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opposite shore is safely gained, the extremities of the conducting wire are connected on either side with other wires which are in communication with the telegraphic apparatus, and the signals can be at once transmitted. The increased pressure upon the gutta-percha, produced by submersion at great depths, is found to improve the insulating power of the material, the pressure upon each square inch being increased nearly 1 ton for each mile below the surface.*

In cases in which the wires are insulated with gutta-percha, and are then encased in iron tubes, or sunk beneath a body of water, it has been observed that if the wire be connected with the battery, the signal is not instantaneously transmitted to the opposite extremity; and that if the battery-contact be broken, there is not an instantaneous cessation of electric action at the distant point.

Faraday (Phil. Mag. 1854 [4], vii. 197) has shown that this retardation is produced by the inductive action of the current upon the gutta-percha insulator. The insulated wire, in fact, forms a Leyden jar; the gutta-percha is the dielectric; the wire within forms the inner coating, and the iron tube, or water of the ocean which surrounds it, forms the exterior coating. The time lost at first is that which is expended in giving to the guttapercha its charge; and the current which is observed to continue for a short time after the wire has been disconnected with the battery, is produced by the gradual discharge of the electricity which had been communicated by lateral induction to the gutta

The most gigantic submarine cables which have yet been laid, are those which connect the coast of Valentia in Ireland, with Heart's Content, Newfoundland, a distance of 1670 nautical miles. There are three of these cables connecting the two hemispheres; one which, after the unsuccessful attempt in 1865, was successfully completed in 1866; the second, laid at one operation in 1866; and the third from the coast of France, laid in 1869. The cable of 1866 is 1858 knots in length, and is formed of a single copper conductor, consisting of seven strands of copper wire, of a diameter of 0.114 inch (2.9); this is enclosed in alternate layers of gutta-percha and "Chatterton's compound" (a mixture of gutta-percha with wood-tar, and resin), making a core of 0'464 inch (11mm.78) in diameter; this is covered with a serving of jute, soaked in infusion of catechu, and enclosed in ten bright steel wires, each protected with a covering of hemp; the whole forms a cable the entire diameter of which is 1125 inch (28.mm.6); its weight per knot in air is 35 cwt. and in water 14 cwt. (a knot = 6087 feet, or 2029 yards). The cable of 1866 loses half its charge in from 60 to 70 minutes. The total resistance of the 1858 knots of copper wire contained in this cable is equal to 7209 "British Association units" (note, p. 550).-(North Brit. Revier, Dec. 1866.)

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ELECTRIC TELEGRAPH-THE INDICATOR.

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percha the gutta-percha in this case becomes polarized, just in the same manner as the glass of an ordinary Leyden jar. If the conducting wire employed have a diameter of inch, a mile of such wire would expose a surface of 85'95 square feet; so that it is obvious that a cable, even of moderate length only, must be capable of acquiring an extremely powerful charge, as the amount of charge from a battery of uniform power is directly proportioned to the length of the wire. Variation in the degree of conductivity in the wire does not affect the amount of the charge by induction, but the amount of charge is very greatly influenced by the nature of the insulating material used. In the course of the experiments made by Wheatstone for the Government Commission on Electric Telegraphy, published in a report to the House of Commons, in 1860, it was found that the induction produced in gutta-percha was very much greater than that in caoutchouc; whichever material is employed, the amount of induction varies inversely as the square root of the thickness of the insulating envelope: so that the induction of a coating inch thick is only twice that of a coating inch thick.

Further, by increasing the diameter of the wire and the thickness of the covering in the same proportion, the amount of induction remains unaltered. A wire inch thick, covered with gutta-percha thick, experiences the same induction as a wire of inch coated with gutta-percha thick. The rate of transmission of the current in such coated conductors, when of uniform dimensions, is inversely as the square of their length (Sir W. Thomson).

These observations do not affect the fundamental conclusions deduced from Ohm's law; in consequence of which it is found that the quantity of electricity transmitted is directly as the difference in its potential at the two ends of the wire; and from a source of uniform power, it is inversely as the length of the wire, but directly as the square of its diameter. When the wires are suspended in air, no retardation of this kind is observed; and no after-current is perceived. The gutta-percha in such a case cannot assume the polarized condition, owing to the absence of any conducting communication between its external surface and the earth by which the induced electricity could be carried off.

Supposing that the line of communication has been established, we have now to consider ::

3. The Instrument for Exhibiting the Signals.-The ordinary ndicator, or instrument by which the signals are exhibited, is

FIG. 252.

