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Having secured the means of transmitting at will a current of electricity with great velocity, it remains to determine what phenomenon we shall cause it to produce at the distant station.

The reader can select whichever of the ex- the universal practice of expositors of these pheplanations now given he prefers, or can devise nomeną, write as if there were but a single curtheories for himself, or dispense with any. But rent of positive electricity flowing at once along the ultimate and only important fact in reference a telegraph wire. The other and opposite negato the telegraph is, that by the marvellously simple tive current may conveniently be disregarded, just device of dissolving a few pieces of metal con- as in navigation a compass-needle is referred to as nected with a long wire, we can develop instan- if it had but one pole, pointing to the north. taneously, a thousand miles off, a force which will speak for us, write for us, print for us, and, so far as the conveyance of our thoughts is concerned, annihilate space and time. This annihilation is not of course complete, but in reference to practice it may be called so. Shakspeare's Juliet refers to "the lightning which doth cease to be, ere one can say it lightens." The exact velocity of electricity along a copper wire is 288,000 miles in a second. It is calculated, accordingly, that we could telegraph to our antipodes in rather less than the five hundredth part of one second of time!

The phenomena most easily produced by electricity are magnetic ones; and these, accordingly, are now preferred as the sources of signals. Thẹ electric telegraph, indeed, remained an unrealized idea in the minds of ingenious men, till the famous Danish philosopher, Oersted, discovered that a current of electricity, even though of very small intensity, if passing near a compass-needle poised on a pivot, will cause the needle to change its position, and point in a new direction. Let the telegraph-wire, for example, whilst connected with a battery, be placed so that the needle of a mariner's compass shall be directly below or above and parallel to the wire, and the needle, no longer "true to the pole," will whirl round and stand development of one force, as of electricity antagonizing electricity, magnetism antagonizing magnetism. Polar- east and west, instead of as before, north and ity, in short, implies unity quite as much as duplicity, and south. It depends upon the direction in which the may literally be said to exhibit a force or power "divided current of electricity is sent, which pole of the against itself." This division, however, never becomes

The most impatient of correspondents may be satisfied with this velocity; and we may now inquire in what way electricity is made to produce signals. In discussing this we shall recur to the provisional theory adopted at the outset, that electricity flows in currents; and in conformity with

schism. The one twin is never found detached and alone, compass-needle points east or west. Let the telbut always side by side with the other, and when permit-egraph-wire stretching from London to Edinburgh ted they combine, neutralize each other--and then polarity and back again, be considered as consisting of an ceases. We have likened a body exhibiting polarity to a or to Jove's two-forked thunderbolt. upper and a lower wire. If the London end of the It might also be compared to the conventional zodiacai upper wire be connected with the copper extremsign Pisces with its connected fishes, to the heraldic

double-headed arrow,

double eagle, to the Siamese Twins, or perhaps best of ity of the battery, whilst the termination of the all, though the comparison is a homely one, to two hunting lower wire is connected with the zinc, the current dogs of the same breed, size, shape, and power, held in of positive electricity (the only one of which we couples, and pulling against each other. All those objects

agree with each other, and with bodies or forces exhibiting now take cognizance) will flow along the upper polarity, in being double unities. Similes, however, may wire to Edinburgh, and return by the lower one to be pushed too far. The idea of polarity is best based on

the spectacle of the compass-needle-with its opposite London. If the upper wire be now attached to powers at its opposite ends, and its one magnetism deter- the zinc, and the lower to the copper, the current inining the tendencies of both.

