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ment, shall represent a letter of the alphabet, a numeral, a word, a sentence, a paragraph, or the like. A source or fountain of electricity, conductors to carry it, and a dial plate on which it shall cause an index to exhibit signals, are thus the essential elements of an electric telegraph.

Our present object is to discuss chiefly what is electrical in the telegraph-without much reference to the mechanical devices or subsidiary arrangements which it involves. Our first concern, then, is with the source of electricity; and, as our space is limited, we shall confine ourselves to the voltaic battery, the apparatus chiefly in use along the telegraph lines. A voltaic battery, in

to intensity, simply on account of its brevity, and | application of the electricity so conveyed, to proits convenience in forming a double noun with duce at the distant station some striking phenomelectricity. Electricity of intensity then, or ten-enon, which, according to a preconcerted arrangesion-electricity, is electricity characterized by the greatness of its intensity-or whose intensity is greater than its quantity. Electricity of quantity, on the other hand, has its quantity greater than its intensity. The intensity diminishes as the quantity increases; but the ratio which the one bears to the other differs through a very wide scale, so that a knowledge of the degree of the one does not often enable us to predicate the amount of the other. Practically, we have no difficulty in reducing both to a minimum, or in exalting the one whilst we reduce the other; but we cannot at once exalt both intensity and quantity. The discovery of a method of effecting this will make a new era in the science; and admit its simplest form, consists of two dissimilar solids of the most important applications to the useful-generally metals-arranged side by side, witharts. Meanwhile we may compare electricity of out touching each other, in a liquid which distension, as we have done already, to high-pressure steam issuing in small jets under great pressure; and electricity of quantity to the thousands of cubic feet of invisible vapor which arise softly every moment from the surface of the sea. Or the former may be likened to a brawling, gushing mountain brook, rushing with great force but little volume of water; and the latter to the slow rolling Amazon or Mississippi, silently moving onwards to the sea. Or the first to a swift, sudden hailstorm or avalanche, and the second to the inexhaustible glacier, constantly melting, but as constantly increasing. Or the one to an instantaneous gust or white squall, passing off in a moment, and the other to the unceasing trade wind, forever sweeping gently over the bosom of the waters.

We

solves only one of them. One of the solids is almost invariably a plate of zinc, rubbed over with quicksilver, or, as it is called, amalgamated. The other is copper, iron, silver, gold, or platinum; the last being preferred for very powerful batteries, and admitting of being replaced by coke. For telegraph-batteries, amalgamated zinc and copper, or zinc and silver, are generally employed; and the liquid in which they are dipped is diluted sulphuric acid-which dissolves the zinc, but does not affect the copper or silver. Let us suppose copper and zinc to be the metals selected. have it in our power to take all the copper we propose to employ, in one large sheet, and all the zinc in another; or we may cut down each sheet into many small ones. The quantity of electricity evolved by a voltaic battery is chiefly determined It depends upon the purpose to which electricity by the size of the plates made use of; but if we is to be applied, whether it should be chosen take a single sheet of zinc, however large, and a great in quantity, or great in intensity. If the single sheet of copper, we find the intensity of the chemist desires to analyze a gaseous mixture by electricity they evolve exceedingly feeble. If, on detonation, he will use the friction-machine, to the other hand, we cut down each of the large supply a momentary spark of great intensity. plates into several smaller ones, and arrange these But the electro-plater, who has constantly to de- so that the copper and zinc shall be placed compose a compound of gold or silver, employs alternately, in a way to be presently described, the magneto-electric machine, or a small voltaic we find the quantity of the electricity much diminbattery-which furnishes great quantities of elec-ished, but its intensity greatly increased. Unless tricity of considerable intensity. The electric the intensity be considerable (although it need not light requires both quantity and intensity to be very great. For the electric clock the intensity may be at a minimum, and the quantity need only be moderate. The electric telegraph demands great quantity, but the intensity need not be very high.

