horizontal axis (a b), to which is attached the vertical pallet (i). This pallet acts upon an escapement-wheel (O), toothed in a peculiar way, the thickness of the teeth being only half the thickness of the wheel, and the teeth on one-half of the thickness being opposite the spaces on the other half. The total number of teeth is 26, thirteen on each half of the thickness.
When no current is passing, the pallet (i) is engaged with one of the teeth on the remote side, as represented in Fig. 6. When a current passes, the armature is attracted, and the pallet is moved over to
Fig. 7.
Breguet's Manipulator.
the near side, thus releasing the tooth with which it was previously engaged, and becoming engaged with the next tooth on the near side of the wheel. The wheel, which is urged by a clock-movement, thus advances 1⁄26 of a revolution; and the hand on the dial, being attached to the wheel, moves forward one letter. When the current ceases, the pallet moves back to the remote side, and the hand is advanced another letter. If the hand is initially at the cross, it will be advanced to any required letter by so arranging matters that the number of currents plus the number of interruptions shall be equal to the number denoting the place of the letter in the alphabet. To effect this arrangement is the office of the sending instrument.
This is represented in Fig. 7. There is a dial inscribed with 25 letters and a cross, like that of the receiving instrument, and an arm
Fig. 8.
Vibrating Alarum.which can be carried round the dial by a handle (M). There are 26 notches cut in the edge of the dial, in which a pin attached to the movable arm catches; and the arm is allowed sufficient play to and from the face of the dial to admit of this pin being easily released or inserted. When the pin is in one of the notches, the instrument is in position for transmitting the corresponding letter. The action is as follows: A toothed or rather undulated wheel is fixed on the same axis as the revolving arm, and turns with it. There are 13 projections and 13 hollows on its circumference, a few of which are shown in the figure where the face is cut away. A bent lever (T), movable about an axis at a, bears at one end against the circumference of the undulated wheel, while its other end plays between two points (P, Q), and is in contact with one or other of these points whenever its upper end bears against a hollow or a projection. P is in connection with a battery, and Q with the earth, the undulated wheel being in connection with the line-wire. The movement of the handle thus produces the requisite number of currents and interruptions.
Besides the sending and receiving apparatus above described, each station has an alarum, which is employed to call attention before sending a dispatch. There are several different kinds. Fig. 8 represents the vibrating alarum, which is one of the simplest. It contains an electro-magnet (e), with an armature (f), fixed to the end of an elastic plate. When no current is passing through the coil, the armature is held back by the elasticity of this plate, so as to press against a contact-spring (g) connected with the binding-screw (m). The terminals of the coil are at the binding-screws (p, p'), the former of which is in connection with the armature, and the latter with the earth. As long as the armature presses against the spring (g), there is communication between the two binding-screws (m and p') through the coil; but the passing of a current produces attraction of the armature, which draws it away from g, and interrupts the current. The electro-magnet is thus demagnetized, and the armature springs back against g, so as to allow a fresh current to pass. The armature is thus kept in continual vibration; and a hammer (K), which it carries above, produces repeated strokes on a bell (T).
Morse's apparatus, first tried in America about 1837, is now perhaps the most extensively used of all.
His receiving instrument, or indicator, in its primitive simplicity, consists (Fig. 9) of an electro-magnet, a lever movable about an axis, carrying a soft-iron armature at one end, and a pencil at the other, and a strip of paper which is drawn past the pencil by a pair of rollers.
Fig. 9.
Morse's Telegraph.
As the pencil soon became blunt, and was uncertain in its marking a point, which scratched the paper, was substituted. This has now, to a great extent, been superseded by an ink-writer, which requires the exertion of less force, and at the same time leaves a more visible trace.
Fig. 10 represents Morse's indicator as modified by Digney. A train of clock-work, not shown in the figure, drives one pair of rollers (n m), which draw forward a strip of paper (p p) forming part of a long roll (K). The same train turns the printing-cylinder (H), the surface of which is kept constantly charged with a thick, greasy ink by rolling-contact with the ink-pad (L). The armature (B B') of the electro-magnet (A) is mounted on an axis at C, and carries a style at its extremity just beneath the printing-cylinder. When a current passes, the armature is attracted, and the style presses the paper against the printing-cylinder, causing a line to be printed on it, the length of which depends on the duration of the current, as the paper continues to advance without interruption. The lines actually employed are of two lengths, one being made as short as possible (-), and called a dot, and the other being about three times as long (), and called a dash. The opposing spring (D) restores the armature to its original position the moment the current ceases.
