We come now to one of the most important inventions of the nineteenth century, the electric telegraph. From very early times men have felt the necessity for some means of rapidly communicating between two distant points. The first really practical method of signalling was that of lighting beacon fires on the tops of hills, to spread some important tidings, such as the approach of an enemy. From this simple beginning arose more complicated systems of signalling by semaphore, flags, or flashing lights. All these methods proved incapable of dealing with the rapidly growing requirements of commerce, for they were far too slow in action, and in foggy weather they were of no use at all. We are so accustomed to walking into a telegraph office, filling up a form, and paying our sixpence or more, that it is very difficult for us to realize the immense importance of the electric telegraph; and probably the best way of doing this is to try to imagine the state of things which would result if the world’s telegraphic instruments were put out of action for a week or two.
The earliest attempts at the construction of an electric telegraph date back to a time long before the discovery of the electric current. As early as 1727 it was known that an electric discharge could be transmitted to a considerable distance through a conducting substance such as a moistened thread or a wire, and this fact suggested the possibility of a method of electric signalling. In 1753 a writer in Scott’s Magazine brought forward an ingenious scheme based upon the attraction between an electrified body and any light substance. His telegraph was worked by an electric machine, and it consisted of twenty-six separate parallel wires, every wire having a metal ball suspended from it at each end. Close to each ball was placed a small piece of paper upon which was written a letter of the alphabet. When any wire was charged, the paper letters at each end of it were attracted towards the metal balls, and in this way words and sentences were spelled out. Many other systems more or less on the same lines were suggested during the next fifty years, but although some of them had considerable success in an experimental way, they were all far too unreliable to have any commercial success.
With the invention of the voltaic cell, inventors’ ideas took a new direction. In 1812 a telegraph based upon the power of an electric current to decompose water was devised by a German named Sömmering. He used a number of separate wires, each connected to a gold pin projecting from below into a glass vessel filled with acidulated water. There were thirty-five wires in all, for letters and numbers, and when a current was sent along any wire bubbles of gas formed at the pin at the end of it, and so the letters or numbers were indicated. This telegraph, like its predecessors, never came into practical use. Oersted’s discovery in 1829 of the production of magnetism by electricity laid the foundation of the first really practical electric telegraphs, but little progress was made until the appearance of the Daniell cell, in 1836. The earlier forms of voltaic cells polarized so rapidly that it was impossible to obtain a constant current from them, but the non-polarizing Daniell cell at once removed all difficulty in this respect. In the year 1837 three separate practical telegraphs were invented: by Morse in the United States, by Wheatstone and Cooke in England, and by Steinheil in Munich.
The first telegraph of Wheatstone and Cooke consisted of five magnetic needles pivoted on a vertical dial. The letters of the alphabet were marked on the dial, and the needles were deflected by currents made to pass through wires by the depression of keys, so that two needles would point towards the required letter. Fig. 26 is a sketch of the dial of this apparatus. This telegraph was tried successfully on the London and North-Western Railway, over a wire a mile and a half in length. Wheatstone and Cooke afterwards invented a single-needle telegraph in which the letters were indicated by movements of the needle to the right or to the left, according to the direction of a current sent through a coil of wire. Wheatstone subsequently produced an apparatus which printed the letters on paper.
In the United States, Morse had thought out a scheme of telegraphy in 1832, but it was not until 1837 that he got his apparatus into working order. He was an artist by profession, and for a long time he was unable to develop his ideas for lack of money. After many efforts he succeeded in obtaining a State grant of £6000 for the construction of a telegraph line between Baltimore and Washington, and the first message over this line was sent in 1844, the line being thrown open to the public in the following year. Amongst the features of this telegraph were a receiving instrument which automatically recorded the messages on a moving paper ribbon, by means of a pencil actuated by an electro-magnet; and an apparatus called a relay, which enabled the recording instrument to be worked when the current was enfeebled by the resistance of a very long wire. Morse also devised a telegraphic code which is practically the same as that in use to-day.
The great discovery of the German Steinheil was that a second wire for the return of the current was not necessary, and that the earth could be used for this part of the circuit.
In reading the early history of great inventions one is continually struck with the indifference or even hostility shown by the general public. In England the electric telegraph was practically ignored until the capture of a murderer by means of it literally forced the public to see its value. The murder was committed near Slough, and the murderer succeeded in taking train for London. Fortunately the Great Western Railway had a telegraph line between Slough and London, and a description telegraphed to Paddington enabled the police to arrest the murderer on his arrival. In the United States too there was just the same indifference. The rate for messages on the line between Baltimore and Washington was one cent for four words, and the total amount taken during the first four days was one cent!