Another misapplication of the term Polarity is to the will travel north by the lower wire, and come opposite effects which the same force exhibits when its south by the upper. Now, without entering into intensity varies. Thus heat of a certain intensity causes details for which we have not room, and which are quicksilver to combine chemically with oxygen; and heat of a greater intensity causes the combined oxygen and not essential to the comprehension of the telegraph, quicksilver to separate again from each other. A slight it may suffice to say, that the pole of the compassmechanical impulse increases cohesion; a more powerful needle, which points east if the electrical current impulse destroys it. But those are not exhibitions of polarity, Heat, for example, either entirely decomposes, or passing near it be sent in one direction, points west entirely combines; it does not do both at once-as elec- if it be sent in the opposite one; while, if the tricity does, when it decomposes (electrolyzes) chemical compounds. Variation in intensity, moreover, is not the passage of electricity be discontinued, the needle cause of the opposite powers of the poles of a magnet, or a resumes its original position. We have it thus in voltaic battery. The northern magnetism, on the contra- our power to cause a compass-needle to move to ry, always possesses the same intensity as the southern magnetism-positive electricity the same intensity as neg- either side at will; and we can bring it in a moative electricity. If the word polarity signified only dual-ment to rest. All those effects are produced still ism, it should be struck out of the language; for it is more strikingly if the wire, instead of being obscure to ordinary readers, and very far-fetched. But if we discard it as implying one kind, and one kind only of dualism, we must introduce some new term to denote the duplex unity, which those who employ it wisely, intend it alone to signify.

stretched above or below the compass-needle, be coiled many times around the compass-box, or case containing the magnetic needle. The wire, in that Some have inconsiderately sought to render the main case, is covered with thread, which allows its coils truth under notice more distinct, by referring to a body to be put close together, without risk of the elecsuch as a magnet exhibiting polarity, as bi-polar. This, however, is a useless and vicious tautology. We might tricity passing across from coil to coil where they as well speak of a four-sided square, or a three-angled tri- touch, as it would do, if the thread, which is a angle. A polar body is, by its very definition, bi-polar-non-conductor, did not insulate the electricity. It just as a square is necessarily four-sided, and a triangle three-angled. is more convenient that the magnetic needle should

originally stand vertically, so as to move from right to left, or vice versâ—like the index of a wheel barometer, than that it should revolve in a horizontal plane like the mariner's compass. It is also much more easily moved, if the effect of the earth's magnetism on it be neutralized. This is done by placing two magnetic needles on the same axis, with their poles reversed, so that the north pole of the one is opposite the south pole of the other. Such an arrangement, if the needles are of equal power, has no tendency towards one point of the compass more than another; and by making what are to be the lower ends of the needles somewhat heavier than the opposite extremities, the needles, when not under the influence of electric currents, will at once resume their vertical position.

Now, the one needle which is to act as the visible index, appears in front of a dial plate; the other surrounded by the coil of covered wire, which is continuous with one of the telegraph-wires, is placed behind the dial. An arrangement of this kind is provided at Edinburgh, the upper telegraph-wire being drawn out there into a long loop, which consists of soft copper wire covered with thread. This is wound round the concealed magnetic needle, so that a current of electricity moving along the upper wire follows the coiled loop, moves the needles in passing, and returns to London. At London, for a reason to be mentioned immediately, there is a similar loop or coil of covered copper wire surrounding a double magnetic needle, and then rejoining the upper main wire from which it proceeded. From the copper end of the battery, a wire is conducted to one of the strands of this coil, and soldered to it. From the zinc end a wire also is conducted, which is soldered to the lower telegraph-wire. The current setting off from the one end of the London battery, deflects the needles at London and at Edinburgh before it returns to the former. That the needles may be deflected to either side at will, a contrivance is supplied for cutting off and letting on, as well as for reversing, the electric current from the battery. It is a little difficult, without a diagram, to explain distinctly this important portion of the telegraph. The following description, however, will perhaps make it sufficiently clear. Let the upper end of the double telegraph-wire at London be marked A, and the lower end B. If A be connected to the copper of the battery, and B to its zinc, the current of electricity setting off from A, and returning to B, moves the index-needle to one side, for example to the left. If the arrangement be now reversed, so that A is connected to the zinc, and в to the copper, the current flows from B to A, and moves the needle to the right.

signal is to be transmitted, a metallic cylinder is moved by a handle so as to fill up the gap, and establish continuity between the wires and the copper and zinc respectively, of the battery. This bridge, however, is so contrived, that when the handle which controls it is moved to the left, it stretches in such a manner as to connect the end A of the telegraph-wire with the copper, and the end B with the zine, and the needle moves to the left. When the handle is moved to the right, it shifts the cylinder or bridge so as to establish a communication between A and the zinc, and between B and the copper; and the needle moves to the right. When the handle is placed vertically the current is cut off from both wires.

