308.]

ELECTRO-MAGNETIC ROTATIONS.

623

to revolve around the fixed wire, a b; ff are the north ends of two bar magnets, which are united below, and terminate in a pivot, g; this pivot works upon a hard steel plate in the board, A B; c d is a wooden ring which contains mercury, and is in metallic communication with the cup, e. At the centre of each of the magnets is a small brass hook which dips into the mercury of the trough, c d, for conveying the current transmitted through the wire, a b, which is supported by the arm, c. As soon as the connexion of the cups a and e is made with the battery, the magnet begins to rotate round the wire, a b, and continues to do so as long as the current passes; if the direction of the current be reversed, the direction of the rotation is reversed likewise. No. 2 is a similar arrangement for showing the rotation of the wire, g h, around the north end of the magnet, a b; the current enters at the cup, f, divides itself, and passes down g and h into the ring, c d, which contains mercury, and is supported above the board, c D, by the stand, A B: the circuit is completed by means of the cup, e: reversal of the current reverses the direction of the rotation. If the current descend in the wire around the north end of the magnet, the direction of the rotation is the same as that of the hands of a watch lying with the face upwards. The current may be passed through the upper half of the magnet itself, and if delicately poised, the bar may thus be made to rotate rapidly upon its own axis. These rotations may also be exhibited by liquid and by gaseous conductors; if the wires from a powerful voltaic battery be made to dip into mercury, the mercury over the point where the wires terminate will rotate rapidly if a magnet be held above or below the spot. The flame of the voltaic arc revolves with equal regularity and distinctness under magnetic influence; for instance, by making a powerful horse-shoe magnet a part of the circuit, and passing the current through the magnet itself, the voltaic arc of flame which may be drawn from one of its poles will rotate in the opposite direction to the flame which may be drawn from the other pole. This magnetic rotation of the electric discharge is also well exhibited when the induced current of Ruhmkorff's coil is passed through an exhausted globe immediately over the pole of an electro-magnet, the direction of the rotation being reversed with each reversal of the magnetism. (De La Rive, Electricity, 1856, ii. 248.)

A beautiful proof of the magnetic condition of the liquid part of the circuit so long as the current is passing, is exhibited by the rotation of the battery itself, in obedience to the action of a magnet. The experiment may be made as follows:-Let a double

624

ELECTRIC TELEGRAPH.

[308.

cylinder of copper, shown in section at c, fig. 250, of about two inches (5 centimetres) in diameter and three inches (7.5 centimetres) high, be formed into a cell capable of conFIG. 250. taining liquid, and be supported by a point attached to a connecting strip of copper, over one end of a bar magnet; let a cylinder of zinc, z, be supported on a second point in metallic communication with the copper as soon as a little dilute acid is poured into the cell, the zinc will begin to revolve around the magnet in one direction, whilst the copper rotates in the opposite; the current is ascending in the copper, whilst in the zinc it is descending around the same magnetic pole: round the north end of the magnet, the cylinder of zinc will move in the same direction as the hands of a watch which is lying with its face upwards.

Ampère has explained these rotations by means of the theory to which allusion has already been made; but it will not be needful to pursue this part of the subject further.

(309) Electric Telegraph.-The most important and remarkable of the uses which have been made of electricity, consists in its application to telegraphic purposes; an application which has not only brought distant towns upon the same island or continent within the means of instantaneous communication with each other, but which has spanned the seas, and placed an insular metropolis like London within momentary reach of the distant capitals of the Continent, of America, of India, and of Australia.

It would be impossible in a work like the present, to give even a sketch of the numberless modifications and improvements in the apparatus which have been suggested or practised for carrying out telegraphic communications by means of electricity, since the year 1837, which is memorable as the period at which Cooke and Wheatstone took out their first patent for electric telegraphing, and proved to the world the possibility of transmitting and receiving signals produced by electricity, with facility and with certainty, through insulated wires of great length. On the present occasion, an outline of the essential parts of the telegraphic system which is generally adopted in this country is all that can be attempted

The electric telegraph may be regarded as consisting of three parts-viz. : 1. The battery, or source of electric power. 2. The line, or the means of transmitting the signals.

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ELECTRIC TELEGRAPH-THE LINE.

625

3. The telegraphic indicator, or instrument for exhibiting the signals.

1. The Battery.-—The apparatus for producing the signals is simply a voltaic battery, any form of which may be used; the one formerly in general use consists of a series of alternate pairs of copper and amalgamated zinc plates arranged in wooden troughs, subdivided into compartments, similar to those used with Smee's battery (fig. 221). These compartments, after the plates have been introduced, are filled with sand, which is then moistened with dilute sulphuric acid. In this form of instrument the risk of leakage is diminished and the amount of evaporation is lessened : the charge requires renewing once in ten days or a fortnight, according to the frequency with which the telegraph is used. Another form of battery which has been found to be effective for a long period consists of plates of amalgamated zinc, and gas coke, excited by solid mercuric sulphate moistened with water; the plates are arranged in compartments, similar to those used for the moistened sand. But modifications of Daniell's and Leclanché's batteries are now generally preferred to any other form.