be very great) the electricity cannot force its way along a great length of conductors; and, if its quantity be not great, its effect will be but momentary. Plates, however, a few inches square, supply a sufficiency of electricity for the longest telegraph line; and from twelve to sixty pairs of This much premised, we may now consider its such plates are as many as are required. The application to the construction of the telegraph. exact number needed will be determined by the An electric telegraph consists essentially of three distance which the electricity is to travel. By things. First, a voltaic battery or other appara- varying the number and size of the plates, as well tus to evolve, when required, electricity. Sec- as the strength of the acid in which they are ondly, an arrangement of metallic wires or other dipped, the quantity and intensity of the electricgood conductors, to convey the electricity to the ity may be modified through very wide limits. distant places with which telegraphic communication is to be carried on, and to bring it back to the machine from which it set off. Thirdly, the

A voltaic battery, strictly speaking, consists of associated pairs of dissimilar solids, such as zinc and copper. A single pair, or simple voltaic cir

solids, immersed in a rapid solvent of one of them, will yet be found sufficient for working the longest existing or conceivable telegraph line. As it is, the intermediate pairs of the voltaic batteries in actual use are introduced only to give the requisite intensity to the electricity generated. They may be ignored in our further discussion; and our telegraph-battery will resolve itself into a piece of copper and a piece of zinc, immersed, without touching each other, in the same vessel of acid

cle, like a single cannon in an artillery battery, is | was very short; and it is quite within possibility but an elementary portion of a voltaic battery, that a single voltaic pair of strongly contrasted which is constructed by arranging several pairs together. The simplest voltaic battery, then, will consist of at least two pairs, i. e. of four plates, two of zinc and two of copper. In arranging these, two glass beakers or drinking tumblers are taken, and placed side by side, half full of diluted sulphuric acid. A wire is then soldered to one of the zinc plates, and a corresponding wire to one of the copper plates, and one of these plates is placed in each of the tumblers. The second zinc plate is thereafter soldered by one edge to the sec-ulated water. ond copper plate, so as to form one continuous sur- For the sake of simplicity and clearness in our face of metal. The compound plate thus produced further description, we shall suppose the battery is then bent over, so that the soldered edges form described, as locally situated in London; and that the summit of an arch, which resembles a saddle, our object is to send messages to Edinburgh, withwith one flap consisting of copper and the other of out communicating with any intermediate place. zinc. This metallic saddle is placed astride of the An iron wire, plated with zinc to keep it from approximated edges of the tumblers, so that the rusting, is connected with the copper plate of the zinc flap dips into the vessel in which the first battery, and then stretched all the way from copper plate with the wire is immersed, and the London to Edinburgh, along wooden poles, erected copper flap into the tumbler containing the zinc some sixty yards apart. In order that the elecplate with its wire. If we wish to enlarge the tricity, which is to travel along this wire, may not battery, we take additional tumblers, and such go elsewhere than to the northern metropolis, the copper-zinc arches as have been described, connect-zinc is insulated, i. e. prevented from coming in ing the vessels, half filled with dilute acid, by the contact with metallic conductors, moist wood, or metallic bridges which dip on either side into the other surfaces which would transfer the electricity liquid; taking care, also, that all the zinc semi- along the poles to other wires that are generally circles or saddle-flaps shall be turned in one direc- stretched upon them, or to the earth. The insution, and all the copper ones in the opposite, so lation is effected by passing the wire through rings that zinc and copper succeed each other alternately, or short tubes of glazed porcelain, attached to the from the first tumbler at one end of the range to posts, so that the electricity has no choice but to the last at the other. In actual practice, porce- move along the wire. At Edinburgh the wire is Jain, or wooden, or gutta percha cells or troughs placed in connection with the signal apparatus, to are generally substituted for glass vessels, and the be afterwards described; and then is brought back pieces of zinc and copper are not soldered together, to London through separate porcelain tubes along but only connected by movable wires and binding the poles as before, and finally terminates at the screws. But these mechanical adjustments are detached zinc plate of the battery. In the arrangeonly for greater economy and convenience; and ment described, which is the earliest and most the battery remains, in principle, identical with the easily understood form of telegraph, it will be arrangement described. observed that the zinc and copper plates of the battery at London are connected by one unbroken metallic wire, which extends to Edinburgh, bends back there, and returns to London.