It only remains that an arrangement be made between the parties in Edinburgh and London, as to the signification of these deflections of the needle. This having been settled, the message-sender in the metropolis, seated before his dial, moves the handle which determines the transmission and direction of the electricity along the wires. Every motion of the handle to the right or to the left causes the index-needles at London and Edinburgh to move simultaneously to the same sides. We may suppose, for example, that an answer in the negative is to be telegraphed from London to an interrogation from Edinburgh. It has been prearranged that one movement of the needle to the left shall signify N, and one to the right, o. The respondent accordingly moves his handle to the left; thereby transmits the current of electricity in such a direction as to move the index-handle at Edinburgh to the left also; and so represents N. He then places the handle verticaliy so as to cut off the current, and permit the needle to resume its vertical position; and, after a brief pause, carries his handle to the right, which moves the Edinburgh needle also to the right-which indicates o, and thus completes the answer.

The signal-dial at London is not essential, if London is not to receive messages; but as it must be provided with a view to their reception, it is so arranged that the electricity moves its index-needle before it passes on to Edinburgh. The party transmitting a message, has thus figured before him deflections of the index-needles, identical with those which his correspondent is watching and deciphering, at the same moment, hundreds of miles away.

Only two movements, it will be observed, can really be effected; but it is easy to make them rep resent the whole alphabet, and to telegraph rapidly, although every word be spelled letter by letter. Man, moreover, is by his natural-history definition one of the bimana. Two dials can therefore be arranged side by side, with coils and indexIn actual practice, however, the wires are not needles for each, and handles to be managed by shifted from the zinc to the copper, but are cut either hand. Four movements are thus made posacross between the battery on the one hand, and sible; and for most purposes these supply an ample the telegraph-wires and coil round the magnetic abundance of signals. It does not, however, form needles on the other. The gap thus made is left part of our present purpose to explain these-as vacant when no message is to be sent. When a their employment to represent letters, numerals,

words, paragraphs, or the like, is quite arbitrary, ring. Electric currents not only deflect permanent and involves nothing electrical. We give a speci- magnets, such as the compass-needle, but confer men, however, of one of the telegraph alphabets :- magnetism upon non-magnetic iron. If a copper

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wire, therefore, be coiled round a rod of malleable iron, and a current of voltaic electricity be sent along the wire, the rod becomes a magnet so long as the current passes; and loses magnetism when the current ceases. This magnetizing power of electricity is turned to account in the telegraph. An ordinary alarum, or the striking machinery of a common clock, wound up so that the hammer would strike and ring the bell if one of its wheels were not locked, is placed at every station. But this wheel is only locked by an iron rod which is balanced on a centre, and so arranged that one end falls into one of the notches between the teeth on the circumference of the wheel. The other extremity of the rod is placed opposite, and close to the ends of a horse-shoe of malleable iron, which is surrounded by a coil of covered copper wire closely twisted round it, and connected by its ends with one of the telegraph-wires. And now, if a current of electricity be sent along the telegraph-wire, it circulates round the horse-shoe, and converts it, for the time, into a powerful magnet, which accordingly pulls towards it the free extremity of the iron rod, and thereby shifts the other end out of the notch in the toothed wheel. The bell immediately begins to ring, as the unlocked wheels revolve by the action of a spring or a weight; but as soon as the current is stopped, the horse-shoe ceases to be a magnet; the rod is no longer attracted, but falls back into the notch and stops the bell. Under this arrangement, the bells at every station would ring simultaneously, although only one was intended to be warned; and the current that rings the bells would also move the

