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the cells of a voltaic battery, the united current is equal to the sum of the currents from each brick. Dr. Brard has also constructed an electro-generative torch or candle by making a brick of coal dust and treacle, and dipping it in melted nitrate. The power of yielding heat as well, is, of course, the peculiar merit of this electro-generative fuel.

The electric current produces many curious effects, nearly all of which have been usefully applied; but we shall only enumerate the most important of these. It decomposes chemicals, and hence has given rise to the electro-plating industry and many other processes, such as the storage of electricity, the rectification of alcohol, the manufacture of aniline colours, and the reduction of watery tumours in the art of medicine. In overcoming the resistance of a wire, or, better still, some semi-conductor, such as carbon, it produces light and heat, hence we have the electric light and the redhot platinum wire used in the actual cautery. When it is caused to flow through a wire coiled round a pivoted magnetic needle it causes the needle to move, and if a bar of soft iron is put within the coil in lieu of the needle, that bar will become a magnet. From the first of these two important effects we have the needle telegraph of Cooke and Wheatstone and the reflecting galvanometer for measuring currents, while from the second we have an infinite variety of appliances, such as the printing telegraph, the electric bell, the telephone, and the magneto-electric motor.

CHAPTER III.

INDUCTION.

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WE come now to the principle known as electrical induction, a principle which is of the highest importance, and on which all mechanical generators of electricity are based. The name is derived from the fact that an electrified body induces a charge of electricity in a nonelectrified body brought within its influence. For example, let P, Fig. 11, be a body charged with positive electricity, and let it be brought near a nonelectrified body n p which, for the sake of demonstration, is usually a brass cylinder mounted on an insulating stem of glass, and having a pair of pith balls hung from its two ends. As the cylinder approaches P the pith balls will be seen to diverge, thereby showing that the ends of the cylinder have become electrified; and it will be found that the balls at n indicate a negative charge at that end, and the balls at p indicate a positive charge at the other. In short, the positive electricity on P has apparently separated the two fluids, which before were neutral in the cylinder, and attracted the negative or opposite fluid to the nearest point, n, while it has

FIG. 11.

repelled the positive or similar fluid to the farthest point, p. This is what we should expect from the law that unlike electricities attract and like electricities repel each other.

It would seem, however, that there is here an "action at a distance," were it not for the presence of the air between the two bodies; and Faraday has shown that every molecule of air between the two bodies is acted on in the same way as the cylinder itself, and has become "polarised" like it--that is to say, has its side nearest P showing a negative charge, and its side nearest the cylinder a positive charge, according to the imaginary Fig. 12, where P and N are the two bodies,

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and the intervening circles are supposed to be the air molecules. Should the mutual attraction between P and N become too great for the air molecules to bear, the molecular bridge between them will break down under the stress, and the electricities will rush together through the air with a cracking sound and flash of light. This is the action which equally takes place when a spark is drawn from the prime conductor of an electric machine, or when a flash of lightning passes between an electrified cloud moving over the surface of the earth and inducing opposite electricity on the fields, trees, and steeples below. As electricity tends to discharge from points, it is generally through some prominent object in the landscape that the lightning discharge takes place, and hence the necessity of having all high buildings protected.

We have said that this principle of induction is utilised in the construction of machines for generating electricity-notably in the Holtz frictional machine. And although we need not enter into the details of that complex apparatus, it will be requisite to explain how the principle is applied. Returning, then, to Fig. 11, it will readily be understood that if the charged body, P, be withdrawn from the cylinder, the two separated electricities will at once recombine, and the transient separation will exist no more. But if, while p is near the cylinder, we touch the remote end p, and thus take away the positive charge thereon, then on withdrawing our finger again we shall still leave the negative charge at the end n; and on removing P, this negative charge, having no longer an equal positive charge to combine with, will remain upon the cylinder as a free and permanent charge. In this way, then, can induction be made to generate new charges of electricity.

Not only, however, does a body with a fixed charge of electricity induce an opposite charge in another body standing near it, but a current of electricity flowing in a wire induces an opposite current in another wire close by. This is the greatest discovery of the immortal Faraday, and upon it are founded all the "induction coils" used by medical men for giving shocks to patients, and the modern dynamo-electric machines for generating the currents to feed electric lights and drive electric motors. In the year 1831 Faraday found that whenever an electric current is suddenly sent along a wire w, Fig. 13, as shown by the arrow, it instantly excites an opposite current in a second wire, w' held parallel to the first. This induced current,

however, is only momentary, and is evidently due to the first passage of the primary current through its wire. Though the primary current is kept flowing, the induced, or secondary current disappears; but when

W

w'

FIG. 13.

the primary current is suddenly stopped the induced one reappears, but flowing in a direction contrary to that which it followed before. In fact, the disturbance caused by starting and stopping the current in the primary wire has the effect of setting up a contrary disturbance in the neighbouring wire.

The apparatus known as the "Induction Coil," which is useful for giving a constant stream of sparks, is based upon this discovery. Its action will be readily understood from the form shown in Fig. 14, which we have chosen because of its simplicity. There B is a coil wound of a short length of stout copper wire insulated with silk and varnish; and B' is a coil wound of a long length of fine copper wire similarly insulated. These two coils are shown apart, but в' can be slid over в so as to bring one within the other. When the current from a voltaic battery is started in the coil в a momentary current is induced in B'; flowing in a reverse direction to the current in B; and when the current in в is stopped, another momentary current is induced in B' flowing in a contrary direction to the current induced in it when the current in B was started. The coil в is called the "primary," or induc

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