Such, then, in its most skeleton and simple form, is the apparatus which is to furnish the primum mobile of our telegraph. Although each zinc and copper pair contributes to the power of the battery, the whole electricity generated by it manifests itself only at the detached zinc plate at the one end of the battery, and the detached copper plate at the other. A battery thus resembles a compass-needle or bar-magnet, which appears to manifest its inherent magnetism only at its opposite poles; although, in reality, it is magnetic throughout its entire length. In the practical application of such a battery, accordingly, no account is taken of any portion of it but the terminal zinc and copper plates, to each of which a wire is attached. To these plates all the intermediate ones convey the electricity which they respectively set free; so that we may, after all, properly enough conceive the battery as consisting of a single plate of zinc and one of copper. Such an embryo battery-or, rather, voltaic pair-might, indeed, be

The wire, however, does not return to the latter city, in order to provide a channel for messages being sent from Edinburgh to London, as well as from London to Edinburgh. Without this returning double wire, (as we shall call it,) or an equivalent arrangement of conductors, it is impossible to telegraph from either town to the other, even if it were thought sufficient or desirable to send messages only from one of them. It will appear from this, that there must be something peculiar in the way in which electricity travels along the telegraph-wire. We have compared it to the transmission of a fluid; but the wires cannot convey it as pipes do gas or water, otherwise there would be no occasion for the return-wire. A tube extending from London to Edinburgh, and filled with air or water, might be employed to telegraph from the metropolis to the northern capused for working the telegraph, where the distance | ital, as an air-tube is actually employed at the rail

way tunnels near termini; and but one tube only would be needed, if messages were sent only from London. It is very different with electricity; it must not only travel to Edinburgh, but it must come back to London-otherwise nothing can be recorded at Edinburgh; so that the communication must be as complete between Edinburgh and London, although the latter only is to send messages, as between London and Edinburgh.

to zero-or what we may call the northern magnetism neutralizes the southern magnetism, and all indications of free magnetic force cease.

Electricity exhibits exactly similar phenomena. In the very act of becoming free, as when it is evolved from a voltaic battery, it separates into two forces-identical in nature, but opposite in the direction of their manifestation-whose intensities and powers are equal, and which, like the northThe explanation of this peculiarity, if we avoid ern and southern magnetisms when they meet, the niceties of electrical theory, may be said to be instead of yielding a double electrical force, neufound in the fact, that no electricity leaves the bat- tralize and annihilate the powers of each other. To tery till its terminal zinc and copper plates are the two electricities the names have been given of connected (after a long détour) by a wire or other positive and negative respectively-an unfortunate electrical conductor. It is not as if one wire were nomenclature, as it almost unavoidably conveys the sufficient at least to carry the electricity from Lon-impression that the one is more positive or potent don to Edinburgh. Our electrical messenger is like than the other; whereas the negative electricity a government courier-who does not start till he has as positive an existence, and as substantial is satisfied that there are relays of horses to make powers, as the opposite electricity-and neither, certain his homeward, as well as his outward in fact, can be produced without the development journey. If there be not a return-wire, or equiv of the other. The terms in question, like the older alent arrangement, the electricity never sets off ones, vitreous and resinous, are to be regarded, in from London! or, rather, there is in truth no elec- short, as quite arbitrary, and might be replaced by tricity to set off in any direction, till the zinc and any other words or signs ;—though we leave medcopper at that starting place are connected. Till ical men to explain the account which a wilfully a communication is effected between them, the bat- ambiguous critic has given of their electrical actery is equivalent only to a loaded gun. The com- quirements: viz., that their knowledge of elecpletion of the connection is like the fall of the tricity is chiefly of the negative kind! trigger which fires the charge. In a moment the battery discharges its electricity, which, with inconceivable rapidity, passes, by the shortest route it can find, from the copper plate at the one end of the battery, to the zinc plate at the other. No shorter route, however, is provided for it than the insulated wires, so that in the case supposed, although the plates to be connected are only a few inches apart, the electricity which leaves one of them must travel from London to Edinburgh and back again, before it can arrive at the other! Our newest telegraph, in this respect, is like Noah's most ancient one. His raven "went to and fro," and his dove “returned" to the ark with the oliveleaf in her mouth.