We have provided hitherto only for messages being despatched from London. To secure Edinburgh the same privilege, it is only requisite to deposit a battery there also, and to attach one of the wires from the battery (controlled by the handle for reversing and arresting the current) to the coil round the magnetic needle, and the other wire to the telegraph-wire with which the coil is not connected, as more fully described with reference to the London arrangement. If intermediate stations are to receive messages, then one of the telegraph-wires is cut across opposite the station, and an insulator of porcelain inserted between the divided surfaces. A thin wire is then soldered to the main wire on one side of the insulator, led into the station, covered with thread wound round the magnetic index-needle, and led out again and soldered to the main wire on the other side of the insulator. This arrangement is equivalent to a loop on the telegraph-wire; and it must be bent so that the current shall flow in the same direction, round the intermediate station-needles as it does round the terminal ones, otherwise the indices will not be moved to the same side by the same elec-index-needles, though only for a moment. On trical current. A battery at each station, with wires connected in the way already described, enables it to send messages in its turn.

From what has been said, it will be understood that signals telegraphed from any one station to any one other, will be contemporaneously exhibited at every station. For the whole of the stations are included in one circle of conductors, which carry the electricity round all the indicating apparatus within its circuit; and the current cannot move one index without moving all. It is impossible, therefore, if a common alphabet be used along the line, to conceal from the whole of the stations what may be intended only for one. All that can be done, unless a separate series of wires or other conductors were supplied for every station, is to signify what place the message is directed to, so that other stations need not be at the trouble of deciphering the signals.

In addition to the arrangements for producing and interpreting signals, it is plainly necessary that we should also have some contrivance for calling the attention of the parties in attendance to the dials, when a message is about to be sent. For this purpose, warning is given by a bell, which a very ingenious application of electricity is made to

most telegraph-lines, however, a separate set of wires is now provided for the bells, so that they are rung without affecting the needles. A separate wire, also, is sometimes furnished for every station, so that each bell can be rung independently of the others; but such arrangements necessarily add much to the cost of the entire telegraph.

The magnetizing power of electricity is also applied to produce visible as well as audible signals.

The following is one of many such arrange

ments. A horse-shoe which becomes alternately magnetic and non-magnetic, as an electrical current does, or does not, circulate round a copper wire coiled about it, alternately lifts and lets fall an iron lever, which, like the beam or piston of a steam-engine, gives a rotatory motion to a wheel. This wheel carries an index which travels over a dial, round which the letters of the alphabet are engraved. The current must be alternately interrupted and continued to keep the wheel revolving. When the current passes along the wire, the index moves from the letter at which it is pointing, to the next. The current is then cut off; and, when it is restored, the index moves on to the succeeding letter. A key, like those of the organ or piano-alternately depressed and

next.

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allowed to ascend-furnishes the means of inter- and back again. In actual practice, however, one rupting and renewing the current. This arrange-half of the wire is now commonly dispensed with, ment has been called the step by step telegraph; and its place supplied-by the earth! as for each touch of the key the index makes only tury has elapsed since the very curious discovery one step; namely, from the letter it is at, to the was made, that the electricity of a charged LeyIt has the convenience, too, of using the den jar or battery will pass instantaneously through old familiar alphabet, instead of arbitrary deflec- a great length of moist earth. Voltaic electricity tions of needles, and is alleged to possess other has more recently been discovered to possess the advantages, which will presently be referred to. same power; and advantage has been taken of it in the following way. A wire is led from the last copper plate of a battery placed, let us suppose at London, along the telegraph posts, in the way already described, to Edinburgh, and is there bent backwards towards London. Instead, however, of being carried along the posts a second time, the wire is now cut short and soldered to a large plate of metal, which is buried in the ground at some little depth. A comparatively short wire is also attached to the last zinc of the London battery, and soldered to a metallic plate which is likewise buried in the ground. arrangement is equivalent to a great gap or breach several hundred miles long, in the double wire, filled up by moist earth. When the battery is in action, the electricity (positive) flows from the copper along the wire to Edinburgh, descends there to the one earth-plate, (as it has been called,) passes from it through the earth to the similar plate near the London station, and from it reaches the zinc of the London battery. The circulation Their of the electricity in this way is found to be even more rapid than when the double wire is furnished for its passage.