2. The Line. The conducting wire was formerly made of copper, but is now generally made of iron wire about 8mm. or onethird of an inch thick, coated with zinc to protect it from oxidation. For the purpose of insulation, this wire is supported upon wooden posts, which are firmly sunk into the earth, and which are kept dry at the upper extremity by means of a cap or case of wood fourteen or sixteen inches (35 or 40 centimetres) long, between the sides of which and the post is an interval of air. To the sides of this cap short tubes of porcelain, or supports of glass, are attached, and through these insulating tubes the wire passes. Suppose that a message is to be transmitted from London to Manchester; a continuous insulated conducting wire must extend between the instrument or battery in London and the instrument at Manchester which is to receive the signals, and there must also be a continuous conducting communication to complete the circuit between Manchester and London. This return conductor may consist of a second metallic wire which must be insulated from the earth and from the first wire, although it may be suspended upon the same posts side by side with the first. The earlier telegraph lines were all made in this way.

It was, however, discovered by Steinheil that the second metallic wire may be dispensed with, and that the earth itself may be employed as the conductor for completing the return communication between the two distant stations. The possibility of

626

ELECTRIC TELEGRAPH-THE LINE.

[309. doing this arises from the law of conduction in solids-viz., that the conductivity increases in proportion to the area of the section of the conductor. As a conductor of electricity, the earth is many thousand times inferior in power to any of the metals, if columns of each metal and of the earth of equal diameter be compared. But it is possible to multiply indefinitely the area of the conducting portion of the earth between the two stations, and thus a line of communication may be obtained which actually offers a far smaller amount of resistance than the metallic part of the circuit. In practice all that is found necessary, in order to take advantage of this conducting power of the earth, and to substitute it for the return wire of the telegraph, consists in leading a wire from the telegraphic apparatus at one end, into the earth, the wire being attached to a plate of copper which exposes a square metre or more of surface, this copper plate being buried in the ground, as represented at P (figs. 251, 252, 253). By increasing the size of this plate, any extent of surface of contact with the earth may be obtained, and thus, notwithstanding the intrinsic inferiority of the earth to the metals as regards its conductivity, the resistance which it offers, according to Breguet's experiments, is insignificant when compared with that of the telegraph wire itself. Another way of making an earth-plate is by soldering the return wire to the gas- or water-pipes of the telegraph office.

The general plan of this arrangement will be understood from fig. 251, in which м and I represent two telegraphic instruments,

one stationed, we will suppose, in Manchester, the other in London. L is the metallic line or wire of communication which connects the stations; E is the earth; and P, Q, copper plates attached to wires, one of which proceeds from each instrument. Suppose, for example, a message to be in the act of transmission from 1, the instrument in London, to м, the instrument in Man

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ELECTRIC TELEGRAPH-SUBMARINE CABLE.

627

chester. If c z represent the battery at the London station, the current will take the course indicated by the arrows: it will pass from c to a wire connected with the earth-plate, P, thence it will pass through the 200 miles of earth between the two cities; at Q it will be taken up again, and be transmitted by the wire to the instrument, м, thence it will be conveyed along the metallic wire, L, and back again to London, where it will pass through the instrument, 1, and so return to the end, z, of the battery.

When it is impossible to insulate the conducting wire by supporting it in the air on posts, the whole length of the wire requires to be covered with an insulating material.* Caoutchouc and gutta-percha are found to be well adapted to this purpose. In this case, it is usual to substitute copper wires for the iron ones, as owing to the superior conductivity of copper, a wire of much smaller diameter can be employed without adding to the resistance, and a saving of space and of insulating material is thus effected, as well as a reduction in the inductive action, to which allusion will be made almost immediately. The wires, after having been covered with a coating of gutta-percha about 3mm. or of an inch thick, may be enclosed either singly, or several of them side by side, in iron tubing, to protect them from mechanical injury: they are then placed under ground, in the same manner as pipes for the conveyance of gas or water. In the submarine telegraphs, copper wires coated with gutta-percha are carefully enveloped in tarred hemp, so as to form a compound rope, which contains several strands of conducting wire; the whole is protected by enclosing it in a flexible metallic covering, formed by carefully twisting several iron wires around the compound conducting rope already described: the exterior is often further protected by an outer covering of tarred hemp or other analogous material. The cable having been previously coiled up in the hold of a vessel, and one of its extremities having been properly secured upon the shore, is carefully lowered into the sea; from its weight, the electric rope at once sinks to the bottom as it is gradually paid out over the ship's side. When the

*The insulating power of different materials is very differently affected by temperature. It is always highest when the temperature is lowest. The insulating power of caoutchouc is diminished but slightly by a rise of temperature from 32° to 92° (33°3 C.), whilst in gutta-percha the insulation is reduced more than half between the same points of temperature. It must be remembered that the effect of heat upon metallic conductors is exactly the reverse, a rise in temperature of copper from 32° to 92°, being attended with a diminution in its conductivity of nearly 10 per cent.