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Originally appearing in Volume V26, Page 525 of the 1911 Encyclopedia Britannica.
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AMERICAN CODE A B C D E F G H Open circuit, single-current system. plates. It will be observed that the circuit is not in this case actually open; the meaning of the expression " open circuit " is " no battery to line." In normal circumstances the instruments at both ends are ready to receive, both ends of the line being to earth through the receiving instruments. A signal is sent by de-pressing the key K, and so changing the contact from a to b, and w thus putting the battery to line. On circuits where the traffic is small it is usual to make one wire serve several stations. At an intermediate or wayside station W, a " switch " S, consisting of three blocks of brass fixed to an insulating base, is sometimes used current System. W may be made the terminal station of LI by inserting plug 3, and of L2 by inserting plug 2, or the instruments may be cut out of circuit by inserting plug 1. In ordinary circumstances the messages from all stations are sent through the whole line, and thus the operator at any station may transmit, if the line is free, by manipulating his key. The connexions for single-current working on the " closed-cir- cuit " system are shown in fig. 17. It differs from the open circuit in only requiring one battery (although, as in the figure, closed- half of it is often placed at each end), in having the re- circuit ceiving instrument between the line and the key, and in system. having the battery continuously to the line. The battery is kept to the line by the bar c, which short-circuits the keys. When signals are to be sent from either station the operator turns the switch c out of contact with the stop b, and then operates precisely as in open circuit sending. This system is more expensive than the open-circuit system, as the battery is always at work; but it offers some advantages on circuits where there are a number a of intermediate stations, as the circuit is under a constant electromotive force and has the same resistance no matter which station is sending or receiving. The arrangement at a wayside station is shown at W. When the circuit FIG.17.—Closed Circuit, Single- is long and contains a large current System. number of stations, the sending battery is sometimes divided among them in order to give greater unirmity of current along the line. When only one battery is used the current at the distant end may be considerably affected by the leakage to earth along the line. If long circuits were worked direct with ordinary instruments, high battery power would be required in order to send- sufficient stogie- current to actuate the apparatus. In such cases it is current usual to employ a local battery to produce the signals, relay and to close the local battery circuit by means of a working. circuit-closing apparatus called a relay, which is practi- cally an electromagnetic key which has its lever attached to the armature of the magnet and which can be worked by a very weak current. The arrangement at a station worked by relay on the " single-current " system is shown in fig. 18, where L is the line wire, joined through the key K to one end of the coil of the relay magnet R, the other end of which is put to earth. When a current passes through R the armature A is attracted and the local circuit is closed through the armature at b. The local battery B, then sends a current through the in- Working. the signal. In the form of relay indicated in the figure the armature is held against the stop a by a spring S. " Single-current " working by means of a non-polarized relay (fig. 18), although general in America, is not adopted in England. Double- In the latter country, when such working is resorted to, The Siemens polarized relay, shown in fig. 19, consists of an armature a, pivoted at one end h in a slot at one end N of a permanent magnet m, the other pole s of which is fixed to the yoke y of a horse-shoe electromagnet M. The armature is placed between the poles of the electromagnet, and being Polarized magnetized by the magnet m it will oscillate to the Pool right or left under the action of the poles of the electro- ys. magnet M according as the current passes through M in one direction or the other. This form of relay is largely used, but in Great Britain it has been entirely displaced by the form shown in fig. 20, which is the most modern pattern of relay used by the British Post Office, known as the " Post Office Standard Relay." In this instrument there are two soft iron tongues, n, s, fixed upon and at right angles to an axle a, which works on pivots at its ends. These tongues are magnetized by the inducing action of a strong horse-shoe permanent magnet, S N, which is made in a curved shape for the sake of compactness. The tongue plays between the poles of two straight electromagnets. The coils of the electromagnets are differentially wound with silk-covered wire, 4 mils ( = "004 inch) in diameter, to a total resistance of 400 ohms. This differential winding enables the instrument to be used for " duplex " working, but the connexions of the wires to the terminal screws are such that the relay can be used for ordinary single working. Although • the relay is a " polarized " one, so that it can be used for " double-current' working, it is equally suitable for " single-current " purposes, as the tongue can be given a bias over to the " spacing " side, i.e. to 8 Iii(---- the side on which no current passes through the local circuit. The standard relay will work single current with a current of 3 milliamperes, though in practice about io would be used. Worked double current—that is, with the tongue set neutral, having no bias either to the spacing or marking side—the relay will give good signals with I2 milliampere of current, though in practice 10 milliamperes are provided. The lightness of the moving part enables great rapidity of action to be obtained, which for fast speed working is very essential. The relay tongue, being perfectly free to move, can be actuated by a comparatively weak current. Normally a switch attached to the key cuts the battery off, and connects the line direct through the receiving relay; this switch is turned to " send " when transmission commences, and is moved back to " receive " when it ceases: this movement is done quite mechanically by the telegraphist, and as it is practically never forgotten, automatic devices (which have often been suggested) to effect the turning are wholly unnecessary. Fig. 21 shows the general arrangement of the connexions for double-current working; the galvanometer G is used for the purpose of th h e, current a " polarized relay " (fig. 20) with a bias is used, but on system. all important lines worked by sounders the " double- current " system is employed. In this the tongue of the relay is kept over to the spacing side by means of a current flowing in one direction, but on the depression of the signalling key the cur-rent is reversed, moving the relay tongue over to the marking side. Standard Relay. indicating whether a station is calling, in case the relay sticks or is out of adjustment. The key K (shown in general plan), when worked, sends reversed currents from the battery B. In cases where " universal battery " working, i.e. the working of several instruments from one set of batteries or accumulators, is adopted, the positive and negative currents have to be sent from independent batteries, as shown by fig. 22. The stop a of the key K is connected through a switch S with one pole of the battery B, and the stop b in the usual way with the other pole. Suppose the arm c of the switch S to be in contact with 2; then when the key is manipulated it sends alternately positive and negative currents into the line. If the positive is called the signalling current, the line will be charged positively each time a signal is sent; but as soon as the signal is completed a negative charge is communicated to the line, thus hastening the discharge and the return of the relay tongue to its insulated stop. When a local instrument such as a sounder (fig. 15) is worked from a relay, the dying away of the magnetism in the iron cores of the electromagnet, when the relay tongue moves from the Spark marking to the spacing side, i.e. when the local battery is coils• cut off, sets up an induced current of high tension, which causes a spark to jump across the contact points of the relay, and by oxidizing them makes it necessary for them to be frequently cleaned. In order to avoid this sparking, every local instrument in the British Postal Telegraph Department