A third method of electric signalling, which promises well, but has not as yet been fully tried, is to effect chemical decompositions by the current. One such electro-chemical process is the following. A ribbon of paper, soaked in an acid solution of the yellow prussiate of potash, and pressed upon by two metallic springs placed side by side-which are in connection with the telegraph-wires-is wound off a roller by a piece of clockwork. When the current circulates, it passes, according to the direction in which it is sent, by the one spring or the other, across the wet ribbon, and decomposes the salt with which it is impregnated, producing blue marks at either of the points where the spring touches the paper. The blue spots or lines thus produced are longer or shorter, in proportion to the period during which the current flows, and at the one side or the other of the ribbon, according to the spring by which the electricity passes; and these blue marks or lines may be made to represent letters, according to their length and position on the paper. variations in both respects are determined either by the movements of a handle at the station sending messages, by means of which the current from a battery is interrupted, renewed, or reversed at pleasure; or by a mechanical arrangement of great ingenuity, which we have not left ourselves room to describe.

The

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Good people have perplexed themselves with speculations as to why the electricity never wanders, misses its road, or fails to find its way back. But, as has been implied already, in the case of the double wire, electricity, like a prudent genLastly, it may be mentioned, on this topic, eral, always takes care that a retreat be provided that, from the first, much attention has been di- for before it begins its march. Till an unbroken rected to the arrangement of an apparatus which circuit of conductors connect the terminal plates should print as well as signal its messages. of the battery, no electricity can be set free. Many beautiful contrivances for this purpose have is not essential, however, that those conductors been devised and tried-and in no long time we should be metallic; a column or stratum of moist may expect to see some of them in use. Descrip- earth, we have seen, will do quite as well as an tions of them, however, would scarcely be intel-iron or zinc wire. One half in length of the conligible without drawings; and their consideration necting conductors must, however, be insulated; may be deferred till their adoption is ratified by public approval. The question, What is the best method of applying electricity to produce signals? is at present undergoing the keenest discussion; nor will it be speedily settled. The telegraph has not been long enough in use to enable us to decide what arrangement is best; but all competent parties are satisfied that, wonderful as its achievements are, they will yet be greatly exceeded. Our immediate object, however, is to record its present condition, not to speculate on its future improvements.

so that the electricity may be compelled to travel to the farthest point to which messages are to be telegraphed. But the other half of the conductors need not be insulated, and cannot be too large. The quicker the current can pass the better; and it will pass most quickly when conveyed by one or other of the two great electrical conductors which man has at his disposal—the solid mass of the globe, and the ocean with its tributary waters.

The last allusion leads us directly to the Marine Telegraph. It requires, however, no detailed description-as it differs from the Land Telegraph only in having the space between the buried plates occupied by water instead of by earth. Broad estuaries or channels do not perinsulated wire to be carried across by The wire therefore proceeding from the

In the preceding description we have purposely referred to the simplest and most easily understood form of electric telegraph, where there is a wire reaching from the terminus at the one end mit the of the telegraph-line, to the terminus at the other, bridges.

copper end of the battery is embedded in gutta percha, or any other water-proof insulator, and sunk in the waters to a depth sufficient to secure it against fishing-nets, ships, anchors, or large sea animals. In this way it is conveyed from one shore to the other, and bending backwards after beng connected with the index needles, terminates in a broad plate of metal sunk in the waves, close to the further shore. A second uninsulated wire proceeds from the zinc end of the battery to a metal plate sunk below low water mark, at the side from which the insulated wire set off. Between the immersed plates on the opposite shores the mass of water, though ever changing, acts in relation to electricity as if it were an undisturbed gigantic metallic wire. Theoretically, there is no limit to the ocean spaces which electricity may traverse in this way. Already, accordingly, schemes for telegraphing across the Atlantic and the Pacific have been triumphantly expounded to the wonder-loving public.

