Popular Science Monthly/Volume 9/May 1876/Recent Advances in Telegraphy
|RECENT ADVANCES IN TELEGRAPHY.|
THE improvements in telegraphy, about which the public has lately been learning a good deal through the newspapers, really constitute a remarkable element of progress, and are deserving of separate consideration. With the fire-alarm, domestic, and district telegraphs in our cities, the reduced rates and increased efficiency of the great lines and the further improvements promised us, it does not seem too much to expect that the telegraph will soon rival the post-office and the press as a bearer and diffuser of intelligence.
The failure of the English postal telegraph to fulfill the sanguine prophecies of its advocates will hardly be held to militate against this view, when it shall be shown what the nature of these improvements is. Prof. Jevons, in a late number of the Fortnightly Review, has indicated the causes of this failure. It was taken for granted by the promoters of the scheme, he asserts, that, as in the case of the Post-office, a vast increase of business might be done with but little more expense. Accordingly, to gain the increased business they reduced the rates one-half, and succeeded—but not in a pecuniary sense. Prof. Jevons ascribes this disappointing result to the great cost of erecting and maintaining the lines; to their small carrying capacity when compared with that of a railroad-train; and to the number of hands and heads which each telegraphic message has to pass through before reaching its destination, and which must all be paid. But the progress of the last five years, made principally in this country, has demonstrated that these difficulties are not insuperable.
In order of time, the first important step toward this end was the Duplex Telegraph of Mr. Joseph Stearns, of Boston, Massachusetts. Its object is to allow of two operators using the same wire to send messages in opposite directions simultaneously. To persons having only a general acquaintance with the ordinary working of the telegraph, this at first seemed impossible; and, when it was accomplished, it was held by many—some scientific men among the number—to furnish an indubitable proof of the theory that the electric waves, or currents, or whatever they might be held to be, necessarily passed each other in contrary directions over the wire. That they do not will be evident from the subjoined explanation.
It must be remembered that the galvanic battery gives birth to a force which returns in a circuit to where it was generated, and accelerates the liberation of more force, being like a steam engine employed partly in fanning its own fire. This circuit can be performed much more easily through great lengths of some substances, such as the earth and metals, than through very small spaces of others, as the air and the dilute acid of the battery. Galvanic electricity is, therefore, strictly confined in a sort of mill-round; or it may best, for our present purposes, be represented by water flowing through such a system of water-courses as is shown in the annexed cut. We will
suppose them to include a reservoir and a secondary circuit at each end. Let the reservoirs A and B have water pumped into them by force-pumps, and distributed by them to both the main and secondary circuits, in equal quantities and in the direction of the arrows, so as to maintain the water-wheels X and W in the same positions. The highest points in the system must be supposed to be at the front of the reservoirs, and the lowest at the back of them.
If an additional volume of water come from A, being equally divided on each side of W, it will not move that wheel, but it will move the wheel X by destroying the balance which previously existed there. But, if a similar extra volume be at the same time sent from B, the pressure in that part of the circuit between W and X will overcome the opposing forces at each of the points, and both wheels will be worked, each virtually by the distant reservoir and not by its own.
If we substitute galvanic batteries for the reservoirs, wires for the water-courses, and electricity for the water, this gives us the principle of the duplex telegraph, and it is obvious that no currents passing one another in contrary directions are necessary to it. It will be well to keep this in mind when we come to describe the quadruplex system.
Following the duplex, the American Automatic system may be said to have been perfected in 1873. The great rapidity with which messages are transmitted and recorded by it is its principal advantage, but it has others—as requiring a smaller force of operators and less specially skilled. The usual work of a Morse operator is acknowledged to be about 1,500 words an hour, and European operators do not average half as much; but, by the automatic method, to receive and print double that number of words per minute is an ordinary feat, and as many as 7,000 words—fourteen pages of this magazine—have been legibly recorded in that time. As every word contains, on an average, five letters, and as each letter is represented by a varying
number of dots and dashes, each formed by a separate discharge, the circuit, it is calculated, must be "closed" and "broken," and the chemicals in the battery must cease and recommence their action 60,000 times per minute, in the ordinary working of the automatic system.
