Part 22

Oersted discovered in 1820 that a magnetic needle poised horizontally is deflected when the galvanic current passes above it (parallel to the needle’s length) or below it. If the current passes above it, the north end of the needle turns towards the east when the current travels from north to south, but towards the west when the current travels from south to north; on the other hand, if the current passes below the needle, the north end turns towards the west when the current travels from south to north, and towards the east when the current travels from north to south. The deflection will be greater or less according to the power of the current. It would be very slight indeed in the case of a needle, however delicately poised, above or below which passed a wire conveying a galvanic current from a distant station. But the effect can be intensified, as follows:—

Suppose _a b c d e f_ to be a part of the wire from A to B, passing above a delicately poised magnetic needle N S, along _a b_ and then below the needle along _c d_, and then above again along _e f_ and so to the station B. Let a current traverse the wire in the direction shown by the arrows. Then N, the north end of the needle, is deflected towards the east by the current passing along _a b_. But it is also deflected to the east by the current passing along _c d_; for this produces a deflection the reverse of that which would be produced by a current in the same direction above the needle—that is, in direction _b a_, and therefore the same as that produced by the current along _a b_. The current along _e f_ also, of course, produces a deflection of the end N towards the east. All three parts, then, _a b_, _c d_, _e f_, conspire to increase the deflection of the end N towards the east. If the wire were twisted once again round N S, the deflection would be further increased; and finally, if the wire be coiled in the way shown in Fig. 1, but with a great number of coils, the deflection of the north end towards the east, almost imperceptible without such coils, will become sufficiently obvious. If the direction of the current be changed, the end N will be correspondingly deflected towards the west.

The needle need not be suspended horizontally. If it hang vertically, that is, turn freely on a horizontal axis, and the coil be carried round it as above described, the deflection of the upper end will be to the right or to the left, according to the direction of the current. The needle actually seen, moreover, is not the one acted upon by the current. This needle is inside the coil; the needle seen turns on the same axis, which projects through the coil.

If, then, the observer at the station B have a magnetic needle suitably suspended, round which the wire from the battery at A has been coiled, he can tell by the movement of the needle whether a current is passing along the wire in one direction or in the other; while if the needle is at rest he knows that no current is passing.

Now suppose that P and N, Fig. 2, are the positive and negative poles of a galvanic battery at A, and that a wire passes from P to the station B, where it is coiled round a needle suspended vertically at _n_, and thence passes to the negative pole N. Let the wire be interrupted at _a b_ and also at _c d_. Then no current passes along the wire, and the needle _n_ remains at rest in a vertical position. Now suppose the points _a b_ connected by the wire _a b_, and at the same moment the points _c d_ connected by the wire _c d_, then a current flows along P _a b_ to B, as shown in Fig. 2, circuiting the coil round the needle _n_ and returning by _d c_ to N. The upper end of the needle is deflected to the right while this current continues to flow; returning to rest when the connection is broken at _a b_ and _c d_. Next, let _c b_ and _a d_ be simultaneously connected as shown by the cross-lines in Fig. 3. (It will be understood that _a d_ and _b c_ do not touch each other where they cross.) The current will now flow from P along _a d_ to B, circuiting round the needle _n_ in a contrary direction to that in which it flowed in the former case, returning by _b c_ to N. The upper end of the needle is deflected then to the left while the current continues to flow along this course.

I need not here describe the mechanical devices by which the connection at _a b_ and _c d_ can be instantly changed so that the current may flow either along _a b_ and _d c_, as in Fig. 2, circuiting the needle in one direction, or along _a d_ and _b c_, as in Fig. 3, circuiting the needle in the other direction. As I said at the outset, this paper is not intended to deal with details of construction, only to describe the general principles of telegraphic communication, and especially those points which have to be explained in order that recent inventions may be understood. The reader will see that nothing can be easier than so to arrange matters that, by turning a handle, either (1), _a b_ and _c d_ may be connected, or, (2), _a d_ and _c b_, or, (3), both lines of communication interrupted. The mechanism for effecting this is called a _commutator_.

Two points remain, however, to be explained: First, A must be a receiving station as well as a transmitting station; secondly, the wire connecting B with N, in Figs. 2 and 3, can be dispensed with, for it is found that if at B the wire is carried down to a large metal plate placed some depth underground, while the wire at _c_ is carried down to another plate similarly buried underground, the circuit is completed even better than along such a return wire as is shown in the figures. The earth either acts the part of a return wire, or else, by continually carrying off the electricity, allows the current to flow continuously along the single wire. We may compare the current carried along the complete wire circuit, to water circulating in a pipe extending continuously from a reservoir to a distance and back again to the reservoir. Water sucked up continuously at one end could be carried through the pipe so long as it was continuously returned to the reservoir at the other; but it could equally be carried through a pipe extending from that reservoir to some place where it could communicate with the open sea—the reservoir itself communicating with the open sea—an arrangement corresponding to that by which the return wire is dispensed with, and the current from the wire received into the earth.