At first they

The twofold magnetism in a bar-magnet has been likened to a double-headed arrow at rest, pointing in two opposite directions, like a windvane. The two-fold electricity liberated from a battery may be likened to a similar double-headed arrow-not at rest however, but rapidly elongating itself in opposite directions, so as to separate its two heads or points further and further from one another. The one arrow-head represents positive, the other negative, electricity. Though they separate, they are never disunited. move straight away from each other; but their paths are equivalent to semicircles of the same radius, and are in the same plane, so that they ultimately meet-and in the act of meeting, each arrow-head destroys the other, and a harmless nonelectric circle is completed. The Egyptian hieroglyphical serpent, devouring its tail, might be accepted as the symbol of the closed electric circuit. If we apply what has now been said to the telegraph, the necessity for the two wires will appear in a new light. When the plates of the battery, consisting of amalgamated zinc and copper, are

If we look, however, a little more closely into what happens, we shall find something still more curious than we have yet indicated, in the movements of the electricity produced by the battery. We have hitherto represented matters, as if only one current of electricity swept along the wires; but, in reality, if we are to speak of currents at all, we must acknowledge at all times two, moving in opposite directions. Electricity, like mag-merely placed apart from each other in dilute sulnetism, always displays itself as a two-fold force. phuric acid, no change of any kind occurs. But A bar-magnet, or compass-needle, has magnetism if they are connected, as by attaching the zinc to at each pole or extremity. The magnetism of its the one end of the double telegraph wire, and the north pole has the same powers and intensity as copper to the other end, the zinc immediately bethe magnetism of its south pole, if we test these gins to dissolve in the acid; and simultaneously by their action on a third body, such as a piece of with this solution of the metal, and the evolution non-magnetic iron. But if we try two bar-mag- of hydrogen from the water, electricity in its twonets against each other, we find that the south pole fold form is developed. At the middle point in of the one attracts the north, but repels the south the liquid between the two immersed plates we pole of the other, and vice versû; and if a north may suppose the electricity to come into existence, and south pole be placed together, instead of the likening it as before to a double-headed arrow. magnetism being doubled in intensity, it is reduced Elongating themselves in directly opposite direc

tions through the liquid, the one arrow-head | ence bears testimony to the fact that we cannot speedily reaches the copper plate on the one side, alter the arrangement of the component parts of a and the other arrow-head the zinc on the other. The arrow at the copper is positive electricity. If we speak of it as before, we shall say that a current of positive electricity flows from the copper along the telegraph wire to Edinburgh, and then returns to the zinc plate, where it may be regarded as stopping at the same time that a current of negative electricity travels from the zinc plate along the same telegraph wire, in an opposite direction to that taken by the positive current, and may be considered as ending at the copper plate.

According to this view, the narrowest telegraph wire may be compared to a railway with two sets of rails, along which trains (of positive and negative electricity) travel in opposite directions-in obedience to a statute which requires that there shall always be two opposite trains moving at the same time along the rails. We must further regard the wire, whilst conveying electricity, as traversed, not by solitary engines or a few carriages, but by trains occupying the entire length of the railway-fresh carriages constantly setting off at the one end, and being detached at the other.

The necessity, however, for the double wire, is best seen when we revert to the notion of electricity travelling like a flying arrow. The route of the arrow is the wire, and the latter must be double, because the arrow itself is not an English clothyard shaft, which flies only in one direction; but such a two-forked thunder-bolt as the Greek sculptors placed in the clenched hand of Jupiter Tonans, which shoots east and west or north and south at the same time, and the one bolt of which will not fly in one direction unless the other is equally free to move in the opposite direction.

What evidence, it may here be asked, is there to show that anything substantial moves along the telegraph-wires? To this, as already implied, there is but one answer. No actual proof can be given of the passage of anything material. The flowing currents and the flying arrows are both purely imaginary-though the one is an hypothesis, and the other but an illustration. But there is yet another mode of explaining the apparent passage of this invisible agent. It is, to be sure, quite as hypothetical as the other two; but it is, on the whole, more likely to be true; and it is therefore now preferred by most men of science. Our discussion would consequently be incomplete if we did not refer to it.

mass, without inducing a corresponding change in the qualities of the mass those atoms build up. Soot and wood-charcoal, coke and black lead, owe their different properties merely to a different arrangement of identical particles of carbon; and a further modification of these invests them with the utterly diverse and characteristic attributes of the diamond. But the electrical differences between two wires, one acting as an electrical conductor, and the other not, surely are not greater than the optical differences between a lump of coke and a diamond crystal-or between carbonate of lime, uncrystallized in chalk, and crystallized in pellucid Iceland spar. We can set no limits, indeed, to the extent to which modification of molecular arrangement will affect the properties of a mass.