has a " spark " coil connected across the terminals of the electromagnet. The spark coil has a resistance about ten times as great as that of the electromagnet it shunts, and the wire of which it is composed is double wound so as to have no retarding effect on the induced current, which circulates through the spark coil instead of jumping in the form of a spark across the contact points. The device is a most effectual one. On long circuits wcrked by the Wheatstone fast-speed apparatus, and especially on those in which a submarine cable is included, it Repeaters. is found necessary to introduce " repeaters " half-way, in order to enable a high speed to be maintained. The speed at which a circuit can be worked depends upon what is known as the " KR " of the line, i.e. the product of the total capacity and the total resistance, both the capacity and the resistance having a retarding effect on the signals. By dividing a line into two halves the working speed will be dependent upon the KR of the longest half, and as both K and R are directly proportional to the length of the line, the KR product for the half of a circuit is but one quarter that of the whole length of the circuit, and the retardation is correspondingly small. Thus the speed on a line at which the repeater is situated exactly midway will be four times that of the line worked direct. Repeaters (or translators, as they are some-times termed) are in Great Britain only used on fast-speed circuits; they are in no case found necessary on circuits worked by hand. or at " key speed " as it is called. Duplex telegraphy consists in the simultaneous transmission of two messages, one in each direction, over the same wire. The solution of this problem was attempted by J. W. Gintl of Vienna in 1853 and in the following year by Frischen and by Siemens and Halske. Within a few years several methods had been proposed by different inventors, but none was at first very successful, not from any fault in the principle, but because the effect of electrostatic capacity of the line was left out of account in the early arrangements. The first to introduce a really good practical system of duplex telegraphy, in which this difficulty was sufficiently overcome for land line purposes, was J. B. Stearns of Boston (Mass.). In order that the line between two stations may be worked on the duplex system it is essential that the receiving instrument shall not be acted on by the outgoing currents, but shall respond to incoming currents. The two methods most commonly employed are the differential and bridge methods. In fig. 23, representing the " differential " metjhod, B is the sending battery, Bl a resistance equal to that of the battery, R a rheostat and C an adjustable condenser. Suppose the Differ-winding to be depressed, then a current flows through one Differ-winding of the differential relay to line and through the method. other winding and rheostat to earth. Now if the values of the rheostat and condenser are adjusted so as to make the rise and fall of the outgoing current through both windings of the relay exactly equal, then no effect is produced on the armature of the relay, as the two currents neutralize each other's magnetizing effect. Incoming currents pass from line through one coil of the relay, the key, and either the battery or battery resistance, according as whether the key is raised or depressed. The result is that the armature of the relay is attracted, and currents are sent through the sounder from the local battery, producing the signals from the distant station. When the key is in the middle position, that is, not making connexion with either the front or back contacts, the received currents pass through both coils of the relay and the rheostat; no interference is, however, felt from this extra resistance because, although the current is halved, it has double the effect on the relay, because it passes through two coils instead of one. Line Earth In the " bridge " method (fig. 24), instead of sending the currents through the two coils of a differentially wound relay or receiving instrument as in Frischen's method, two resistances a and b are inserted, and the receiving instrument is joined between P and Q. The currents thus divide at the point D, and it is clear that if the difference of potential between P and Q is unaffected by closing the sending key, then no change of current will take place in the instrument circuit. The P Line Receiving Instrument R -IIIIIIilIl relative potential of P and Q is not affected by the manipulation of the sending key if the resistance of a bears the same proportion to that of b as the resistance of the line does to that of the resistance R; hence that is the arrangement used. One very great advantage in this method is that the instrument used between P and Q may be of any ordinary form, i.e. relay, Hughes, siphon re-corder, &c. Most important cables, such as those of the Eastern Telegraph Company and the various Atlantic cables, are worked duplex on Muirhead's plan. What may be called a mechanical method of duplexing a cable was described by Lord Duplex Kelvin in a patent taken out by him in 1858. In this, as working in the ordinary methods, a differentially wound receiving oncahieS. instrument was used, one coil being connected with the cable and the other with the earth; but it differed from other methods in requiring no " artificial " or balancing cable. The compensation was to be obtained by working a slide resistance included in the circuit of the compensating coil, either by the sending key or by clockwork released by the key, so as to vary the resistance in that Duplex telegraphy. Bridge method. circuit according to any law which might be required to prevent the receiving instrument being affected by the outgoing current. Four years later Varley patented his artificial cable, which was the first near approach to a successful solution of the duplex problem on the principle now adopted. It was not, however, a sufficiently perfect representation of a laid cable to serve for duplexing cables of more than a few hundred miles in length. By a modification of the bridge method, applied with excellent results by Dr Muirhead to submarine work, condensers are substituted for a and b, one being also placed in the circuit between P and Q. In this case no current flows from the battery through the line or instruments, the whole action being inductive. As we have already stated, the distribution of the capacity along the resistance R must in submarine cable work be made to correspond very accurately with the distribution of the capacity along the resistance of the cable. This is accomplished by Dr Muirhead in the following manner. One side of a sheet of paraffined paper is covered with a sheet of conducting substance, say tinfoil, and over the other side narrow strips of the same substance are arranged gridironwise to form a continuous circuit along the strip. The breadth and thickness of the strip and the thickness of the paraffined paper are adjusted so that the relative resistance and capacity of this arrangement are the same as those of the cable with which it is intended to be used. A large number of such sheets are prepared and placed together, one over the other, the end of the strip of the first sheet being connected with the beginning of the strip of the second, and so on to the last sheet, the whole representing the conductor of the cable. In the same way all the conducting sheets on the other side of the paper are connected together and form the earth-plate of this artificial cable, thus representing the sea. The leakage through the insulator of the cable is compensated for by connecting high resistances between different points of the strip conductor and the earth coating. Faults or any other irregularity in the cable may be represented by putting resistances of the proper kind into the artificial line. This system of duplexing cables has proved remarkably successful. Quadruplex telegraphy consists in the simultaneous trans-mission of two messages from each end of the line. The only new problem introduced is the simultaneous transmission of Quad- two messages in the same direction; this is sometimes ruplex called " diplex transmission." The solution of this tele- problem was attempted by Dr J. B. Stark of Vienna graphy. in 1855, and during the next ten years it was worked at by Bosscha, Kramer, Maron, Schaak, Schreder, Wartmann and others. The first to attain practical success was Edison, and his method with some modifications is still the one in most general use. The arrangement is shown in fig. 25, and indicates the general principle involved. Kl and K2 are two transmitting keys; the former reverses the