The discovery that the return wire may be dispensed with was made by Steinheil in 1837.

The actual arrangement, then, is in essentials that represented in Fig. 4.

A and B are the two stations; P N is the battery at A, P´ N´ the battery at B; P´ P´ are the positive poles, N´ N´, the negative poles. At _n_ is the needle of station A, at _n´_ the needle of station B. When the handle of the commutator is in its mean position—which is supposed to be the case at station B—the points _b´ d´_ are connected with each other, but neither with _a´_ nor _c´_; no current, then, passes from B to A, but station B is in a condition to receive messages. (If _b´_ and _d´_ were not connected, of course no messages could be received, since the current from A would be stopped at _b´_—which does not mean that it would pass round _n´_ to _b´_, but that, the passage being stopped at _b´_, the current would not flow at all.) When (the commutator at B being in its mean position, or _d´ b´_ connected, and communication with _c´_ and _a´_ interrupted) the handle of the commutator at A is turned from its mean position in _one_ direction, _a_ and _b_ are connected, as are _c_ and _d_—as shown in the figure—while the connection between _b_ and _d_ is broken. Thus the current passes from P by _a_ and _b_, round the needle _n_; thence to station B, round needle _n´_, and by _b´_ and _d´_, to the earth plate G´; and so along the earth to G, and by _d c_, to the negative pole N. The upper end of the needle of both stations is deflected to the right by the passage of the current in this direction. When the handle of the commutator at A is turned in the other direction, _b_ and _c_ are connected, as also _a_ and _d_; the current from P passes along _a d_ to the ground plate G, thence to G´, along _d´ b´_, round the needle _n´_, back by the wire to the station A, where, after circuiting the needle _n_ in the same direction as the needle _n´_, it travels by _b_ and _c_ to the negative pole N. The upper end of the needle, at both stations, is deflected to the left by the passage of the current in this direction.

It is easily seen that, with two wires and one battery, two needles can be worked at both stations, either one moving alone, or the other alone, or both together; but for the two to move differently, two batteries must be used. The systems by which either the movements of a single needle, or of a pair of needles, may be made to indicate the various letters of the alphabet, numerals, and so on, need not here be described. They are of course altogether arbitrary, except only that the more frequent occurrence of certain letters, as _e_, _t_, _a_, renders it desirable that these should be represented by the simplest symbols (as by a single deflection to right or left), while letters which occur seldom may require several deflections.

One of the inventions to which the title of this paper relates can now be understood.

In the arrangement described, when a message is transmitted, the needle of the sender vibrates synchronously with the needle at the station to which the message is sent. Therefore, till that message is finished, none can be received at the transmitting station. In what is called duplex telegraphy, this state of things is altered, the needle at the sending station being left unaffected by the transmitted current, so as to be able to receive messages, and in self-recording systems to record them. This is done by dividing the current from the battery into two parts of equal efficiency, acting on the needle at the transmitting station in contrary directions, so that this needle remains unaffected, and ready to indicate signals from the distant station. The principle of this arrangement is indicated in Fig. 5. Here _a b n_ represents the main wire of communication with the distant station, coiled round the needle of the transmitting station in one direction; the dotted lines indicate a finer short wire, coiled round the needle in a contrary direction. When a message is transmitted, the current along the main wire tends to deflect the needle at _n_ in one direction, while the current along the auxiliary wire tends to deflect it in the other direction. If the thickness and length of the short wire are such as to make these two tendencies equal, the needle remains at rest, while a message is transmitted to the distant station along the main wire. In this state of things, if a current is sent from the distant station along the wire in the direction indicated by the dotted arrow, this current also circuits the auxiliary wire, but in the direction indicated by the arrows on the dotted curve, which is the same direction in which it circuits the main wire. Thus the needle is deflected, and a signal received. When the direction of the chief current at the transmitting station is reversed, so also is the direction of the artificial current, so that again the needle is balanced. Similarly, if the direction of the current from the distant station is reversed, so also is the direction in which this current traverses the auxiliary wire, so that again both effects conspire to deflect the needle.