Fig. 10.
Morse's Telegraph—Modified Form.
Morse's key (Fig. 11) is simply a brass lever, mounted on a hinge at A, and pressed up by the spring f. When the operator puts down the key, by pressing on the button (K) with his finger, the projections
Fig. 11.
Morse's Key.
(c d) are brought into contact, and a current passes from the battery-wire (P) to the line-wire (L). When the key is up, the projections (a b) are in contact, and currents arriving by the line-wire pass by the wire R to the indicator or the relay. By keeping the key down for a longer or shorter time, a dash or a dot is produced at the station to which the signal is sent. The dash and dot are combined in different ways to indicate the different letters, as shown in the following scheme, which is now generally adopted both in Europe and America:
Morse's alphabet.
| A | ‐ | — | J | ‐ | — | — | — | T | — | 1 | ‐ | — | — | — | — | ||||||||||||
| Ä | ‐ | — | ‐ | — | K | — | ‐ | — | U | ‐ | ‐ | — | 2 | ‐ | ‐ | — | — | ‐ | ‐ | ||||||||
| B | — | ‐ | ‐ | ‐ | L | ‐ | — | ‐ | ‐ | Ü | ‐ | ‐ | — | — | 3 | ‐ | ‐ | ‐ | — | — | |||||||
| C | — | ‐ | — | ‐ | M | — | — | V | ‐ | ‐ | ‐ | — | 4 | ‐ | ‐ | ‐ | ‐ | — | |||||||||
| D | — | ‐ | ‐ | N | — | ‐ | W | ‐ | — | — | 5 | ‐ | ‐ | ‐ | ‐ | ‐ | |||||||||||
| E | ‐ | O | — | — | — | X | — | ‐ | ‐ | — | 6 | — | ‐ | ‐ | ‐ | ‐ | |||||||||||
| É | ‐ | ‐ | — | ‐ | ‐ | Ö | — | — | — | ‐ | Y | — | ‐ | — | — | 7 | — | — | ‐ | ‐ | ‐ | ||||||
| F | ‐ | ‐ | — | ‐ | P | ‐ | — | — | ‐ | Z | — | — | ‐ | ‐ | 8 | — | — | — | ‐ | ‐ | |||||||
| G | — | — | ‐ | Q | — | — | ‐ | — | Ch | — | — | — | — | 9 | — | — | — | — | ‐ | ||||||||
| H | ‐ | ‐ | ‐ | ‐ | R | ‐ | — | ‐ | 0 | — | — | — | — | — | |||||||||||||
| I | ‐ | ‐ | S | ‐ | ‐ | ‐ | Understood | ‐ | ‐ | ‐ | — | ‐ | |||||||||||||||
A space about equal to the length of a dash is left between two letters and a space of about twice this length between two words.
In needle-telegraphs, the dot is represented by a deflection to the left, and the dash by a deflection to the right.
Fig. 12 represents Morse's indicator in connection with what is
Fig. 12.
Morse's Apparatus, with Relay.
called a relay; that is to say, an apparatus which, on receiving a feeble current from a distance, sends on a much stronger current from a battery on the spot. The key (B) being up, a current arriving by the line-wire passes through the key from c to a, thence through another wire to the coil of the electro-magnet belonging to the relay, and through this coil to earth. The electro-magnet of the relay attracts an armature, the contact of which with the magnet completes the circuit of the local battery, in which circuit the coil belonging to the indicator is included. The armature of the indicator is thus compelled to follow the movements of the armature of the relay.
Relays are used when the currents which arrive are too much enfeebled to give clear indications by direct action. They are also frequently introduced at intermediate points in long lines which could not otherwise be worked through from end to end. The analogy of this use to change of horses on a long journey is the origin of the name. Relays are also frequently used in connection with alarums when these are large and powerful.
The employment of Morse's alphabet requires on the average about three currents to be sent per letter. The extension of telegraphic service has stimulated the industry of inventors to devise means for obtaining more rapid transmission. Hughes, about 1859, invented a system which requires only one current to be sent for each letter, and which, accordingly, sends messages in about a third of the time required by Morse's method. Hughes's machine also prints its messages in Roman characters on a strip of paper. These advantages are, however, obtained at the expense of extreme complexity in the apparatus employed. It is only fit for the use of skilled hands; but it is extensively employed on important lines of telegraph. We will proceed to indicate the fundamental arrangements of this marvellous piece of ingenuity.