direction of the line current, the latter increases the strength irrespective of direction, by joining on another battery when the key is depressed. RI and R2 are relays for receiving the currents; the former is polarized and .responds to reversals of current, while the latter is non-polarized and responds only to the increased current from K2 irrespective of the direction of that current. This arrangement can be duplexed in the way already explained, by providing differential relays and arranging for the outgoing currents to divide differentially through the two relays at each end. The " multiplex " system devised by Patrick B. Delany (which was adopted to a limited extent in Great Britain, but has now been entirely discarded) had for its object the working of a num- ber of instruments simultaneously on one wire. The general principle of the arrangement of the apparatus is shown by fig. 26. Arms a and b, one at each station A and B, are connected to the line wire, and are made to rotate simultaneously over metallic segments, I, 2, 3, 4, and i', 2', 3', 4', at the two stations, so that when the arm a is on segment I at A, then b is on segment 1' at B, and so on. At each station sets of telegraph apparatus are connected to the segments, so that when the arms are kept rotating the set connected to I becomes periodically con- nected to the set connected to 1', the set connected to 2 to the set connected to 2', and so on. In practice the number of segments actually employed is much greater than that indicated on the figure, and the segments are arranged in a number of groups, as shown by fig. 27, all the segments i being connected together, all the segments 2, all the segments 3, and all the segments 4. To each group is connected a set of apparatus; hence during a complete revolution of the arms a pair of instruments (at station A and station B) will be in communication four times, and the intervals during which any particular set of instruments at the two stations are not in connexion with each other become much smaller than in the case of fig. 26. In practice this subdivision of the segments is so far extended that the intervals of disconnexion become extremely A B 2 -~ t' tine small, and each set of apparatus works as if it were alone connected to the line. As many as 162 segments in eight groups are practically used. The arm which moves round over the segments rotates at the rate of three revolutions per second, and is kept in motion by means of an iron toothed wheel, the rim of which is set in close proximity to the poles of an electromagnet. Through this electromagnet pass impulses of current regulated in frequency by a tuning-fork contact breaker; these impulses, acting on the teeth of the iron wheel, by a series of pulls keep it in uniform rotation. If the rates of vibration of the two tuning-forks at the two stations could be maintained precisely the same, the two arms would rotate in synchronism, but as this uniform vibration cannot be exactly A B / 4~ t +. 3/ .fl 1 3j 2 1 Line Cl ~ 2 4 / 2 3 4 --I. retard the rate of rotation of one or other of the arms. This is effected by means of " correcting " segments, of which there are six sets containing three each. Should the rotating arms fail to pass over these correcting segments at their synchronous positions, correcting currents pass to a relay which cuts off momentarily the current actuating the tuning-fork, thereby altering the rate of vibration of the latter until the arms once more run together uniformly. The actual number of sets of apparatus it was possible to work multiplex depended upon the length of the line, for if the latter were long, retardation effects modified the working conditions. Thus between London and Manchester only four sets of apparatus could be worked, but between London..and Birmingham, a shorter distance, six sets (the maximum for wl ich the system is adapted) were used. Chemical Telegraphs.—A method of recording signals in the Morse code, formerly used to a considerable extent, was to use a chemically prepared ribbon of paper. Suppose, for instance, the paper ribbon to be soaked in a solution of iodide of potassium and a light contact spring made to press continuously on its surface as it is pulled forward by the mechanism. Then, if a current is sent from the spring to the roller through the paper, a brown mark will be made by the spring due to the liberation of iodine. This was the principle of the chemical telegraph proposed by Edward Davy in 1838 and of that proposed by Bain in 1846. Several ingenious applications of his method were proposed and practically worked, as, for example, the copying telegraph of Bakewell and of Cros, by means of which a telegram may be transmitted in the sender's own handwriting; the pantelegraph of Caselli; the auto-graphic telegraphs of Meyer, Lenoir, Sawyer and others; and the autographic typo-telegraph of Bonelli; all forms of the apparatus have, however, fallen into disuse. Automatic Telegraphs.—It was found impossible to make the Morse ink writer so sensitive that it could record signals sent over land lines of several hundred miles in length, if the speed of transmission was very much faster than that which could be effected by hand, and this led to the adoption of automatic methods of transmission. One was proposed by Bain as early as 1846, but it did not come into use. That now employed is, however, practically a development of his Multiplex telegraphy. idea. It consists in punching, by means of " a puncher," a series of holes in a strip of paper in such a way that, when the strip is sent through another instrument, called the " transmitter," the holes cause the circuit to be closed at the proper times and for the proper proportionate intervals for the message to be correctly printed by the receiving instrument or recorder. The most successful apparatus of this kind is that devised by \Vheatstone; others were devised by Siemens and Halske, Garnier, Humaston, Siemens, and Little. In the Wheatstone automatic apparatus three levers are placed side by side, each acting on a set of small punches and wheat- on mechanism for feeding the paper forward a step stone after each operation of the levers. The punches are system. arranged as shown in fig. 28, and the levers are adjusted so that the left-hand one moves a, b, c and punches a row of holes across the paper (group r in the figure), the middle one moves b only and punches a centre hole (2 in the figure), while the right-hand one moves a, b, d, e and punches O4' Ob Od O' 0• Fig. 28.—Wheatstone Punching Apparatus. four holes (3 and 4 in the figure). The whole of this operation represents a dot and a dash or the letter " a." The side rows of holes only are used for transmitting the message, the centre row being required for feeding forward the paper in the transmitter. The perforation of the paper when done by hand is usually performed by means of small mallets, but at the central telegraph office in London, and at other large offices, the keys are only used for opening air-valves, the actual punching being done by pneumatic pressure. In this way several thicknesses of paper can be perforated at the same time, which is a great convenience for press work, since copies of the same message have often to be transmitted to several newspapers at the same time. The mode of using the paper ribbon for the transmission of the message is illustrated in fig. 29. An ebonite beam B is rocked up and down rapidly by a train of mechanism, and moves the cranks A and A' by means of two metal pins P, P'. A and A' carry two light vertical rods S, M, the one as much in front of the other as there is space between two successive holes in the perforated ribbon. To the other ends of A, A', rods H, H' are loosely hinged, their ends passing loosely through holes in the end of the bar L. By means of two collars K, K', the lever L is made to oscillate in unison with the beam B. The operation is as follows : the paper ribbon or perforated slip is moved forward by its centre row of holes at the proper speed above the upper ends of the rods S, M ; should there be no holes in the ribbon then the cranks A, A' will remain stationary, although the beam B continues to rock, since the rods S, M are pressing- against the ribbon and cannot rise. Should, however, a row of holes, like group t, fig. 28, be in the ribbon, the rod M will first be allowed to pass through the paper, and the corresponding movement of crank A' will, through the agency of collet K, throw over lever L, and the battery zinc will be put to the line; at the next half stroke of the beam, S will pass through, and crank A by its movement will, through the agency of collet K', throw over lever L in