electrified. By the terms positive and negative is implied, that in one case the substance electrified contains more, and in the other less, than its ordinary proportions.

Many common substances used by us in the common affairs of life are susceptible of electrical excitation, and we often produce electrical phenomena without being conscious of it. We may give an example or two. In cleaning glass mirrors with an old silk handkerchief, or a very dry linen duster, it generally happens that small fibres and particles of dust accumulate on their surfaces, the more rapidly in proportion to the labour bestowed in removing them. The same thing occurs in wiping decanters and other articles of glass, and especially the glass chimneys used on gas-burners. In all these cases electrical excitement is produced by friction, and the fibres disengaged from the duster, as well as the dust floating in the surrounding atmosphere, are attracted by the glass, and adhere to it, as already shewn with the glass tube and feather.

Silks of all kinds are highly electric, as are also most of the precious stones, and a great variety of resinous substances, the paste of which false gems are made, the hair and fur of animals, paper, sulphur, and some other minerals, India rubber (caoutchouc), and certain descriptions of wood, when thoroughly dried by baking.

Among domesticated animals the cat furnishes a remarkable instance of electrical excitability. When dry and warm the back of almost any full-grown cat (the darker its colour the better) can be excited by rubbing it with the hand in the direction of the hair, a process which is accompanied by a slight snapping sound, and in the dark by flashes of pale blue light.

The substances, which were just now mentioned as highly electric, must be understood as being intended merely as specimens. All subjects, without exception, are undoubtedly capable of being electrically excited; but some require more complicated arrangements than others. Those which allow the electric fluid (as it is called) to pass over them most easily are called conductors.

All metals are good conductors, and so you will see often on the top of a church steeple a lightning conductor, of metal wire, to attract the electric fluid, and conduct it down to the ground without any damage to the building. Other substances hold it fast, and do not let it pass freely. These are called non-conductors, or insulators. All resinous substances are non-conductors. This is why, if you rub a brass rod on your sleeve as you did the sealing-wax, you will not find it attract or repel the feather. No electric fluid remains on its surface, because it has rapidly been conducted along the rod to your arm, and through your body down into the ground.

THE ELECTRIC TELEGRAPH.

THE most useful practical purpose to which electricity is applied is that of sending messages by telegraph, or, as they are called, telegrams. You have seen those wires stretched from post to post along the sides of a railway, and you know that telegrams are sent by them, so that almost as soon as they are sent from London they are received in America. But you must not think that a letter is sent along this wire, as it is through the post, and that the same paper you give in at a telegraph office is delivered to the person to whom you sent your message; and you must not imagine that the clerk who receives it from you really sends anything. He reads your message, and shakes a wire in a particular way, and the clerk at the other end of the wire knows that one particular shake means, we will say, the letter A, and another means B, and so on.

Now, let us see what causes this shake or vibration, and how people can tell one shake from another. If ever you have looked about you, either in a railway station or a post office, you may have seen a thing standing on a shelf, and looking very like a small American clock, with two faces and a hand to each, and two little handles below them, or, as is now more common, only one face with its hand and one handle.

C

There is a kind of electricity produced by metals in contact with chemical liquid, which was discovered by an Italian, called Galvani, from whom it gets its name, Galvanism. I daresay some of you have been galvanized by taking hold of two brass handles fastened to two wires, and have felt the shaking vibration making all the nerves of your arms quiver. This is caused by putting a plate of zinc, B, and another of copper, c, in a vessel containing sulphuric acid and water. They must not touch one another in the liquid, but, if connected at the edges, which are dry, by a piece of wire, a current of electricity runs constantly through the liquid from one plate to the other, and comes back again along the wire, A. It does not at all matter what the distance is between the plates, as by lengthening the wire the current

Fig. 1.

still passes through any distance, and it travels at the rate of 288,000 miles in a second of time.

But as a

single pair of plates

would not afford a

sufficiently strong current for the transmission ofmessages to a great distance, the former can be increased to any extent by multiplying

Fig. 2.

the number of pairs of plates. For this purpose a long trough is made, in which these pairs are placed side by side (fig. 2). This is called a galvanic battery. A wire is attached to one end of the battery and continued to a distant station, being supported by high posts along the side of the railway, say, from London to Edinburgh, where its other end is attached to a similar battery. It has been found that, if a wire from the other end of each

of these batteries be attached to a metallic plate sunk in the ground, the electric current is conveyed through the

Battery in London.

Edinburgh.

Fig. 3.

earth to any distance, so completing the circuit. Thus the electricity travels along the wire from London to Edinburgh, and returns, through the earth, from the latter to the former, as shewn in fig. 3.

You know that a magnet hung on a pivot will turn always towards the north, because of a constant natural current of electricity going round the earth from east to west. You can see this in a mariner's compass, which is a magnet needle, or by rubbing a needle with load-stone, and then floating it on a tumbler of water. About the year 1819 it was discovered that if such a needle were to be hung over a wire along which a current of electricity could be made to run, this needle would be made, by the electric current, to turn across the electric wire more or less at right angles to it, moving to the right hand or left, according to the direction in which the electricity was sent backwards or forwards along the wire. B This has been applied to form electric telegraph machines. Here is a coil of wire, covered with silk so as to prevent the coils from touching In the centre a steel needle, A, is hung on an axle which is lengthened at one end so as to hold a similar needle outside, B. The lower ends of these two needles are made rather heavier than Fig. 4. the upper ends, so as to make them swing back again to the perpendicular position, when they have been moved by the current. As soon as the wire

from the copper plate is connected with the end of the coil of wire to the right hand, and the zinc plate with the end towards my left hand, the current speeds like lightning along the coil from right to left, and makes the needle move towards the left. On reversing the communication by connecting the right end of the coil with the zinc and the left with the copper plate, the needle turns towards the right. It is evident that the needle, A, moving inside will also turn the outer needle, B, in the same direction, but B is only the pointer to shew how the inner one, A, is moving.

A

Now, if we look inside the clock-like machine, which I mentioned as being often seen in a railway station, we shall find one of these coils behind the clock-face, and the finger we see is the pointer; below we shall find a pair of wires, one descending to the battery and the other running on to the terminus at the distant point to which messages are to be sent. The outer finger or pointer, B, is that which is seen on Fig. 5. the clock-face, and marks the direction in which the inner finger, A, turns.

When the finger, A, in the machines we are working, is made by the current to turn towards the right, as in fig. 5, the current is continued along the whole of that

wire, however far it extends, and sơ turns all the fingers on every machine in the line in the same direction and at the same instant. Thus, if we have a number of coils placed at different towns along the line, as London, Peterborough, York, Edinburgh, represented by 1, 2, 3, 4 (fig. 6), and the left hand wire of the coil (1) in London is attached to the copper plate, and the wire of its right