F

essentially a galvanometer, in which the astatic needles are suspended vertically, instead of being placed in a horizontal position. A side view of the coil is shown at G, fig. 252. One of the needles is shown vertically suspended within it; the other needle, ns, is represented in front of the dialplate, F, F, of the instrument. The needles are slightly heavier at their lower extremities than at their upper ones, in order that when disturbed from the vertical line, they may again resume it when the disturbing force ceases to act. The motions of the needle to the right or to the left Hare limited by a little ivory stud, which projects on either side from the face of the dial; loss of time, which would otherwise be occasioned by the unnecessary length of the oscillations of the needle, is thus prevented. L and P are the wires which com

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municate with the distant station; c z is

the battery; H is the handle by which the instrument is worked. Fig. 253 is intended to illustrate the principle upon which such

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an instrument is made to exhibit the signals; the details of its construction have been slightly modified in the diagrams, in order that the course of the electric current may be more clearly traced.

309.]

ELECTRIC TELEGRAPH-THE INDICATOR.

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No. I represents a back view of the essential parts of the instrument, when at rest and in a position to receive a message from the distant extremity. In this position, supposing the current to originate from the distant battery, and to enter the galvanometer G by the wire L, it will pass through the coil, will make its exit by the wire upon the right hand, which is attached to the metallic spring t; thence it will pass along the brass crosspiece, d, into the metallic spring, v, and complete the circuit through the wire attached to the plate, P, and the earth, E, by which it is returned to the distant station. The battery shown. at cz is inactive during the whole of this stage; the wires which proceed from its two extremities are attached to insulated pieces of brass at either end of the vertical piece which is connected with d. No current therefore can, in this position, be transmitted from this battery, since the wire proceeding from c is completely insulated. But suppose it be desired to transmit a signal from this instrument to the distant station :—by means of the handle, н (fig. 252), the piece to which d is attached can be pressed against one of the springs at t (fig. 253, 2), whilst its lower extremity, by the same movement, is pressed against the other spring, v; the current now passes from the battery in the direction shown by the arrows. From c it proceeds to v, thence, through the wire attached to P, into the earth; then through the distant station, where the instrument is arranged for receiving the signal, as in No. 1, and it then produces a deflection of its needle. Thence the current returns by L to the galvanometer coil, G, and then deflects the needle, returns through the wire attached to the spring, t, and by the metallic piece, d, completes the circuit through the wire attached to z.

It is obvious that by reversing the movement given to the handle, н, the direction of the current and the motion of the needles in the coil will be reversed both in the near and in the distant instrument, as shown at No. 3. As soon as the operator has finished making his signals, the springs, v and t, restore the crosspiece, d, to the position shown in No. 1, and thus the instrument at once adjusts itself for receiving the signals from the distant station; the battery at c z being thrown out of action, and the conducting communication with the line being restored through the crosspiece, d, by the self-acting power of the instrument itself.

By this arrangement a corresponding motion of the needle is always produced at the same instant at both stations, so that both the giver and the receiver of the message perceive the signal.

Since the needle admits of being moved either to the right or to the left, it is clear that by combining together on a definite plan a certain number of these movements, any letter or word may be transmitted for instance, two movements of the upper end of the needle to the right may show the letter A; three movements in the same direction the letter B; four might indicate C; one to the right and one to the left D and so on.

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By employing two or more needles in each instrument, a greater number and variety of signals can be transmitted in the same time, but each needle requires a separate conducting wire, although the number of batteries need not be increased.

cuits.

SV. MAGNETO-ELECTRICITY.

(310) Volta-Electric Induction.-The term volta-electric induction was given by Faraday to the production of secondary currents, or currents in closed wires obtained by inductive action from wires conveying currents in the vicinity of such closed cirThe circumstances under which these currents are formed will be best understood by a description of an experiment. If a wire through which a voltaic current is passing be placed parallel to a second wire, the two extremities of which are connected with the ends of a sensitive galvanometer, no perceptible effect is produced in the second wire, so long as the current passes without interruption through the first wire; but if the current through the first wire (or primary current, as it may for the sake of distinction be termed) be suddenly stopped by interrupting the connexion with the battery, a secondary current of momentary duration is produced in the second wire, and this current is direct, that is to say, it is in the same direction as that in the battery wire. On again completing the communication between the first wire and the battery, a momentary current or wave of electricity again passes through the second wire, but it is now inverse, or in the opposite direction to the primary

current.

These effects may be much increased, if instead of employing simple wires, the wires be coiled into the form of two concentric helices the wire which is to convey the primary current, or primary coil, being placed in the axis of the coil for the secondary current, and the ends of the secondary coil being connected as before with the extremities of the galvanometer. Under these circumstances the needle will receive a powerful impulse at the moment the primary coil is connected with the battery, but after

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