One of the simplest forms of telegraph is the single-needle instrument. This consists of a magnetic needle fixed to a spindle at the back of an upright board through which the spindle is passed. On the same spindle, but in front of the board, is fixed a dial needle, which, of course, moves along with the magnetic needle. A coil of wire is passed round the magnetic needle, and connected to a commutator for reversing the direction of the current. By turning a handle to the left a current is made to flow through the coil, and the magnetic needle moves to one side; but if the handle is turned to the right the current flows through the coil in the opposite direction, and the needle moves to the other side. Instead of a handle, two keys may be used, the movement of the needle varying according to which key is pressed. A good operator can transmit at the rate of about twenty words a minute with this instrument. The Morse code, which consists of combinations of dots and dashes, is used, a movement of the dial needle to the left meaning a dot, and one to the right a dash. The code as used in the single-needle instrument is shown in Fig. 27.
Needle instruments are largely used in railway signal cabins, but for general telegraphic work an instrument called the Morse sounder is employed. This consists of an electro-magnet which, when a current is passed through it, attracts a small piece of iron fixed to one end of a pivoted lever. The other end of this lever moves between two stops. At the transmitting station the operator closes a battery circuit by pressing a key, when the electro-magnet of the sounder at the receiving station attracts the iron, and the lever flies from one stop to the other with a sharp click, returning again as soon as the circuit is broken. A dot is signalled when the lever falls back immediately after the click, and a dash when it makes a short stay before returning. Fig. 28 shows the code of signals for the Morse telegraph.
In passing through a very long wire an electric current becomes greatly reduced in strength owing to the resistance of the wire. If two telegraph stations are a great distance apart the energy of the current thus may be unequal to the task of making the electro-magnet move the lever of the sounder so as to produce a click, but this difficulty is overcome by the use of an ingenious arrangement called a “relay.” It consists of a very small electro-magnet which attracts a light bar, the movement of the bar being made to close the circuit of another battery at the receiving station. The feeble current works the relay, and the current in the local circuit operates the sounder.
The word “telegraph,” which is derived from the Greek tele, far off, and grapho, I write, strictly signifies writing at a distance. The needle instrument and the sounder do not write in any way, but by modifying the construction of the sounder it can be made to record the messages it receives. A small wheel is fitted to the free end of the lever of the sounder, and an ink-well is placed so that the wheel dips into it when the lever is in the normal position. When the circuit is closed the lever moves just as in the ordinary sounder, but instead of clicking against a stop it presses the inked wheel against a paper ribbon which is kept slowly moving forward by clockwork. In this way the wheel continues to mark a line along the paper as long as the circuit remains closed, and according to the time the transmitting key is kept down a short mark or dot, or a long mark or dash, is produced. The clockwork which moves the paper ribbon is started automatically by the current, and it continues working until the message is finished.
Fig. 29.—A Morse Message.
(a) Perforated Tape. (b) Printed Tape.
TRANSLATION.
Series of alternate dots and dashes indicating commencement of message.
Sec (section) A. D. T. (Daily Telegraph) Fm (from) Berri, Antivari.
Then follow the letters G. Q., signifying fresh line.
They hd (had) bn (been) seen advancing in t (the) distance and wr (were) recognised by thr (their) usual uniform wh (which) consists o (of) a white fez.
Finally double dots indicating full stop.
A good Morse operator can maintain a speed of about thirty words a minute, but this is far too slow for certain kinds of telegraphic work, such as the transmission of press news, and for such work the Wheatstone automatic transmitter is used. First of all the messages are punched on a paper ribbon. This is done by passing the ribbon from right to left by clockwork through a punching machine which is provided with three keys, one for dots, one for dashes, and the other for spaces. If the left-hand key is pressed, two holes opposite to one another are made, representing a dot; and if the right-hand key is pressed, two diagonal holes are punched, representing a dash. In Fig. 29, which shows a piece of ribbon punched in this way, a third line of holes will be noticed between the outside holes representing the dots and dashes. These holes are for the purpose of guiding the paper ribbon steadily along through the transmitting machine. The punched ribbon is then drawn by clockwork through a Wheatstone transmitter. In this machine two oscillating needles, connected with one pole of a battery, are placed below the moving ribbon. Each time a hole passes, these needles make contact with a piece of metal connected with the other pole of the battery, thus making and breaking the circuit with much greater rapidity than is possible with the Morse key. At the receiving station the messages are recorded by a form of Morse inker, coming out in dots and dashes as though sent by hand. Below the punched ribbon in Fig. 29 is shown the corresponding arrangement of dots and dashes. The same punched ribbon may be used repeatedly when the message has to be sent on a number of different lines. The Wheatstone automatic machine is capable of transmitting at the rate of from 250 to 400 words a minute. Fig. 29 is a fragment of a Daily Telegraph Balkan War special, as transmitted to the Yorkshire Post over the latter’s private wire from London to Leeds. In the translation it will be seen that many common words are abbreviated.