In every form of electric telegraph the signals are given by an intermittent now of electricity. In the Morse system a "key" is used which, in its normal position, "breaks" the circuit, but when depressed by the linger of the operator allows the electricity to pass through it on its mission. Arrived at the distant station, it is converted, by means of an electro-magnet, into mechanical motion, which is utilized either to produce indentations in a moving slip of paper by means of a style, or, more commonly, to give a series of taps, which the operator understands, by an instrument called a "sounder."
In the automatic system the means employed are altogether different. The message is, first of all, prepared by punching holes in a narrow ribbon of paper. These perforations are so grouped as to represent the dots and dashes of the telegraphic alphabet, and by the punching-machine, which is very complicated, all that are required to form a letter are punched at one stroke. In comparing the two systems this must not be lost sight of, as the time taken in punching must, of course, be added to the time of transmission. The machine, however, does its work more quickly than the Morse operator with his key, and, the time occupied in transmitting being so vastly less, the "automatic" may claim to have rendered old-fashioned telegraphy comparatively slow.
After the perforated slip of paper has been prepared, it is taken to the operator's table, where it is made to move forward rapidly between a metallic drum and a needle carrying two small steel wheels which rest upon it. Drum and wheels form part of the circuit, which is broken by the non-conducting paper interposed and closed when the holes permit of the wheels and the metallic cylinder beneath coming into contact. At the receiving-station a very similar arrangement does duty as a register. The paper slip is there saturated with a certain chemical solution which renders its whole substance a good conductor, and, instead of the wheels, there is an iron style or "pen." When electricity arrives over the line, it decomposes the moisture of the paper into oxygen and hydrogen, and oxidizes or rusts the pen.
A little of this oxide is rubbed off by the quickly-moving paper, and enters into combination with the chemical still contained in it, producing a stain in the form of a dot or dash which corresponds with the holes punched in the paper at the sending-station. Where three holes come together, both wheels form a contact, and a dash is produced, because the second wheel touches the cylinder while the first passes over the paper between the upper holes.
The germ of the automatic system, as we have described it, was contained in the "Chemical Telegraph" invented by Alexander Bain, a Scotchman, in 1846. Bain was the first to use the perforated paper to transmit and the chemically-prepared paper to receive the message. But his invention, from a practical point of view, bears about the same relation to the American system which the steam-engine as known to the ancients does to that of James Watt. Bain's system, improved by the late Sir Charles Wheatstone and known as Wheat-stone's automatic system, is employed to a limited extent in Great Britain; but, thus improved, its speed does not exceed 60 to 100 words a minute. It is therefore proper to regard the American Automatic Telegraph as a distinct American invention. In its present form, we owe it to Mr. Thomas A. Edison, of Newark, New Jersey.
The accompanying cut (Fig. 4) illustrates the results of attempting
high speed on the Bain telegraph. Instead of recording themselves by decided dots and dashes, the electric discharges leave indistinct and elongated traces, which, when the speed amounts to 300 words or over, run into one another and make a continuous line. This effect is due to the property which all electrified bodies have of inducing electricity in neighboring bodies. The earth, reacting on the line wire suspended above it, induces in it what is called an extra current, both on closing and breaking the circuit. On first closing the circuit the extra current runs in the contrary direction to the primary, and retards and weakens its action, so that, if suffered to record itself, it would do so by a mark like this: the long after-part being caused partly by the accumulated electricity and partly by the second extra current which is in the same direction with the primary one.
