Class X.] INSULATION OF THE FORCE REQUISITE FOR TIIEIR EFFICIENCY.

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at the receiving station by turning on pan* after pair ofplates in the reverse direction until the needle comesback to zero, when the number of cells required to dothis gives the letter or signal. The idea is original; butin order to carry it out the batteries must be precisely ofequal power, and no portion of the force must be lost intransit.

The earth itself has been made to furnish a supply ofelectric force; in other words, a single pair of zinc andcopper plates have been buried sufficiently below the sur-face to be in the wet subsoil, when the earth, saturatedwith water, represents the sand saturated with acid water,of an ordinary battery cell; by this means a current oflow intensity is obtained, even when the plates are somemiles apart. It was thought by some that this feeblecurrent might be made available for telegraphic purposes,by laying one metal at one station and the other metal atanother, and very exaggerated propositions were putforth. Steinheil rejected this mode of obtaining thecurrent, which is only available, and that very indiffer-ently, at the place where both plates are buried.

The means of turning on the electric force, or settingthe current in motion, are as various as are the differentinventions: in one case, consecutive depressions of asingle stud are made, and a current is sent in one direc-tion only; in another, handles or levers are moved ineither direction, and two directions of current ensue; inanother, the current is turned on in a constant direction,and the action of the instrument breaks and reneAvs thecircuit; in another, the whole force is sent along one wire,or is divided betAveen two, and is directed this Avay andthat as the case may be.

The magneto-electric machine is a constant fountain offorce; it does not a ary in power, as do batteries, norultimately become exhausted, but remains for an un-limited time capable of generating the induced electriccurrent. It consists essentially of a set of poAverful per-manent magnets, and a coil or coils of copper wire woundupon iron cores; the ends of the copper wire being ledoff to adjustments proper for distributing the force in theright direction. The current is obtained by brisklymoving the coils in presence of the magnet, making, infact, the conducting wires to move among the lines ofmagnetic force. Where single currents are required, onemotion of a lever on the other arm of which the coils arefixed, gives an instantaneous current. Where, as inWheatstone's instrument, a number of consecutive cur-rents are required in rapid succession, the coils aremounted on an axis which can be made readily to rotate,and to rest at any required point. Where, as in Henley’sinstrument, the force is required along one or other, orboth of tAvo wires, coils arc mounted on le\-ers near eachend of bar magnets. Steinheil, Dujardin, and Hatcherare among those avIio have employed the currents fromthis source. When the current of high tension, inducedfrom an ordinary current of low tension, is wanted, a cellor tw r o of the more constant form of battery is employedto pass a current along a coil of stout Avire surrounded bya long coil of thin wire, Avhich latter is permanently inthe telegraph circuit, and out of which is generated theactive force.

II. The Insulation of the Force .—This is unquestionablythe most difficult part of electro-telegraphy. It has en-gaged the attention of practical men from the outset.It is in vain that an abundant supply of electricity is ob-tained ; it is in A-ain that the best measures are contrivedfor turning on this supply, and vain is the expenditure ofthought, and contrivances for converting this electricityinto a representation of our ideas, if the means are defec-tive for conveying it in its integrity to its destination.Electricity of high tension, such as Avas employed for theearly illustrations of signalling, might have been suffi-ciently Avell passed by Lomond from room to room, ifhis wires Avere carefully suspended, and the atmospheretolerably dry; but Avhen Ronalds proposed to pass thesame agent along a distance of only 175 yards, he wascompelled to surround his wires by thick glass tubes,well coated with wax at the joints, and placed under-ground in wooden troughs lined within and Avithout Avithpitch. Frictional electricity, as is well known, requires

perfect insulation to prevent it passing off in the form ofthe electric spark. Weber, in 1S.33, had found the voltaiccurrent to be retained in wire of about a mile and a halfin length, which was merely suspended from the steeplesand house-tops of Gottingen . Steinheil, in 1S37, erectedand Avorked his telegraph at Munich , through a distanceof about six miles, with no other insulation than a pieceof felt placed between the wire and the support; but hefound the insulation imperfect, and the more so in wetweather; and saw clearly that the force would have beenaltogether dissipated, had the circuit been longer.

Tiie suspended wires are, at the present time, occa-sionally insulated from their supports by glass, but morecommonly, as in England, by hard, Avell-glazed stone-ware. Much ingenuity has been exercised in the form tobe given to the insulators, of which there are many inthe Exhibition. The barrel-shape, pierced longitudinally,universally prevailed here until long circuits called forsome more perfect form; and now in the bell or mush-room, the bold, open-mouthed cone, or other analogousforms, Ave can discover that the actuating idea has beento keep the points of contact dry, and to let the intervalbetween the wire and the pole be a maximum. Walksh(N o. 430) has gone a step further, and has placed a closedroof OA'er his insulators. Cases occurred in Avhich it wasfound convenient, even in the early days of telegraph, tolay the Avire on or near the ground, and eventually underthe public streets, when it was covered with cottonsaturated Avith tar and pitch, and protected by leadenpipes. This was found to be very inefficient, nor didmuch better success attend the use of caoutchouc, Avhichwas not only expensive, but neither manageable nordurable under the conditions to which it was subjected.By a most happy coincidence, gutta-percha appeared inthe market at the time, when resins and varnishes, andthe known gums had been tried and found wanting. Itwas appropriated by Siemens in Prussia, who employsburied wire very largely, and by Walker in England, whoused it at first betAveen Dover and Folkestone , and nowextensively in tunnels and under water, and who demon-strated its value for submarine purposes, by sendingsignals to London witli tAvo miles of gutta-percha wire(a piece of which Avire is exhibited) in circuit, in the seaat Folkestone , in January, 1849. The street wires andtunnel wires, as Avell as those submerged in England, Bel-gium, America , and elsewhere, are hoav perfectly insulatedby a coating of this valuable material. It is applied warm,either by poAverfully pressing it upon the wire, or bycausing it to follow the wire through a hole of the givensize. In the streets the wires are sunken, and protectedby being enclosed in iron-pipes, which proves to be verynecessary, for in Prussia, Avhere they are buried withoutprotection, they suffer so frequently from the attacks ofvermin, that it is in contemplation to suspend them aswe do.

Under rivers or harbours they are protected by pipes,or are secured in a mass of timber, or otherwise. In allthe tunnels on the South-Eastern line, Mr. Walker liaslaid them iu grooA r ed boards, which method has beensince followed by the Telegraph Company, and is alsoadopted by the Belgian Government on their lines. It isunquestionably the simplest and safest of all methods.

The protection required for submarine Avires is verygreat. The Avire, by means of Avhich a telegraph com-munication was obtained for a few hours betAveen thecoasts of England and France , was not calculated, neitherwas it expected to remain perfect for many days. It Avasclearly not laid doAvn with the intention of remainingpermanently, but for some special object. ItAvas a copperwire, coated in the ordinary way, but thickly, withgutta-percha, and where it reached the shore, protectedby a leaden pipe.

McNair (No. 421, p. 455) has exhibited some excel-lent specimens of his process of covering gutta-perchawire, first Avith cotton and then with an outer coat ofgutta-percha, and finally with lead, in the application ofAvhich latter the cotton becomes impregnated Avith gutta-percha from the outer coating.

Wrought iron chain-pipes, Avith SAvivel joints, haA r ebeen constructed byWisnAAY (No. 419), and are exhi-