the reverse direction, so that the battery copper will be put to the line. Thus for a dot, first a negative and then a positive current is sent to the line, the effect of the current continuing during the time required for the paper to travel the space between two holes. Again, suppose groups 3 and 4 to be punched. The first part will be, as before, zinc to the line; at the next half stroke of the beam M will not pass through, as there is no hole in the paper; but at the third half stroke it passes through and copper is put to the line. Thus for a dash the interval between the positive and the negative current is equal to the time the paper takes to travel over twice the space between two successive holes. Hence for sending both a dot and a dash, reverse currents of short duration are sent through the line, but the interval between the reversal is three times as great for the dash as for the dot. In the receiving instrument the electromagnet is constructed in precisely a similar way to the relay (fig. 20), so that the armature, if pulled into any position by either current, remains in that position, whether the current continues to flow or not, until a reverse current is made to act on the magnet. For the dot the armature is deflected by the first current, the ink-wheel being brought into contact with the paper and after a short interval pulled back by the reverse current. In the case of the dash the ink-wheel is brought into contact with the paper by the first current as before and is pulled back by the reverse current after three times the interval. The armature acts on an inking disk on the principle described above, save only that the disk is supplied with ink from a groove in a second wheel, on which it rolls: the grooved wheel is kept turning with one edge in contact with ink in an ink-well. By this method of transmission the battery is always to the line for the same interval of time, and alternately with opposite poles, so that the effect of electrostatic induction is reduced to a minimum. Although it is quite possible to obtain good signals at a rate corresponding to 600 letters per minute, in practice it is found that such a high speed is not advisable, as it is difficult or impossible for even the most skilled operators properly to handle and transcribe from the " slip" on which the signals are recorded. In Squier and Crehore's " Synchronograph " system " sine waves " of current, instead of sharp " makes and breaks," or sharp reversals, are employed for transmitting signals, the waves being Squlerand produced by an alternating-current dynamo, and regu- Grehore lated by means of a perforated paper ribbon, as in the system. Wheatstone automatic system. The arrangement has been found under certain conditions to give better results than those obtained with sharp reversals. In the undulator apparatus, which is similar in general principle to the " siphon recorder " used in submarine telegraphy, a spring or falling weight moves a paper strip beneath one end of The un- a fine silver tube, the other end of which dips into a dulator. vessel containing ink. The siphon is supported on a vertical axle carrying two armatures which are acted upon by two electromagnets. It is in fact the electromagnet and spindle of a telegraph relay with a siphon in place of the tongue. Screw adjustments are provided for closing or opening the air gap between the electromagnets and armatures, for raising or lowering the siphon, and for adjusting the point of the siphon to the centre or side of the paper strip. The received signals are recorded on the paper strip in an undulating continuous line of ink, and are distinguished by the length of deviation from zero. The amplitude of the signals can be varied in several ways, either by a shunt across the electromagnet, or by altering the tension of the controlling springs or by altering the air gap between electromagnets and armatures. Up to too words per minute the signals are easily readable, but beyond that speed they are more difficult to translate, although experts can read them when received at 20o words per minute. Pollak-Virag System.—In the improved Pollak-Virag system the received signals are recorded in characters similar to ordinary hand-writing. The operator actuates a typewriter form of perforator which punches varying groups of holes, representing the different characters, in a paper strip about one inch wide. This slip is then passed through a transmitter fitted with brush contacts and connected to the two line wires of a metallic loop. One circuit is formed by the loop itself, and a second, quite independent, by the two wires in parallel, earthed at each end. At the receiving end there are two telephone receivers, one joined in the loop circuit, the other in the earth return circuit. The diaphragms of these are mechanically connected to a small mirror and control its movement in accordance with the strength and direction of the received currents. One diaphragm gives the mirror a movement in a vertical direction while the other gives it a horizontal motion. The two acting together can thus give the mirror any desired movement within limits. A ray of light is directed upon the mirror, and the motion of the latter, due to the varying strengths and direction of the received currents, is made to write the transmitted signals upon a strip of bromide photographic paper about three inches wide. The line of writing is of course continuous, there being no break, although there is a space between words. The writing, although not well formed, is sufficiently distinct for ordinary messages; the figures 3, 5, and 8 are, however, liable to be mistaken for each other, being very similar in appearance. The bromide paper is automatically passed through a developing bath, a fixing bath, and drying rollers. This operation occupies about twelve seconds, giving a message written in column form ready for delivery. It is not a system likely to have general application. Type Printing Telegraphs.—The first considerable improvement in type printing telegraphs was made by D. E. Hughes in 1855. Hughes In the Hughes instrument two trains of clockwork iau Hughes one at each end of the line, are kept moving meat. at the same speed. Each instrument is provided with a keyboard, resembling that of a small piano, the key levers of which communicate with a circular row of vertical pins. A horizontal arm fixed to a vertical shaft in gear with the mechanism sweeps over these pins at the rate of about two revolutions per second. When a key is depressed, slightly raising one of the pins, the horizontal arm will pass over it and in doing so will momentarily join the battery to the line. The current thus sent to the line may be made either to act directly on the printing instrument or to close a local circuit by means of a relay For simplicity we will suppose direct action. The current then passes through the coils of an electromagnet, which releases the printing mechanism. The electromagnet consists of two coils, each wound on a soft iron core fixed to the poles of a strong permanent horse-shoe magnet. The armature of the electromagnet is normally attracted by the effect of the permanent magnet, but it is furnished with two antagonistic springs tending to throw it upwards. These springs are so adjusted that they are not quite "able to release the armature. When a current comes in from line it passes through the electromagnet in such a direction as to weaken the effect of the permanent magnet; hence the springs are able to release the armature, which rises smartly and in its turn releases the printing mechanism. Either a weight or a motor is used for making the movements of the mechanism required to effect the printing of the signals. The type-wheel is carried round continuously by the mechanism to which it is attached by a friction disk and ratchet drive. An axle carrying four cams is normally at rest, but it is thrown into gear with the mechanism when the armature rises, makes one complete revolution, and comes to rest ready for the next signal. In its revolution one of its cams engages with the correcting wheel attached to the type-wheel in order to ensure that the latter is in the correct position for printing a complete letter; the second cam lifts the paper against the type-wheel and prints the letter; the third moves forward the paper tape one space to be ready for the printing of the next letter; and the last cam replaces the armature on the cores of the electromagnet. This complete operation occupies about one-twelfth of a second. It is of course necessary that two instruments working together should have the same speed. This is obtained by causing one of them to send a series of