One weak point of telegraphy with Wheatstone instruments is that the messages are received in Morse code, and have to be translated. During recent years telegraphs have been invented which actually produce their messages in ordinary written or printed characters. A very ingenious instrument is the Hughes printing telegraph, which turns out messages in typewritten form. Its mechanism is too complicated to be described here, but in general it consists of a transmitter having a keyboard something like that of a typewriter, by means of which currents of electricity are made to press a sheet of paper at the right instant against a revolving type-wheel bearing the various characters. This telegraph has been modified and brought to considerable perfection, and in one form or another it is used in European countries and in the United States.
In the Pollak-Virag system of telegraphy the action of light upon sensitized photographic paper is utilized. An operator punches special groupings of holes on a paper ribbon about 1 inch wide, by means of a perforating machine resembling a typewriter, and the ribbon is then passed through a machine which transmits by brush contacts. The receiver consists of a very small mirror connected to two vibrating diaphragms, which control its movements according to the currents received, one diaphragm moving the mirror in a vertical direction, and the other in a horizontal direction. The mirror reflects a ray of light on to photographic bromide paper in the form of a moving band about 3 inches in width, and the combined action of the two diaphragms makes the mirror move so that the ray of light traces out the messages in ordinary alphabetical characters. As it moves forward after being acted upon by the light, the paper is automatically developed and fixed, and then passed through drying rollers. Although the writing is rather imperfect in formation it is quite legible enough for most messages, but trouble occasionally occurs with messages containing figures, owing to confusion arising from the similarity of the figures, 3, 5, and 8. The whole process is carried out with such rapidity that 40,000 or even more words can be transmitted easily in an hour.
One of the most remarkable of present-day telegraphs is the Creed high-speed automatic printing telegraph. This has been devised to do away with hand working as far as possible, and to substitute quicker and more accurate automatic methods. In this system a perforated paper tape is produced by a keyboard perforator at the sending station. This tape is just ordinary Wheatstone tape, its perforations representing in the Morse code the message to be transmitted; and the main advantage of the Creed perforator over the three-key punching machine already described lies in the ease and speed with which it can be worked. The keyboard contains a separate key for each letter or signal of the Morse code, and the pressing of any key brings into operation certain punches which make the perforations corresponding to that particular letter. The perforator can be worked by any one who understands how to use an ordinary typewriter, and a speed of about 60 words a minute can be maintained by a fairly skilful operator. If desired a number of tapes may be perforated at the same time.
The tape prepared in this way is passed through a Wheatstone transmitter, and long or short currents, according to the arrangement of the perforations, are sent out along the telegraph line. At the receiving station these signals operate a receiving perforator. This machine produces another perforated tape, which is an exact copy of the tape at the sending station, and it turns out this duplicate tape at the rate of from 150 to 200 words a minute. There are two forms of this receiving perforator, one worked entirely by electricity, and the other by a combination of electricity and compressed air, both forms serving the same purpose. The duplicate tape is then passed through an automatic printer, which reproduces the message in large Roman characters on a paper tape. The printer works at a speed of from 80 to about 100 words a minute, and the printed tape is pasted on a telegraphic form by a semi-automatic process, and the message is then ready for delivery. Plate XI. shows a specimen of the tape from the receiving perforator, and the corresponding translation as turned out by the printer. This message formed part of a leading article in the Daily Mail. Some idea of the wonderful capabilities of the Creed system may be gained from the fact that by means of it practically the whole contents of the Daily Mail are telegraphed every night from London to Manchester and Paris, for publication next morning.
One of the most remarkable features about present-day telegraphy is the ease with which two or more messages can be sent simultaneously over one line. Duplex telegraphy, or the simultaneous transmission of two separate messages in opposite directions over one wire, is now practised on almost every line of any importance. At first sight duplex telegraphy seems to be an impossibility, for if we have two stations, one at each end of a single wire, and each station fitted with a transmitter and a receiver, it appears as if each transmitter would affect not only the receiver at the opposite end of the wire, but also the receiver at its own end, thus causing hopeless confusion when both transmitters were in use at the same time. This actually would be the case with ordinary telegraphic methods, but by the use of a special arrangement all confusion in working is avoided.
PLATE XI.
By permission of
Creed, Bille & Co. Ltd.
SPECIMEN OF THE WORK OF THE CREED HIGH-SPEED PRINTING TELEGRAPH.