By Mr. Edison's plan the evil is made to cure itself. He simply interposes another wire with a coil, shown at A C E, (Fig. 5). This divides the current, one part of which is again subdivided on reaching the earth, and a moiety of it ascending the ground-line at D' counteracts the first weak installment of the other. Then, as each turn of the coil, C, acts the part of the earth on the turn next it, the whole sets up another powerful extra current, which at first forces the full strength of the main current through the recording instrument, and ultimately counteracts the accumulated electricity and the second extra current due to the earth. In practice, several such lines are used, and magnets, which are preferable, instead of coils. This occasions a great loss of electricity, but the sensitiveness of the receiving apparatus is such that less than one-fourth of the total strength of the current is sufficient to give a good record.
The chemical used by Bain in his sensitized paper was ferrocyanide of potassium, which, with the oxide from the iron pen and an extra equivalent of oxygen, forms Prussian blue. The oxygen of the air, it has been found, protracts this action, and thus arises another source of confusion, which is not affected by the device just described. A preferable combination, requiring only the protoxide of iron, which is formed immediately by the electricity, is used in the American system.
One of the most curious of the recent discoveries respecting the chemical action of electricity is that of its usefulness, under certain circumstances, as a lubricator. During Mr. Edison's experiments on the automatic telegraph he perceived that, when using a paper soaked in a certain solution, the pen was apt to slip whenever a discharge occurred. This effect was found to be so marked that a person drawing a strip of metal along the paper—leaning rather heavily on it—finds his hand obliged to move in a succession of jerks when signals are sent by a current powerful enough to overcome the resistance of his body. On this principle, Mr. Edison has constructed a little instrument in which a style is kept pressed against the paper by springs so as to make a continuous indentation, except when the current is passing. Its record is, therefore, the reverse of that of a Morse register; but the "electromotograph," as it is called, differs also from the "Morse" in being the most sensitive recording instrument known.
Still another of Mr. Edison's inventions is the quadruplex telegraph, the principal aim of which is, not to augment the speed of signaling, but, like the duplex, to allow of several persons using the same wire at one time. In fact, the arrangement may be used as a duplex telegraph, if required, so that the wire is by it made susceptible of either double or quadruple employ.
The instruments used are modifications of those of the Morse system. The "key" has already been shown in Fig. 2, and the changes made to adapt it to the uses of the quadruplex telegraph may be understood from Fig. 5. The essential part of the receiving instrument is an electro-magnet, which is shown in Fig. 2, and consists of a bent bar of soft iron, surrounded at each end by a coil of wire connected with the wire of the line. The current, passing through these coils, communicates to the iron core magnetic properties, and enables it to attract another piece of iron or steel called its armature; but, when the current ceases, the magnetism ceases also, and a spring—too weak to neutralize it—draws back the armature. It is shown in section at M, in Fig. 6. When the armature and the lever carrying it are discarded, and instead of them a jointed tongue of steel, as at P M, is inserted between the poles of the magnet, it will be unaffected by the current except when a change occurs in its direction. It is then called a polarized magnet. Its use will be explained a little further on. One
of the keys, K, in the diagram, is provided with a spring, which is in contact with the metal of the key when this latter is in its normal position, and maintains across the key a circuit including a portion of the battery b'. But when the key is depressed the spring comes in contact with a screw, to which another circuit is connected, applying the full strength of the battery to the line. The circuit across the key is never broken, because the spring remains in contact with the arm of the key until it begins to press against the screw. This key works the magnet M, which has its retractile spring so adjusted as to be overcome only by the full intensity of the current when the key is down. The other key, K', is for changing the direction of the current, and working the polarized magnet, P M. Its construction is such that, when not in use, one pole of the battery, the positive, for example, is in connection with the line, and the negative with the earth, necessitating the passage of the current through the line in the first place; but when the key is touched the negative pole is connected "to line" and the positive to earth, reversing the direction of the current, These reversals of direction operate, as has been said, the polarized magnet P M.
To revert to the illustration we made use of in describing the duplex, let the reader picture to himself a water-course in which both the direction and the volume of the current can be changed at pleasure. He can suppose, in addition to the water-wheels before figured, and which will indicate the force of the stream, a pair of hinged valves or gates, which, whether the current be strong or weak, will be moved only by a change in its direction. The former will represent the ordinary magnets, and the latter the polarized magnets.
