Electric Telegraph. In the description of the electric telegraph, we will lay aside all technical and scientific terms, and explain clearly tin's greatest wonder of our day. The source of the electricity used requires our first attention This is what is called a voltaie, or galvanic battery ; and it is so called from Volta and Galvani, its originators. We can make a very simple battery by means of two tumblers, a little Bait and water, two small pieces of zinc, and two of copper, united in the following manner: - A and B are the tumblers, c c the pieces of copper, z z the pieces of zinc ; the tumblers being partly filled with the salt and water, the battery is complete. It may be observed that the metals used are dissimilar ; that a plate of copper and one of zinc unite at e, and that there are wires fixed to the other two plates, which as yet are in no way connected. Whilst things are in this state, nothing will take place ; the battery is at rest, and no electricity is evolved by it; but if we join the two wires, a current of electricity will immediately pass, and this current will continue till we again separate the wires. If two plates of metal are placed in a solution which will only dissolve one of them, and their upper edges are brought into contact, whilst the others are kept apart, a current will pass from one to the other through the solution, and, passing also from one to the other at the point of contact, will continue thus circulating, till either the soluble matter is consumed, or the liquid itself is saturated - that is, has dissolved as much metal as it is capable of dissolving. This is always the case; but often the effect is so slight, that it is rarely perceptible. Take a piece of silver, and a piece of zinc the size of a half-crown ; place one upon the tip of the tongue, the other under it; bring their edges into contact, and what is called a shock will be perceptible; that is, the saliva acting upon the zinc and not upon the silver, a small battery is made, and the electricity passes from the zinc through the tongue to the silver, thence to the zinc again, and thus circulates till you part the edges of the metals. The shock is very slight, being chiefly known oy an acid taste; nor would it be felt at all, but that the tongue is so acutely sensitive. We have called this a small battery, but it is scarcely a correct term ; it is a single voltaic pair - a battery, in its proper sense, being made up by a union of two or more such pairs, as in the case of the one above. In practice, a battery consists of twelve or more such pairs; and the following sketch represents one commonly used in working the electric telegraph:
It contains twenty-four pairs of zinc and copper plates, about four inches square. Each pair are soldered together by means of a strap of metal, as in Fig. 3. To make it quite clear, we have drawn but twelve pairs in section, and lettered the alternate plates, z standing for zinc, and c for copper. The trough in which they are placed is either made of baked wood, glass, earthe ware, or gutta-percha; the only requisite being that it is a non-conductor of electricity - that is, such a substance as electricity will not readily pass through. The last plate at one end is zinc, and the other copper, and a wire is soldered to each. If these wires are joined, a current of electricity will pass from the zinc plate to which one is attached, through the exciting liquid, which is here sulphuric acid and water, to the copper plate in the same cell, thence by the metal strap to the zinc of the next cell, and thus through the whole series to the other single plate, whence it passes by the wires to the first zinc plate again. But as each pair of plates produce similar currents, their combined power is very great, and a large quantity of electricity passes through the terminal wires. So much for the battery. We will now go a step further, and learn an extraordinary effect which it is capable of producing. You know what a magnet is, and many of you have, we dare say, seen a mariner's compass - if not, we must briefly toll you what it is. It consists of a flat piece of steel, of this shape, which is called aneedle needle.This is suspended on a point, by means of a small hole, or rather conicul indention made in its centre. This needle being magnetised, and thus suspended, will always point in the same direction, one end being directed to the north, the other to the south; and it thus enables the sailor to go in any direction he may desire. For if he knows where these points are, he can tell the east and west; and if, as is always done, a card is placed below the needle, with the intermediate points carefully marked upon it, he has no difficulty in steering exactly to any place of which he knows the position ; and thus the compass 19 to the sailor on the pathless deep just what the direction post is to the traveller, Mow, if we take such a needle as we have described, and suspend it vertically on an axis passing through its centre, and then, by means of a wire, pass a current from our battery round it thus, the needle will take up a new position at right angles to that which it main-tained before, indicated by the dotted lines. Whether the upper point, or north pole, of the needle moves to the right or left in order to attain this position depends on the direction of the current. Thus we have arrived at the principle of an electric telegraph. We have but to agree upon a set of signals that the deflection of the needle shall signify ; and if we can contrive to send the current in the direction we wish, so as to move the north pole of the needle to the right or left at will, the apparatus will be complete. But, in practice, it is necessary that we should be able to move, as we please, a similar needle to our own, at the station to which we desire to send the message. In order to accomplish this, we have but to conduct the current from one station to the other by means of an insulated wire. This will be easily understood by the following diagram, where the battery is represented at A. and the different stations at 13, at each of which the needles have their north poles upwards; and the wire conveying the current passes in the same direction round all, and lastly returns to the other pole of the battery; thus the current, leaving the battery by the copper or positive end, c, will traverse the wire in the direction of the arrows, deflect the needles in the same direction at the different stations, and return to the zinc or negative end of the bat:ery by the same wire.