signals from one particular key, while the operator at the other station adjusts his speed until he receives the same signal after short-circuiting his electromagnet for ten revolutions. Both type-wheels are then set to zero by the lever provided for that purpose, and released by the current from the letter-blank key; then all subsequent signals will be recorded similarly at the sending and receiving ends. If by any chance wrong signals are printed or the instruments get out of phase, the sender is stopped by the receiver sending a few signals, after which both type-wheels are again set to zero and correspondence continued. This system of telegraphic printing has a great advantage over the step-by-step system in avoiding the necessity for the rapidly acting electric escapement, which, however skilfully planned and executed, is always liable to failure when worked too rapidly. In Hughes's instrument almost perfect accuracy and certainty have been attained; and in actual practice it has proved to be decidedly superior to all previous type-printing telegraphs, not only in speed and accuracy, but in less liability to mechanical derangement from wear and tear and from accident. It involves many novel features: the receiving electromagnet is of peculiar construction and remarkable efficiency and the transmitting apparatus has a contrivance to prevent unintentional repetitions of a letter through the operator holding his finger too long on a key. This instrument was for some years extensively used in the United States, until superseded by G. M. Phelps's modification of it, known as the " American combination printing telegraph," because it embodied part of Hughes's and part of House's instruments. With this modified form somewhat greater speed was obtained, but it was found difficult to drive, requiring the use of steam or some such motive-power. In a subsequent modification introduced in 1875 an electromotor was applied to drive the printing mechanism. This allowed a shorter train and stronger wheelwork to be used, securing more certain action, and involving less risk of derangement. Hughes's form was taken up by the French government in 1860, and is very largely in use not only in France but in all European countries, including Great Britain. The system brought out in 1874 by Emile Baudot and since considerably developed is a multiplex system giving from two to Baudot six channels on one wire, each channel giving a working system. speed of thirt y words per minute. The channels can be worked in either direction according to the traffic requirements. The line is joined at each end to distributors which are arranged to maintain uniform speed and to control their respective receivers. Each channel consists of a keyboard and receiver both electrically connected to certain parts of the distributor. The keyboard has five keys similar to those of a piano, and the letters and figures are obtained by the different combinations which can be formed by the raised and depressed keys. In the raised position a negative battery is connected to the distributor and in the de-pressed position a positive battery. At regular intervals a rotating arm on the distributor connects the five keys of each keyboard to line, thus passing the signals to the distant station, where they pass through the distributor and certain relays which repeat the currents corresponding to the depressed keys and actuate electromagnets in the receivers. Each receiver is provided with five electromagnets corresponding to the five keys of the keyboard, and the armatures of the electromagnets can thus repeat the various combinations for all the signals allocated to the different combinations of the keys. When a combination of signals has been received and the armatures have taken up their respective positions corresponding to the transmitting keyboard, certain mechanism in the receiver translates the position of the five armatures into a mechanical movement which lifts the paper tape against a type-wheel and prints the corresponding letter. The movement for any particular combination of armatures can only take place once per revolution of the type-wheel and at one particular place. The signals must therefore be sent at regular intervals, and to ensure this being done correctly a telephone or time-tapper is provided at each key-board to warn the operator of the correct moment to depress his keys. The Baudot apparatus can have certain channels extended so as to form a means of continuous communication between one station and two or three others by means of one line. It can also be duplexed or repeated similar to any other telegraph system. In the Murray system the messages are first prepared in the form of a strip of perforated paper about half an inch wide. Per- forating machines equipped with typewriter keyboards Murray are used for the preparation of the messages, two or system. three keyboard perforators being employed at each end of the telegraph lines on which the Murray system is used. The messages in the form of perforated tape are then passed through an automatic transmitter, something like a Wheatstone transmitter, at a speed of about 10o words a minute. At the receiving station electrical mechanisms record the signals once more as perforations in a paper strip forming an exact replica of the transmitting tape. This received perforated tape is then used to control what is known as the printer or automatic typewriter, a machine that translates the tape perforations into letters and prints the messages in Roman type in page form. This printer is purely mechanical, and its speed is very high. An experimental printer constructed about the middle of 19o8 by the British Post Office, operated successfully at the rate of 210 words (1260 letters) per minute. The usual working speed is from 10o to 120 words per minute. The Murray automatic system was designed specially for dealing with heavy traffic on long lines. As it uses the Baudot telegraph alphabet it has an advantage in theory over the Wheatstone using the Morse alphabet in regard to the speed that can be obtained on a long telegraph line in the ratio of eight to five, and this theoretical advantage is more or less realized in practice. The Murray automatic system is not regarded as suitable for short telegraph lines or moderate traffic, printing telegraphs on the multiplex principle being considered preferable in such circumstances. One of the longest circuits upon which it has been successfully worked is that between St Petersburg and Omsk, a distance of approximately 2400 miles of iron wire, with three repeating stations. As in some other systems retransmission is effected from the received-perforated tape. The Creed system is a development of the Morse-Wheatstone system, and provides a keyboard perforator which punches Morse letters or figures on a paper strip by depressing type- Creed writer keys. The slips are passed through an ordinary system. Wheatstone transmitter and actuate Wheatstone receiving apparatus which in turn controls a " Creed receiving perforator." This machine reproduces a copy of the original transmitting slip, which can be passed on to any other Wheatstone circuit or can be run through a " Creed printer," which is a pneumatic machine actuating a typewriter by means of valves. Messages are thus typed upon a slip which is gummed to the telegraph form. The speed of the receiving perforator ranges from 20 to 150 words per minute. In the Rowland multiple method of telegraphic working, the transmitter consists of a mechanical keyboard provided with a series of levers, which effect certain combinations of Rowlaad positive and negative currents for each letter. These system. currents are furnished by an alternator which transmits sine currents over the line and operates a motor at the distant end of the line, both machines running in synchronism. At the receiving end of the circuit a shaft is coupled to the motor; this is provided with gearing which rotates four combining commutators and four type-wheels, which print the letters on the band of paper. There are four transmitters and four receivers, which are operated independently by means of an adaptation of the multiplex system of working, and each circuit is provided with a number of segments set apart for its own use. Each transmitter is therefore able to transmit a separate series of positive and negative currents in different combinations; these are distributed, by suitably arranged distributors and relays at the receiving end of the line, into their respective receivers. The function of the " combiner " in each receiving instrument is so to group the received combination of positive and negative currents that they operate polarized relays in such a manner that the position of the tongues corresponds with the operation of the levers on the transmitter. Since each letter is represented by a specific combination of positive and negative currents, it is possible, by means of the combinations, to close a local circuit at any given interval, and so cause the paper to be pressed against the periphery of the type-wheel at the time when the letter required is opposite. The paper is also caused to advance automatically for each letter, start a fresh line, and also to commence a fresh form at the completion of each message. Telautograph.