There is, however, another way in which an auxiliary wire may be made to work. It may be so arranged that, when a message is transmitted, the divided current flowing equally in opposite directions, the instrument at the sending station is not affected; but that when the operator at the distant station sends a current along the main wire, this neutralizes the current coming towards him, which current had before balanced the artificial current. The latter, being no longer counterbalanced, deflects the needle; so that, in point of fact, by this arrangement, the signal received at a station is produced by the artificial current at that station, though of course the real cause of the signal is the transmission of the neutralizing current from the distant station.

The great value of duplex telegraphy is manifest. Not only can messages be sent simultaneously in both directions along the wire—a circumstance which of itself would double the work which the wire is capable of doing—but all loss of time in arranging about the order of outward and homeward messages is prevented. The saving of time is especially important on long lines, and in submarine telegraphy. It is also here that the chief difficulties of duplex telegraphy have been encountered. The chief current and the artificial current must exactly balance each other. For this purpose the flow along each must be equal. In passing through the long wire, the current has to encounter a greater resistance than in traversing the short wire; to compensate for this difference, the short wire must be much finer than the long one. The longer the main wire, the more delicate is the task of effecting an exact balance. But in the case of submarine wires, another and a much more serious difficulty has to be overcome. A land wire is well insulated. A submarine wire is separated by but a relatively moderate thickness of gutta-percha from water, an excellent conductor, communicating directly with the earth, and is, moreover, surrounded by a protecting sheathing of iron wires, laid spirally round the core, within which lies the copper conductor. Such a cable, as Faraday long since showed, acts precisely as an enormous Leyden jar; or rather, Faraday showed that such a cable, without the wire sheathing, would act when submerged as a Leyden jar, the conducting wire acting as the interior metallic coating of such a jar, the gutta-percha as the glass of the jar (the insulating medium), and the water acting as the exterior metallic coating. Wheatstone showed further that such a cable, with a wire sheathing, would act as a Leyden jar, even though not submerged, the metal sheathing taking the part of the exterior coating of the jar. Now, regarding the cable thus as a condenser, we see that the transmission of a current along it may in effect be compared with the passage of a fluid along a pipe of considerable capacity, into which and from which it is conveyed by pipes of small capacity. There will be a retardation of the flow of water corresponding to the time necessary to fill up the large part of the pipe; the water may indeed begin to flow through as quickly as though there were no enlargement of the bore of the pipe, but the full flow from the further end will be delayed. Just so it is with a current transmitted through a submarine cable. The current travels instantly (or with the velocity of freest electrical transmission) along the entire line; but it does not attain a sufficient intensity to be recognized for some time, nor its full intensity till a still longer interval has elapsed. The more delicate the means of recognizing its flow, the more quickly is the signal received. The time intervals in question are not, indeed, very great. With Thomson’s mirror galvanometer, in which the slightest motion of the needle is indicated by a beam of light (reflected from a small mirror moving with the needle), the Atlantic cable conveys its signal from Valentia to Newfoundland in about one second, while with the less sensitive galvanometer before used the time would be rather more than two seconds.

Now, in duplex telegraphy the artificial current must be equal to the chief current in intensity all the time; so that, since in submarine telegraphy the current rises gradually to its full strength and as gradually subsides, the artificial current must do the same. Reverting to the illustration derived from the flow of water, if we had a small pipe the rapid flow through which was to carry as much water one way as the slow flow through a large pipe was to carry water the other way, then if the large pipe had a widening along one part of its long course the short pipe would require to have a similar widening along the corresponding part of its short course. And to make the illustration perfect, the widenings along the large pipe should be unequal in different parts of the pipe’s length; for the capacity of a submarine cable, regarded as a condenser, is different along different parts of its length. What is wanted, then, for a satisfactory system of duplex telegraphy in the case of submarine cables, is an artificial circuit which shall not only correspond as a whole to the long circuit, but shall reproduce at the corresponding parts of its own length all the varieties of capacity existing along various parts of the length of the submarine cable.