Fig. 13 is a general view of the machine. It is propelled by powerful clock-work, with a driving weight of about 120 lbs., and with a regulator consisting of a vibrating spring (l) acting upon a 'scape-wheel. A travelling weight on the spring can be moved toward either end to regulate the quickness of the vibrations. The clock-work drives three shafts or axes: 1. The type-shaft, so called because it carries at its extremity the type-wheel (T), which has the letters of the alphabet engraved in relief on its circumference at equal distances, except that a blank space occurs at one place instead of a letter; 2. The printing-shaft, which turns much faster than the type-shaft, making sometimes 700 revolutions per minute, and carrying the fly-wheel (V). These two axes are horizontal, and are separately represented in Fig. 14; 3. A vertical shaft (a) having the same velocity as the type-wheel, which drives it by means of bevel-wheels.
This vertical shaft consists of two metallic portions, insulated from each other by an ivory connecting-piece. In the position represented in Fig. 14, these two metallic pai-ts are electrically connected by means of the screw (V), but they will be disconnected by raising the movable piece (v).
The revolving arm composed of the pieces v' v is called the chariot.
Hughes's Printing Telegraph
It revolves with the vertical shaft, and travels over a disk (D) pierced with as many holes as there are letters on the type-wheel, these holes being ranged in a circle round the base of the shaft, and at such a distance from the shaft that the extremity of the chariot passes exactly over them. In these holes are the upper ends of a set of pins (g), which are raised by putting down a set of keys (B N) resembling those of a piano. When the chariot passes over a pin which is thus raised, the piece v is lifted away from v', and the current from the battery, which previously passed from the pin through v and v' to the earth, is now cut off from v', and passes through v to the electro-magnet, and thence to the line-wire.
Fig. 14.
Type-shaft and Printing-shaft.
This is the process for sending signals. We will now explain how a current thus sent causes a letter to be printed by the type-wheels at both the sending and receiving stations, the sending and receiving instruments being precisely alike.
The current traverses the coils of an electro-magnet (E, Fig. 13), beneath which is a permanent steel horseshoe magnet, having its poles in contact with the soft-iron cores of the electro-magnet. When no current is passing, the influence of the steel renders these cores temporary magnets, and enables them to hold the movable armature (p) against the force of an opposing spring. The current is in such a direction that it tends to reverse the magnetism induced by the steel. It is not necessary, however, that it should be strong enough to produce an actual reversal, but merely that it should weaken the induced magnetism of the cores sufficiently to enable the opposing spring to overpower them. This is one of the most original parts of Hughes's apparatus, and is a main cause of its extreme sensibility.
The printing-shaft consists of two portions, one of which (I, Fig. 14) carries the fly-wheel (V), and turns uniformly under the action of the clock-movement; the other, which is next the front of the machine, remains at rest when no current is passing; but, when the armature of the magnet rises, the two parts of the shaft become locked together by means of the ratchet-wheel and click (i i' ).
The portion of the shaft which is thus turned every time a current passes, carries a very acute cam or tooth (p, Fig. 15), which suddenly raises the lever (a b), movable about an axis at one end (T), and, by so doing, raises the paper against the type-wheel, and prints the letter. In order thus to print a letter from the rim of a wheel which continues turning, very rapid movement is necessary. This is secured by making the opposing spring which moves the armature very powerful, and the cam (p) very acute. The same movement of the lever which produces
Fig. 15.
Mechanism for Printing.
the impression, raises the arm (J U), which carries a spring (r) with a click at its extremity. This click, in its ascent, glides over the teeth of the ratchet-wheel (E), but locks into the teeth and turns the wheel in its descent, and, by so doing, advances the paper through the distance corresponding to one letter. The spacing of the words is obtained by the help of the blank on the type-wheel.
The type-wheel should admit of easy adjustment to restore it to agreement with the chariot when accidental derangement may have occurred. For this purpose the shaft (G) is made hollow, its internal and external portions being merely locked together by the click (m), which is held in its place by a permanent current in either direction. On pressing down the button (Q, Fig. 13), the click (m) is raised by the piece E, so as to leave the type-wheel free, and a pin is provided which catches in a notch corresponding to the blank on the type-wheel. The adjustment can also be made by hand.