—Instruments such as the telautograph and telewriter are apparatus for transmitting a facsimile of hand-writing inscribed on a paper at one end of a line, the reproduction being made automatically at the other end of the line at the same time that the message is being written. A successful apparatus for effecting this was devised by Cowper and was known as the writing telegraph. The telautograph is on a similar principle to the Cowper apparatus, the motion of the transmitting pencil or stylus used in writing being resolved by a system of levers into two component rectilinear motions, which are used to control and vary the currents in two distinct electrical circuits. By the action of the two variable currents on the electromagnetic mechanism in the receiver, the two component motions are reproduced and by their combined action on a second system of levers the receiving pen is caused to duplicate the motions of the transmitting pencil. The motion communicated to rollers by the pencil serves to cut resistance in or out of the two line circuits which are connected to the rollers, and thus two independent variable line currents are obtained. In the receiver there is a strong electromagnet, excited by a local current, which has in its circuit two annular air gaps, across which the magnetic field is practically uniform and constant. In these annular spaces there are suspended by springs two light coils of fine copper wire, capable of being moved vertically, and connected in such a manner as to be traversed by the two variable line currents from the transmitter. These coils are drawn down, by the magnetic action of the field on the currents in the coils, into the annular spaces, against the pull of the springs, more or less strongly, according to the strengths of the two line currents. Each coil is attached to a shaft by a bell crank arrangement, and to these shafts there is secured a system of levers similar to that at the transmitter carrying the receiving pencil at the junction. The shafts are turned by the pull of the magnet upon the coils, and the motions of the transmitting pencil are thus reproduced. The Korn telephotographic apparatus is based on the principle of an apparatus devised by Shelford Bidwell in 1881 for the Kern electrical transmission of pictures to a distance, in which Klehoto- use was made of the change in electrical resistance which telep selenium undergoes when acted upon by light. In the graph system. Korn apparatus the light from a Nernst electric lamp is concentrated to a point by means of a lens on the original picture, which is wound on a glass cylinder in the shape of a trans-parent photographic film. A totally reflecting prism placed inside the glass cylinder projects the light which penetrates the film upon a selenium cell situated at the end of the cylinder. An illumination of variable intensity (according to the deeper or lighter shades of the portion of the picture on which the light falls) thus takes place on the selenium cell. As the glass cylinder, driven by a motor, revolves upon its axis while also advancing (by means of a screw thread on the axis), all portions of the picture are successively brought under the beam or pencil of light and cause a beam of varying intensity to fall on the selenium cell. Owing to the variable illumination of the selenium thus produced, the resistance of the latter, and therefore the intensity of the current sent through the line to the receiving station by the battery, will be altered accordingly. At the receiving station a cylinder—which revolves synchronously with the transmitting cylinder—is covered with a photographic film or paper, upon a point of which a pencil of light from a Nernst lamp is concentrated. Before reaching the paper the light passes through perforations in two iron plates which are in fact, the pole pieces of a strong electromagnet; between these is an aluminium shutter which is attached to two parallel wires or thin strips. When there is no current the shutter covers the perforations and no light passes, but when a current traverses the wires they are depressed by electromagnetic action, carrying the shutter with them, and a quantity of light proportional to the current strength is admitted through the perforations. By means of this " light-relay " the intensity of the light acting at any moment upon the sensitized paper is made proportional to the illumination of the selenium in the transmitter. To eliminate the sluggish action of the selenium transmitter a selenium cell similar to that at the transmitting station is arranged at the receiving apparatus, and exposed to precisely similar variations of light, the arrangementbeing such that the lag of this cell counteracts the lag of the transmitting cell. The synchronous revolutions of the transmitting cylinders are effected by making one cylinder revolve slightly faster than the other; after each revolution the cylinder which is accelerated is arrested for a moment by means of a special relay until the difference of speed is accurately compensated for. This device was originally adopted in the d'Arlincourt copying telegraph. Submarine Telegraphy.—For working long submarine cables the apparatus ordinarily employed on land lines cannot be used, as the retarding effect of the electrostatic capacity of the cable is so marked that signals fail to be recorded except at a very slow speed of working. The transmitted signals or electric impulses, which on a land line are sharply defined when received, become attenuated and prolonged in the case of a long cable, and are unable to actuate the comparatively heavy moving parts of which the land line instruments are formed. Other patterns of apparatus are therefore necessary. The arrangement of the apparatus for working some of the most recent cables is shown in Fig. 30. The cable is supposed to be worked duplex; but, if S, G, C2, and AC are removed and the key connected directly with Cs, the arrangement for simplex working is obtained. The apparatus consists of a sending battery B, a reversing transmitting key K, a slide of small resistance S, three condensers Cl, C2, Cs, an artificial cable AC, the receiving instruments I and G, and one or more resistances R for adjusting the leakage current. The peculiar construction of AC has been already referred to. The conductor of the cable is practically insulated, as the condensers in the bridge have a very high resistance; hence no appreciable current ever flows into or out of the line. Two receiving instruments, a siphon recorder and a mirror galvanometer, are shown; one only is absolutely necessary, but it is convenient to have the galvanometer ready, so that in case of accident to the recorder it may be at once switched into circuit by the switch s. When one of the levers of K is depressed, the condenser Cl and the cable, and the condenser C2 and the artificial cable, are simultaneously charged in series; but, if the capacity of Ci bears the same proportion to the capacity of the cable as the capacity of C2 bears to the capacity of the artificial cable, and if the other adjustments are properly made, no charge will be communicated to Cs. After a very short interval of time, the length of which depends on the inductive retardation of the cable, the condensers corresponding to C, and Cs at the other end begin to be charged from the cable, and since the charge of Cs passes through the receiving instrument I or G the signal is recorded. The charging of Cs at the receiving end will take place, no matter what is the absolute potential of the condensers, consequently the incoming signals are not affected by those which are being transmitted from that end. In actual practice the receiving instrument is so sensitive that the difference of potential between the two coatings of the condenser Cs produced by the incoming signal is only a very small fraction of the potential of the battery B. When the key is released the condensers and cables at once begin to return to zero potential, and if the key is depressed and released several times in rapid succession the cable is divided