Nor is it any objection to such a view, that a metallic wire is a rigid solid, the component particles of which are so locked together as not to admit of motion upon each other, or change of relative position. The opinion once entertained, that only liquids and gases permit the mobility requisite for alteration in molecular arrangement, is now universally abandoned. And, indeed, the expansion and contraction of a mass of metal under the influence of heat and cold is a sufficient refutation of it. The Menai tubular iron bridge creeps, like a huge snake, backward and forward several inches during the twenty-four hours of a midsummer day. The massive glacier changes, from an aggregate of minute crystals of packed snow, into a mountain of clear ice. Every school-boy is familiar with the same phenomenon as developed during the formation of a slide on a surface of snow. In copper mines, an iron hammer, dropped into a pool saturated with cupreous salts, is found, after the lapse of years, converted into a hammer of copper :the whole of the iron has been extracted, and its place supplied, to the very centre, by copperwithout the form or the bulk of the solid having altered during the process of transmutation. During the production of steel from iron, in like manner, the latter is embedded in charcoal powder, and the whole made red hot. The charcoal then penetrates into the solid iron, and impregnates its entire mass.

These examples (and many more might be added) apply to alterations in the structure of solid masses, much greater than we need assume to occur in an electrical conductor. So that we need According to this view, the metallic conductor, not hesitate to admit as possible, molecular changes such as the telegraph-wire which connects the ter- of a more simple character. The change that minal plates of the voltaic battery, is not a high- probably happens in the telegraph-wire is believed way along which electricity travels; but the to resemble what we can pretty confidently affirm wire exhibits electrical phenomena throughout its to take place in magnetized iron, where the characentire length, only because its connection with the teristic phenomena are more readily observed, and zinc and copper wetted by the acid, produces, for are more familiar than in the case of electrical the time, a new arrangement of its own particles conductors. A bar-magnet, or compass-needle, or molecules, which invests the wire with new appears at first sight to possess magnetic powers properties-those, namely, which we call electri- only at each end, or pole. On closer examination, cal. Nor is there anything extreme or anomalous however, it is found to possess the opposite northin this assumption. The whole of physical sciern and southern magnetisms, in alternate succes

sion throughout its entire length. We may com- passed along the line. No man has stirred further pare it to one of the lines or stripes of a chess-than to observe the flag shown him by his neighboard, or tesselated paveinent, made up of alternate bor on the one side, and to show a corresponding colored pieces. The colors, however, must be flag to his neighbor on the other. The flag disonly two-for example, blue and yellow: the first played at Edinburgh was there from the first, square, or tessera, being of the one color, and the though unfurled, and remains there concealed, till last of the other. A piece of non-magnetic iron the next message is telegraphed from man to becomes temporarily magnetic if brought into the man." neighborhood of a permanent magnet, such as a *The arrangement described in the text, of alternate loadstone; and while thus magnetic, the iron oppositely magnetic or oppositely electrical particles, is an exhibits the same alternation of oppositely mag-dle exhibits magnetic polarity; a voltaic battery or elecexample of what is called Polarization. A compass-neenetic particles which the compass-needle does. trical conductor, electric polarity. We have hitherto We may liken non-magnetic iron to an aggregate block rather than a help, in the exposition of the physical avoided the word, because it has proved a stumblingof compound green particles. It becomes magnetic sciences, to those who do not make them a special study. in consequence of each of these separating into a A few words, however, in explanation of the "idea of blue and a yellow particle-which follow each polarity" may be given here. The terms, "to polarize," "polarization," and "polarity," are taken from the comother alternately in rows. When the iron ceases pass-needle, the extremities of which pointing, the one to be magnetic, in consequence of the withdrawal to the north pole of the earth, the other to the south pole, have long been distinguished as the poles of the compassof the loadstone, the result is as if the blue and needle, or magnet. The largest magnet, moreover, apyellow particles united again, and the whole became pears to consist of a multitude of smaller magnets, uniformly green. In like manner the wire which arranged in rows end to end. The magnetic properties of the hugest magnet are thus referred to its consisting connects the zinc and copper of a voltaic battery throughout its entire mass of particles, each of which if is believed, in consequence of its junction with detached would exhibit a north and sorth pole. If this these metals whilst they are affected by the acid, polar arrangement be destroyed, all magnetic phenomena cease. Thus, if a loadstone be approached to a piece of to have induced upon it, throughout its entire soft iron it polarizes it, or induces in it magnetic polarlength, a succession of alternate electro-positive ity. In other words, the loadstone develops alternate north and south poles in the iron, and this polarization of and electro-negative points, or particles possessed the particles of the metal continues as long as the loadof positive and negative electricity respectively. stone is in its neighborhood. The arrangement is of exactly the same kind as that of the magnetic bar-only it is an alternation, not of the opposite magnetisms, but of the opposite electricities. They remain separate so long as the constraining force of the battery is exerted upon them; but the instant the wire is disconnected from it, the separate electricities unite, and all electrical phenomena cease. liken the telegraph-wire, when disconnected from the battery, to a thread on which purple beads are strung together, as on a necklace. When the wire is connected with the battery, each purple bead separates into a red (positively electric) and blue (negatively electric) one. The red and blue beads now succeed each other alternately along the line; and remain separate, whilst, in the language of another theory, electricity is passing; but they coalesce again into the compound purple spheres, so soon as the connection with the battery is inter-exhibits attraction, the other exhibits repulsion, and vice rupted.