It is plain that, so far, this is only another form of duplex, sending two messages in the same direction at once. To make it a quadruplex telegraph it is necessary, in the first place, to add to it Stearns's duplex, or a contrivance similar to it. Even then a dead-lock would happen when the currents sent from each end of the line should be of the same intensity, and opposite in direction; that is, when all eight operators were working together. To remedy this, extra batteries are introduced, which are neutralized by part of the current in the main circuit, when that is in a working condition, but are set free to work the instruments when the currents in the main circuit destroy one another. In the diagram the extra batteries, etc., have been omitted, as also the transmitting apparatus of one station and the recording instruments of the other.
Although not strictly coming under its title, because belonging, as yet, rather to the future, this article would hardly be complete without some reference to a scheme of multiplex telegraphy which promises results of the greatest importance. The ingenious magnetic apparatus used by Prof. Helmholtz, of Berlin, in his researches in acoustics, was too suggestive not to have inspired more than one inventor with the idea of turning it to account in telegraphy. Accordingly, several, both here and in Europe, have been trying to realize it, and it is likely that the magnetically-excited tuning-forks, or the sonorous steel bars which may be substituted for them, will shortly be heard in every telegraph-office. There seems, so far, to be no ascertained limit to the number of distinct musical notes which may be propagated on a single wire at one time; and, when that limit is found, it is likely that it may be doubled or quadrupled by means of the former systems. The reduction in the cost of erection and maintenance of wires which this will bring about will be an enormous saving to telegraph companies, especially to any new ones that may be formed, or to the Government, if it should undertake the control and extension of the service.
An interesting experiment of Sir Charles Wheatstone's on the transmission of sound through solid linear conductors has, perhaps, helped to suggest this approaching transformation of the telegraph. An account of it was published in 1831. A narrow wooden rod was attached at one end to the sounding-board of a piano, and, after passing through two empty rooms, was joined at the other end to a sounding-board alone. Any piece of music played on the piano was distinctly heard by means of the sounding-board in the distant room. And not the least confusion ensued from the crowding together, for a considerable distance, of the multitude of intricately-related vibrations in a rod having a section of but one square inch.
Prof. Helmholtz's apparatus consisted of a number of electro-magnets acting on tuning-forks pitched to particular notes. His object was so to combine those notes as to demonstrate the formation of certain harmonious sounds; but the object of the telegraph-inventors is the reverse of that, namely, to transmit them in the form of electric vibrations to a distance, and then—as in Wheatstone's experiment—to sift them out again to separate instruments. In most of the plans so far made public, a fixed steel bar takes the place of the tuning-fork, and therefore of the armature as well. When attracted by the magnet, on making a signal, it is of course set vibrating; and, at every forward vibratory movement, it closes the circuit and transmits an electric impulse. A number of such magnets, their sonorous armatures sending each a different number of pulsations in a second, may be working away at once, and the corresponding instruments at the other end of the line will be acted on only by those which suit their times of vibration. In other words, of the total number of electric charges sent into the line, only those will act on any particular magnet at the receiving end which suffice to cause in its armature the number of vibrations per second to which it was set. This, of course, is the same number which was sent by the transmitting instrument of the same pair. Practically, the different tones are not reproduced quite unmixed, every armature being capable of responding though in a less degree, to other notes than its own; so that the effect on the ear, at one of the receiving magnets, is like that of a number of persons talking together in different keys: some quite loudly; some in a lower tone; others in a whisper. To remedy this, different forms of resonators are being tried, adapted to swell the special sounds that should be heard.
The "electromotograph," described in connection with chemical telegraphs, is intended, by its inventor, to be used with some form of this acoustic system. Mr. Gray, of Chicago, another well-known telegraph-inventor, is also understood to have made considerable progress in this direction.
It is matter of reasonable pride to find, at the commencement of our second century, the names of Americans so prominently connected with all the great improvements in the art which owes so much to the labors of Morse and Henry.