into sections of varying potential, which travel rapidly towards the receiving end, and indicate their arrival there by pros ducing corresponding fluctuations in the charge of the condenser C2. All cables of any great length are worked by reverse currents. A modification (known as the cable code) of the ordinary single needle alphabet is used; that is to say, currents in one direction indicate dots and in the other direction dashes. The general principle on which the instruments for working long submarine cables are based is that of making the moving parts very light and perfectly free to follow the comparatively slow rise and fall of the electric impulses or waves. The simplest form of receiving instrument (formerly much used) is known as the " mirror." In this instrument a small and very light mirror, about Mirror t in. in diameter, attached to a stretched fibre andhaving a small magnetic needle fixed to its back, is arranged within a ments. galvanometer coil so that the influence of the latter causes the mirror (through the action of the magnetic needle) to be turned through a small angle in one direction or the other according to the direction of the current through the coil. A ray of light from a lamp is thrown on the mirror, whence it is reflected upon a white surface or scale set at a distance of about 3 ft., forming a bright spot on the surface; the slightest angular deflexion of the mirror, owing to its distance from the scale, moves the spot of light a very appreciable distance to the right or left according to the direction of the angular movement. These indications form the telegraph alphabet and are read in the same manner as in the case of the " single needle " instrument used on land. The spark recorder in some respects foreshadowed the more perfect instrument—the siphon recorder—which was introduced some years later. Its action was as follows. To an Spark recorder, indicator, suitably supported, a to-and-fro motion was given by the electromagnetic actions due to the electric currents constituting the signals. The indicator was connected with a Ruhmkorff coil or other equivalent apparatus, designed to cause a continual succession of sparks to pass between the indicator and a metal plate situated beneath it and having a plane surface parallel to its line of motion. Over the surface of the plate and between it and the indicator there was passed, at a regularly uniform speed, in a direction perpendicular to the line of motion of the indicator, a material capable of being acted on physically by the sparks, through either their chemical action, their heat, or their perforating force. The record of the signals given by this instrument was an undulating line of fine perforations or spots, and the character and succession of the undulations were used to interpret the signals desired to be sent. In the original form of the siphon recorder (fig. 31), for which Lord Kelvin obtained his first Siphon order. patent in 1867, the indi- cator consisted of a light rectangular signal-coil of fine wire, suspended between the poles of two powerful electromagnets M, M so as to be free to move about its longer axis, which is vertical, and so joined that the electric signal currents through the cable pass through it. A fine glass siphon tube is suspended with freedom to move in only one degree, and is connected with the signal-coil and moves with it. The short leg of the siphon tube dips into an insulated ink-bottle, so that the ink it contains becomes electrified, while the long leg has its open end at a very small distance from a brass table, placed with its surface parallel to the plane in which the mouth of the leg moves, and over which a slip of paper may be passed at a uniform rate, as in the spark recorder. The ink is electrified by a small induction electrical machine E placed on the top of the instrument; this causes it to fall in very minute drops from the open end of the siphon tube upon the brass table or the paper slip passing over it. When therefore the signal-coil moves in obedience to the electric signai-currents passed through it, the motion communicated to the siphon is recorded on the moving slip of paper by a wavy line of ink-marks very close together. The interpretation of the signals is according to the Morse code,—the dot and dash being represented by deflexions of the line of dots to one side or other of the centre line of the paper. A very much simpler form of siphon recorder, constructed by Dr Muirhead, is now in general use. The magnet between the poles of which the rectangular signal coil moves is built up of a number of thin flat horseshoe-shaped permanent magnets of a special quality of steel, and is provided with adjustable pole pieces. The signal coil is suspended by fibres and is mounted together with a fixed soft iron core on a brass plate affixed to a rack, with which a pinion operated by a milled head screw engages. To the brass plate is attached an arm carrying the bridge piece. A wire or fibre carrying the aluminium siphon cradle is stretched across this bridge piece, and on it is also mounted the small electromagnet, forming part of the " vibrator " arrangement with its hinged armature, to which one end of the stretched wire carrying the siphon is fastened. The ink-box is made adjustable, being carried by an arm attached to a pillar provided with a rack with which a pinion operated by a milled head screw engages. The motor is usually supported on a platform at the back of the instrument, its driving-wheel being connected to the shaft of the paper roller by means of a spirally wound steel band. In what is known as the " hybrid " form of recorder the permanent magnets are provided with windings of insulated copper wire; the object of these windings is to pro-vide a means of " refreshing " the magnets by means of a strong current temporarily sent through the coils when required, as it has been found that, owing to magnetic leakage and other causes, the magnets tend to lose their power, especially in hot climates. Instruments of the siphon recorder type have been made to work both with and without electrification of the ink. In the latter case, which is the standard practice, mechanical vibration of the siphon is substituted in the place of electrification of the ink, so as to eliminate the effect of atmospheric conditions which frequently caused discontinuity in the flow of ink. Fig. 33 shows a facsimile of part of a message received and re-corded by a siphon recorder, such as that of fig. 31, from one of the Eastern Telegraph Company's cables about 83o miles long. As the earth is used for completing the electric circuit, the signals received on such sensitive instruments as these are liable to be disturbed by the return currents of other systems in their immediate neighbourhood, which also use the earth as return, when such are of the magnitude generated by the working of electric tramways or similar undertakings, and to obviate this it is necessary to form the " earth " for the cable a few miles out at sea and make connexion thereto by an insulated return wire, which is enclosed in the same sheathing as the core of the main cable. The heavier cores, with the consequent advance in speed of working attainable, have necessitated the introduction of automatic sending, the instruments adopted being in general a modification of the Wheatstone transmitter adapted to the form of cable signals, while the regularity of transmission thus secured has caused its introduction even on circuits where the speed cannot exceed that of the ordinary operator's hand signalling. The automatic curb sender was originally designed by Lord Kelvin for the purpose of diminishing the effect of inductive re- tardation in long cables. In ordinary hand-sending the end of the cable is put to one or the other pole of the battery and to earth alternately, the relative time during which it is to battery and to earth depending to a great extent on the operator. By the automatic curb sender the cable is put to one or the other pole of the battery and then to the reverse pole for definite proportionate times during t i 2 t e r m, e d i a. t o p FIG. 33.