We may

According to this view, there is no travelling of electricity charged with messages from one station to another. The message telegraphed from London to Edinburgh is not wafted by electricity which speeds from the former, inscribes its hieroglyphics at the latter as it rushes past, and fleets back to London; but the telegraph-wire, with inconceivable rapidity, merely arranges its own constituent particles, from end to end, in alternate electro-positive and electro-negative molecules; and the index on the Edinburgh dial plate is affected only by the small portion of the wire which surrounds the gnomon. It is as if a row of men were placed side by side from Edinburgh to London, with signal-flags in their hands. The flag shown as a signal at Edinburgh has not been

The idea, suggested by magnetic phenomena, of alternate poles, is transferred to electricity as well as to other forces-with an important restriction, however, the overlooking of which is the great cause of the unintelligibility to general readers of all references to polarity.

In its extended sense, the term carries with it only the conception of an alternation of particles, or points, (centres of force,) possessed of opposite powers-without including the idea of those particles having a directive tendency in space, so that they take up positions in relation to the poles of the earth. Thus the polarity of light is evidenced in one class of phenomena, the polarity of heat in another, that of electricity in a third, that of chemical affinity (which, however, is perhaps identical with electrical polarity) in a fourth, that of crystalline affinity in a fifth. ical affinity, and crystalline affinity, all agree with magBut light, (polarized) heat, (polarized) electricity, chemnetism, in manifesting their powers, not as single, but as two-fold forces; and are all 'characterized by the exhibition, side by side, of two agencies, the same in nature, yet opposite in the mode of their manifestation. Thus positive and negative electricity have each a power of atthe same intensity, and regulated by the same laws-extraction and a power of repulsion, of the same kind, of cept that in the circumstances where the one electricity

versa. And, in like manner, a ray of polarized light, or heat, a row of magnetized or electrified molecules, or a row of atoms under the influence of chemical affinity or crystalline agency, agree in the manifestation of a twofold force, exhibiting itself in the alternation of oppositely endowed points or particles.

This common character is now denoted by saying that they all exhibit polarity; nor have we any other term in our language possessing the same signification. It is much to be regretted, therefore, that its value has been lessened by its vague employment. By many it is used to denote the mere antagonism of two forces. Man, for example, is said to exhibit polarity, because he is possessed of soul and body! And though such language might be consistent in the mouth of a pure idealist, or a pure materialist, it is not competent to one who regards mind and matter as essentially distinct. Polarity is not merely the antagonism or dualistic development of two unlike forces. We can keep a mass of iron suspended either by a loadstone or by a spring, or by the muscular exertion of an animal. And here we have three examples of the antagonistic manifestation of two forces-gravity and magnetism, gravity and elasticity, gravity and vital force. But none of those are exhibitions of polarity. It is the antagonistic or dualistic

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