For

and in fogs or snow-storms, it was confidently
anticipated that the system of electric signals
would be available in all states of the weather.
But this expectation has proved fallacious.
hours together the telegraph will not work. This
failure is sometimes owing to the insulation of
the wires along the poles having for the time
been destroyed by moisture. The porcelain insu-
lating tubes, however, are now made of such a
shape, and so well protected from rain by sloping
covers, that non-insulation from moisture occurs
much more rarely than might be expected. There
are certain damp fogs, however, or mists, which
penetrate everywhere; and so thoroughly wet the
porcelain tubes, that they become conductors of
electricity. In those circumstances it travels
from the battery no further than the first wet post,
down which it passes to the earth, and returns, re
infecta, to the battery.

But a much more troublesome cause of inaction, or of irregular action, in telegraphs, is the influence of atmospheric electricity upon them. The door left open that the friend may enter, stands open also for the foe. The insulated wires stretched along the telegraph-posts for hundreds of miles, in order that a special current of electricity evolved by a battery shall travel only in one direction, cannot, like a private road, be barred against electricity evolved from other sources. Nor is this all. When the electrician wishes to collect atmospheric electricity, he insulates a metallic wire, and suspends it in the air. In other words, he acts exactly as the constructer of the telegraph does, though with a very different object in view. The latter, much against his will, finds that his wires not only permit, but invite, atmospheric electricity to employ them as a highway. They act, in short, as lightning-conductors; and lead the formidable meteor into every station, where it deranges or destroys the coils and magnets, and occasionally menaces buildings, and even life, with destruction.

One of these, whether hopeless or not for immense distances, is so very ingenious, and so likely to succeed across limited spaces, that we cannot pass it unnoticed. It dispenses, except to a very trifling extent, with wires, and carries the current both ways through moist earth and water. It is desirable, for example, to telegraph from the right to the left bank of a broad river. From the copper end of a battery on the right bank, a wire is carried to the shore (on the same side) and soldered to a plate buried in the river below water mark. A wire is also led from the zinc end to a long coil of wire which ends in a metallic plate. This likewise is buried in the river below water mark on the same right bank -but at a distance from the battery considerably greater than the breadth of the river across which signals are to be sent. On the left bank two plates are immersed opposite those on the right bank, and connected by a wire. The electricity, on leaving the battery, has therefore the choice of two paths. It may either keep entirely on the right bank, passing from the one buried plate on that side to the other, and so back to the battery by the long coiled wire; or it may cross to the left bank through the water, traverse the wire on that side, return across the water to the right bank, and regain the battery by the shorter coiled wire. The Thames, as we learn, has been act-determined to them rather than to the less exalted ually crossed by electric currents in this way; the resistance to their passage by the water between the banks being less than that between the ends of the wires on the right and left bank respectively. A wire stretched from Land's End to John O'Groat's House, would indeed measure but a small portion of the breadth of the Atlantic -but by twisting the wire into coils, we might include in a short space an enormous length.

It remains to consider some of the imperfections which attend the electric telegraph, and considerably limit its useful application. When it was first suggested as a substitute for the optical telegraph, which was useless in dark nights

To guard against these serious evils, lightningrods, descending to the ground, are fixed at intervals to the telegraph-posts, and at the stationhouses. The sharp spikes in which these rods terminate above, being elevated considerably beyond the telegraph-wires, present points of attraction to the electricity of the clouds, so that it is

and unprojecting wires. It is thus transferred from the atmosphere to the earth without affecting the telegraph. The rods in question, however, only protect the wires in their immediate neighborhood, and that ineffectually.

An additional and more effectual mode of protection is to place a knob of metal on each wire where it crosses the posts. A second and lower knob is then placed close to the first, but without touching it, and connected with a wire led down the post to the ground. If the lightning discharge ran along the wire, it would be cut off at the first knob it reached on the line, on reaching which it would leap across to the lower knob, and descend

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