-Facsimile of Siphon Recorder Message. each signal. The cable is thus charged first positively and then negatively, or vice versa, for each signal. Owing to the difficulty of maintaining perfect balance on duplexed cables, curb sending is not now used, but the signals are transmitted by means of an apparatus similar to the Wheatstone automatic transmitter used on land lines and differing from the latter only in regard to the alphabet employed; the signals from the transmitter actuate a relay having heavy armatures which in turn transmit the signals to the cable; this arrangement gives very firm signals, a point of great importance for good working. The actual speed or rate of signalling is given approximately by the formula, S =120/ (KR), where S is the number of words per minute, R the total resistance of the conductor in ohms, and K the total capacity in farads. The speed of a cable is given in words per minute, the conventional number of five letters per word being understood, though in actual practice, owing to the extensive use of special codes, the number of letters per word is really between eight and nine; and this forms a considerable factor in lowering the earning capacity of a cable. A relay capable of working at the end of a long cable has long been a desideratum. The difficulty experienced is that of securing a good electrical contact under the very slight pressure obtainable from an instrument excited by attenuated arrival-currents. In an Relays. instrument invented by S. G. Brown (Brit. Pat. 1434 of 1899) it is sought to overcome this difficulty by causing the point of a contact-arm, representing the siphon in the ordinary form of recorder, to traverse the cylindrical surface of a rapidly rotating drum. This surface is divided into two parallel halves by a short insulating space on which the arm normally rests, so that two separate conducting surfaces are provided, with either one of which the arm will make contact in its excursions in one direction or the other from the central position, the direction and duration of contact being governed by the motion of the suspended coil. The great reduction in friction and in electrical resistance of the contact thus effected between the recurved end of the arm and the rotating surface secures the transmission of signals at such a high rate of speed that the combination of this relay with a special form of curb sender allows of the re-transmission of signals into a second cable at a speed not less than that of the siphon recorder worked in the usual way. The special form of curb sender mentioned, termed the " Interpolator," has been devised so as to secure the correct re-transmission of any given number of consecutive elements of a letter which are of the same sign, for when signals are received at the end of a long cable the relay arm will not return to its zero position between consecutive elements of the same sign, but will remain on the respective contact surface during the whole time occupied by such consecutive elements. The instrument consists of two cams, the form of which regulates the components of the curbed signal, one cam being for the dot element and the other for the dash element, which by their sequence give the letter signals; these cams, by means of clutches controlled by the relay, are mechanically rotated by clockwork, the speed of rotation being approximately adjusted to the rate of transmission of a single element, so that the requisite number of consecutive elements is transmitted corresponding to the duration of contact of the relay arm with the side controlling that particular element. By a modification of this apparatus the message, instead of being immediately re-transmitted into the second cable, can he punched on a paper slip, which can be inserted in the usual way into an automatic transmitter, so as to send either cable or Morse signals. Fig. 34 shows the effect of the interpolator in dissecting the consecutive elements of any letter combination. Another instrument (see Brit. Pat, No. 18,261 of 1898) is what may be termed a magnifier, since signals so small as to be almost unreadable on direct record are rendered perfectly legible. The recorder coil is connected mechanically to a second similar coil, which is suspended betweenthe poles of a laminated magnet, so that the motions of the two are similar. This magnet is excited by an alternating current, and the current induced in the second coil is after rectification sent through an ordinary siphon recorder. As the direction and intensity of this induced current are a function of the position of the second coil in its field, and as this position is determined by its mechanical connexion with the recorder coil, it is evident that, by a suitable choice of the electrical elements of the second coil and its alternating field, the indications on the siphon recorder can be magnified to any reasonable extent. By means of a " magnetic shunt " Brown succeeded in increasing the working speed of long submarine cables to the extent of io to 15 per cent. The magnetic shunt (which is connected Magnetic across the receiving instrument) consists of a low resist- shunt. ante coil of some 2000 turns of insulated copper wire, enclosed in a laminated iron circuit, and connected at intervals to a number of terminals so that equal increments of inductance may be obtained. The use of the iron core renders it possible to produce a high inductive effect with a low resistance coil, and thus obtain the necessary slow time constant to which is due the success of this type of magnetic shunt on cable signals. The shunts usually employed with the drum relay (referred to above) have each a resistance of about 30 ohms and an inductance of 20, 30 and 40 henrys respectively. The explanation of the action of the shunt is that all slowly varying currents affect the coil of the receiving instrument and its shunt in inverse proportion to their respective resistances; whereas with the comparatively rapid variations of current used in signalling the coil is forced at the beginning of each element of C D A R V V' Improvements. A, slip as received on recorder, using ordinary relays for translating on to second cable; B, slip as received on recorder, when interpolator is used at intermediate station, for sending on to second cable; C (four cells through a line, KR=3.6), signals with recorder under ordinary conditions; D, all conditions the same as in C, but magnifying relay inserted between the end of the line and the recorder. a signal to take more, and at the end of the element less of the total arrival current from the cable than would traverse it if the shunt were non-inductive. For duplex working a " magnetic bridge " is used. This consists of a low resistance coil of copper wire enclosed in a laminated iron circuit similar to the magnetic shunt already de-scribed. scribed. The coil, however, is arranged so that the bad. sending current enters an adjustable mid-point in the coil and passes through the two halves of the winding to the ends connected to the cable and artificial line respectively. The receiving instrument is joined up across these ends in the usual manner. The action of this bridge resembles the magnetic shunt in its effect on the received signals, as the direction of the winding is the same throughout its length, and thus the full inductive action is produced for curbing purposes. To the sending currents, however, the bridge offers only apparent ohmic resistance due to the fact that the current entering the mid-point of the winding flows through the two halves or arms in opposite direction, and, owing to the winding being on the same iron core, the mutual inductive effect of the two arms on one another neutralizes the self-induction to the sending currents. The average total inductive value of these bridges to received signals is about 40 henrys, and the coil is so arranged that the arms cdntain three sections or blocks of winding each, two of which are joined up to strap connexions, and the Automatic curb sender. p u b t o e h e c c it a 0 i e 3 p J h Vv''v .( third divided into small subdivisions to any terminals of which the cross circuit connexions may be affixed. By this arrangement of the coil winding, similar sections can be thrown in or out of circuit with both arms, and also so combined that any amount of inductance suitable to every class of cable may be obtained. The bridge is provided with two adjustments:—(I) a variable "apex,'having several turns of the winding between each stud to permit of the arms being thrown slightly out of balance as a rough compensation for the differences in the cable and artificial line; and (2) an additional " fine " adjustment in one of the arms by which the small daily balance variations may be corrected. As with other duplex systems it is possible to obtain several approximately correct adjustments with the bridge and its accessories, but only one gives a true balance, and careful experiment is required to make sure that this is obtained. The advantage of using the magnetic bridge duplex method is that the maximum current is sent to line or cable, and the receiving system benefits accordingly. (H. R. K.)
End of Article: AMERICAN

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