Several attempts have been made by electricians to accomplish this result. Let it be noticed that two points have to be considered: the intensity of the current’s action, which depends on the resistance it has to overcome in traversing the circuit; and the velocity of transmission, depending on the capacity of various parts of the circuit to condense or collect electricity. Varley, Stearn, and others have endeavoured by various combinations of condensers with resistance coils to meet these two requisites. But the action of artificial circuits thus arranged was not sufficiently uniform. Recently Mr. J. Muirhead, jun., has met the difficulty in the following way (I follow partially the account given in the _Times_ of February 3, 1877, which the reader will now have no difficulty in understanding):—He has formed his second circuit by sheets of paper prepared with paraffin, and having upon one side a strip of tinfoil, wound to and fro to represent resistance. Through this the artificial current is conducted. On the other side is a sheet of tinfoil to represent the sheathing,[29] and to correspond to the capacity of the wire. Each sheet of paper thus prepared may be made to represent precisely a given length of cable, having enough tinfoil on one side to furnish the resistance, and on the other to furnish the capacity. A sufficient number of such sheets would exactly represent the cable, and thus the artificial or non-signalling part of the current would be precisely equivalent to the signalling part, so far as its action on the needle at the transmitting station was concerned. “The new plan was first tried on a working scale,” says the _Times_, “on the line between Marseilles and Bona; but it has since been brought into operation from Marseilles to Malta, from Suez to Aden, and lastly, from Aden to Bombay. On a recent occasion when there was a break-down upon the Indo-European line, the duplex system rendered essential service, and maintained telegraphic communication which would otherwise have been most seriously interfered with.” The invention, we may well believe, “cannot fail to be highly profitable to the proprietors of submarine cables,” or to bring about “before long a material reduction in the cost of messages from places beyond the seas.”

* * * * *

The next marvel of telegraphy to be described is the transmission of actual facsimiles of writings or drawings. So far as strict sequence of subject-matter is concerned, I ought, perhaps, at this point, to show how duplex telegraphy has been surpassed by a recent invention, enabling three or four or more messages to be simultaneously transmitted telegraphically. But it will be more convenient to consider this wonderful advance after I have described the methods by which facsimiles of handwriting, etc., are transmitted.

Hitherto we have considered the action of the electric current in deflecting a magnetic needle to right or left, a method of communication leaving no trace of its transmission. We have now to consider a method at once simpler in principle and affording means whereby a permanent record can be left of each message transmitted.

If the insulated wire is twisted in the form of a helix or coil round a bar of soft iron, the bar becomes magnetized while the current is passing. If the bar be bent into the horse-shoe form, as in Fig. 6, where A C B represents the bar, _a b c d e f_ the coil of insulated wire, the bar acts as a magnet while the current is passing along the coil, but ceases to do so as soon as the current is interrupted.[30] If, then, we have a telegraphic wire from a distant station in electric connection with the wire _a b c_, the part _e f_ descending to an earth-plate, then, according as the operator at that distant station transmits or stops the current, the iron A C B is magnetized or demagnetized. The part C is commonly replaced by a flat piece of iron, as is supposed to be the case with the temporary magnets shown in Fig. 7, where this flat piece is below the coils.

So far back as 1838 this property was applied by Morse in America in the recording instrument which bears his name, and is now (with slight modifications) in general use not only in America but on the Continent. The principle of this instrument is exceedingly simple. Its essential parts are shown in Fig. 7; H is the handle, H _b_ the lever of the manipulator at the station A. The manipulator is shown in the position for receiving a message from the station B along the wire W. The handle H´ of the manipulator at the station B is shown depressed, making connection at _a´_ with the wire from the battery N´ P´. Thus a current passes through the handle to _c´_, along the wire to _c_ and through _b_ to the coil of the temporary magnet M, after circling which it passes to the earth at _e_ and so by E´ to the negative pole N´. The passage of this current magnetizes M, which draws down the armature _m_. Thus the lever _l_, pulled down on this side, presses upwards the pointed style _s_ against a strip of paper _p_ which is steadily rolled off from the wheel W so long as a message is being received. (The mechanism for this purpose is not indicated in Fig. 7.) Thus, so long as the operator at B holds down the handle H´, the style _s_ marks the moving strip of paper, the spring _r_, under the lever _s l_, drawing the style away so soon as the current ceases to flow and the magnet to act. If he simply depresses the handle for an instant, a dot is marked; if longer, a dash; and by various combinations of dots and dashes all the letters, numerals, etc., are indicated. When the operator at B has completed his message, the handle H´ being raised by the spring under it (to the position in which H is shown), a message can be received at B.

I have in the figure and description assumed that the current from either station acts directly on the magnet which works the recording style. Usually, in long-distance telegraphy, the current is too weak for this, and the magnet on which it acts is used only to complete the circuit of a local battery, the current from which does the real work of magnetizing M at A or M´ at B, as the case may be. A local battery thus employed is called a _relay_.

The Morse instrument will serve to illustrate the _principle_ of the methods by which facsimiles are obtained. The details of construction are altogether different from those of the Morse instrument; they also vary greatly in different instruments, and are too complex to be conveniently described here. But the principle, which is the essential point, can be readily understood.