We have seen that a magnetic needle is deflected by a current passing through a coil of wire placed round it, and that the direction in which the needle is deflected depends upon the direction of the current in the coil. Now suppose we place round the needle two coils of wire, wound so that the current in one flows in a direction opposite to that of the current in the other. Then, if we pass two equal currents, one through each coil, it is evident that they will neutralize one another, so that the needle will not be deflected at all. In a duplex system one end of one of these coils is connected to earth, say to a copper plate buried in the ground, and one end of the other to the line wire. The two remaining ends are arranged as branches leading off from a single wire connected with the transmitting key. The whole arrangement of coils and needle is repeated at the other end of the line. If now the transmitting key at station A is pressed, the circuit is closed and a current flows along the single wire, and then divides into two where the wire branches, half of it taking the path through one coil and half the path through the other. Equal currents thus flow through the oppositely wound coils, and the needle at station A is not deflected. Leaving the coils, one of these equal currents flows away to earth, while the other passes out along the line wire. On its arrival at station B the current is able to pass through only one of the coils round the needle, and consequently the needle is deflected and the signal given. In this way the transmitting operator at station A is able to signal to station B without affecting the receiver at his own end, and similarly the operator at station B can transmit to A without affecting the B receiver. Thus there can be no confusion whether the transmitters are worked at different times or simultaneously, for each transmitter affects only the receiver at the opposite end of the line. The diagram in Fig. 30 will help to make clearer the general principle. K and K¹ are the two transmitting keys which close the circuit, and C and C¹ are the points at which the current divides into two. Instead of coils and needles, electro-magnets operating sounders may be used, such magnets having two separate and oppositely wound coils, acting in exactly the same way as the coils round the needles. The above description is of course only a rough outline of the method, and in practice matters are more complicated, owing to the necessity for carefully adjusted resistances and for condensers. There is also another and different method of duplexing a line, but we have not space to describe it. Duplex telegraphy requires two operators at each end of the line, one to send and the other to receive.
Diplex telegraphy is the simultaneous transmission of two separate messages in the same direction over one line. Without going into details it may be said that for this purpose two different transmitting keys are required, one of which alters the direction, and the other the strength of the current though the line wire. The receivers are arranged so that one responds only to a strong current, and the other only to a current in one particular direction. A line also may be quadruplexed, so that it is possible to transmit simultaneously two messages from each end, four operators being required at each station, two to transmit and two to receive. Systems of multiplex telegraphy have been devised by which very large numbers of messages can be sent at once over a single wire, and the Baudot multiplex telegraph has proved very successful.
The wires for telegraphic purposes may be conveyed either above or below the ground. Overground wires are carried on poles by means of insulators of porcelain or other non-conducting material, protected by a sort of overhanging screen. The wires are left bare, and they are generally made of copper, but iron is used in some cases. In underground lines the wires formerly were insulated by a covering of gutta-percha, but now paper is generally used. Several wires, each covered loosely with thoroughly dry paper, are laid together in a bundle, the whole bundle or cable being enclosed in a strong lead pipe. The paper coverings are made to fit loosely so that the wires are surrounded by an insulating layer of dry air. As many as 1200 separate wires are sometimes enclosed in one pipe. In order to keep telegraph lines in working order frequent tests are necessary, and the most important British Postal Telegraph lines are tested once a week between 7.30 and 7.45 a.m. The earth is generally used for the return circuit in telegraphy, and the ends of the return wires are connected either to metal plates buried in the ground to a depth at which the earth is permanently moist, or to iron gas or water pipes. The current for telegraph working on a small scale is usually supplied by primary cells, the Daniell cell being a favourite for this purpose. In large offices the current is generally taken from a battery of storage cells.
During the early days of telegraphy, overhead lines were a source of considerable danger when thunderstorms were taking place. Lightning flashes often completely wrecked the instruments, giving severe shocks to those in the vicinity, and in a few cases operators were killed at their posts. Danger of this kind is now obviated by the use of contrivances known as lightning arresters. There are several forms of these, but only one need be mentioned. The main features of this are two metal plates separated slightly from one another, so that there is a small air gap between them. One plate is connected to the line wire, and the other to earth. Almost all lightning flashes consist of an oscillatory discharge, that is one which passes a number of times backwards and forwards between a cloud and the earth. A very rapidly alternating discharge of this kind finds difficulty in passing along the line wire, being greatly impeded by the coils of wire in the various pieces of apparatus; and although the resistance of this air gap is very high, the lightning discharge will cross the gap sooner than struggle along the line wire. In this way, when a flash affects the line, the discharge jumps the gap between the plates of the arrester and passes away harmlessly to earth, without entering the telegraph office at all. As was mentioned in Chapter III., the prevalence of magnetic storms sometimes renders telegraph lines quite unworkable for a time, but although such disturbances cause great delay and general inconvenience, they are not likely to be at all dangerous. It is often possible to maintain telegraphic communication during magnetic disturbances by using two lines to form a complete metallic loop, so that there is no earth return.