Lastly, the shaft (I) carries a third cam, which, at each revolution of this axis, engages with a very coarse-toothed wheel (T'), set on the same axis as the type-wheel, and pushes it a little forward or backward without detaching it from the driving-gear. Small discrepancies between the velocities of the type-wheel and chariot are thus corrected as often as a letter is printed. This contrivance serves to keep the receiving instrument from gaining or losing on the sending instrument during the transmission of a message. The type-wheel of the receiving instrument must be adjusted before the message begins, so as to make the two instruments start at the same letter.
Suppose a metallic cylinder, permanently connected with the earth, to be revolving, carrying with it on its surface a strip of paper freshly impregnated with cyanide of potassium. Also suppose a very light steel point permanently connected with the line-wire, and resting in contact with the paper. Every time that a current arrives by the line-wire, chemical action will take place at the point of contact, and the paper at this point will be discolored by the formation of Prussian blue. This is the principle of Bain's electro-chemical telegraph, which leaves a record in the shape of dots and dashes of Prussian blue. The apparatus for sending signals is the same as in Morse's system. The paper must not be too wet, or the record will be blurred; neither must it be too dry, for then no record will be obtained.
An autographic telegraph is one which produces at the receiving station a facsimile of the original dispatch. The best known-instruments of this class are those of Bonelli and Caselli. We shall describe the latter.
Fig. 16.
Principle of Caselli's Telegraph
At the sending station a sheet of metallized paper, with the dispatch written upon it in a greasy kind of ink, is laid upon a cylindric surface (M, Fig. 16). At the receiving station there is a similar cylindric surface (R), on which a sheet of Bain's chemical paper is laid. Two styles, driven by pendulums which oscillate with exact synchronism, move over the surfaces of the two sheets, describing upon them very close parallel lines at a uniform distance apart, both styles being in permanent connection with the line-wire. The current is furnished by the battery (P) at the sending station. When the style is on a conducting portion of the paper M, the current takes the course of least resistance (ABCD), no sensible portion of it going to the other station. On the other hand, when the style is on the non-conducting
Fig. 17.
Caselli's Telegraph.
ink in winch the dispatch is written, the circuit ABCD is broken, and the current travels through the line-wire. At this moment the style on the sheet R is in exactly the same position as that on the sheet M, by reason of the synchronism of the pendulums, and a blue line will be produced which will be the exact reproduction of the broken line of the dispatch traversed by the style. Accordingly, when the style of M has described a series of lines close together and covering the sheet, R will be covered with a series of points or lines forming a copy of the dispatch. The tracing point is carried by a lever turning about an axis near its lower end. To this lower end is attached, a connecting rod, jointed at its other end to the pendulum (Fig. 17). While the pendulum swings in one direction, the style traces a line in one direction on the sheet. At the end of this stroke, an action occurs which, besides advancing the style, raises it, so that it does not touch the sheet during the return-stroke.
The synchronism of the pendulums at the two stations, which is absolutely necessary for correct working, is obtained by means of two clocks which are separately regulated to a given rate, the clock-pendulums making two vibrations for one of the telegraphic pendulum. The bob of the latter consists of a mass of iron, and vibrates between two electro-magnets, which are made and unmade according to the position of the clock-pendulum, as the latter makes and breaks the circuit of a local battery. The mass of iron is thus alternately attracted by each of the two magnets as it comes near them, and is prevented from gaining or losing on the clock.
| Fig. 18. | Fig. 19. |
 | |
| Fac-simile of Dispatch. | Copy on Tin-foil. |
It is evident that the Caselli telegraph may be applied to copy not only letters, but a design of any kind; hence the name of pantelegraph which has been given it. Fig. 18 represents a copy thus obtained upon Bain's paper. Fig. 19 represents a copy, obtained at the same time upon a sheet of tin-foil, such as is usually placed beneath the paper. The current decomposes the moisture of the paper, and the hydrogen thus liberated reduces the oxide of tin, of which a small quantity is always present on the surface. If the foil be then treated with a mixture of nitric and pyrogallic acid, the traces are developed, and come out black.
The Caselli system has been used for some years on the telegraphs around Havre and Lyons, but has not realized the hopes of its promoters, its dispatches being often illegible.
Instead of a series of parallel lines, the styles may be made to trace the successive convolutions of a fine helix, the two sheets being bent round two cylinders, which revolve in equal times, and also advance longitudinally.
- ↑ Abridged from Deschanel's "Natural Philosophy," Part III.: "Electricity and Magnetism."