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57. Electricity of Two Kinds.-These attractions and repulsions are conveniently shown by suspending a pith

Fig. 10.

ball by a silk thread, as in the figure. The ball is drawn towards the excited wax, and after touching it, is repelled from it beyond the perpendicular. A glass rod rubbed with a piece of silk produces the very same movements in the pith ball. But there is a difference; for if, while the pith ball is standing aloof from the excited wax, the excited glass rod is brought near it, it rushes to the glass. No sooner, however, has it touched, than it is repelled from the glass and flies to the wax; and thus continues to oscillate between the two so long as the excitement lasts. It thus appears that there are two different kinds of electrical excitement, and that bodies having the same kind repel one another, while those having opposite kinds attract one another. The excitement of a glass rod rubbed with silk used to be called vitreous electricity, and that of sealing-wax rubbed with flannel, resinous; the names now preferred are positive for the former, and negative for the latter.

58. One Kind always accompanied by its Opposite.— A fundamental fact in electricity is, that one of the kinds can never be produced by itself, but is always attended by an equal amount of the opposite kind. Thus when positive (+) electricity is excited in glass. by rubbing it with silk, the silk is found equally charged with negative (-) electricity.

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59. Hypothesis of Two Fluids.-A convenient way of conceiving and speaking of electrical action is the hypothesis that the two principles, or fluids,' as they have been called, exist in all bodies in a state of union; and in this condition they give no indication of their presence the body is neutral. But when the two fluids are separated by friction or otherwise, they immediately seek to be reunited, and the bodies in which the two opposite principles inhere are drawn towards each other. This hypothesis is used merely for convenience, and does not pretend to be an explanation of the real nature of electricity, which, as yet, is uncertain (see par. 91).

60. Whence comes the Attractive Energy ?-When two bodies with opposite electricities are drawn towards each other, we have an instance of work being done. But in the instances above described, the energy that does the work is purchased, as it were, by the exertion of rubbing; and in all cases electrical excitement costs mechanical movement or the action of chemical affinity.

We

61. Conductors and Non-conductors.-If, instead of glass or sealing-wax, we rub a metal rod, holding it in the hand, we fail to get any signs of electricity. are not to conclude from this that none has been produced; for, if the metal be held by a glass handle, it may be readily excited.

The cause of the difference is, that in the case of the sealing-wax or the glass, the electricity is held, so to speak, to the spot where it is excited; in the metal, it instantly spreads over the whole surface, and escapes through the hand to the earth. Bodies that thus allow a free passage to electricity are called conductors. But there is no hard and fast line between conductors and non-conductors. The best conductors offer more or less obstruction to the passage of

it.

electricity, and there is no substance that altogether bars The best conductors are the metals, charcoal, salt solutions, water. The chief non-conductors are dry air, shell-lac (sealing-wax), gutta-percha, india-rubber, glass, silk. A body surrounded on all sides by non-conductors has its electricity inclosed as on an island, and is said to be insulated.

62. Induction. The leading facts of induction may be very simply illustrated thus: A brass cylinder with rounded ends, and fixed on a glass support, has pairs of

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piths hung at each end (fig. 11). If, now, an insulated brass ball be charged positively, and brought near one end of the cylinder, both pairs of piths will instantly diverge, and as quickly collapse when it is taken away. It is easily shown that the two ends of the cylinder are in opposite electrical states, and separated by a neutral line of no electricity. Next the positive ball there is negative electricity induced. And so it is always. A charged body induces an opposite kind of electricity on the side of another next it, and the same kind on the farther side.

Fig. 11.

If, while the cylinder is near the positively charged ball, we touch it for a moment with the finger, its positive electricity passes off through our body into the earth, but the negative remains, being held bound to the positive charge of the ball. Let the ball now be removed, and the cylinder remains charged with negative electricity alone.

63. Instead of letting the induced positive electricity of the cylinder escape to the earth, let us now bring the ball gradually nearer and nearer to the cylinder; the

tendency of the negative electricity of the cylinder to unite with the positive on the ball will become stronger and stronger, until at last it is sufficient to burst through the interval of air. The two flash together with a spark and neutralise each other, and the cylinder is left charged with the positive electricity that was on the other end.

Let the student now try to explain the movements of the pith ball (par. 57) by this theory of induction.

64. Condensed Electricity.-By means of special apparatus (the Leyden Jar'), two conducting surfaces may be charged with opposite electricities highly condensed or in a state of great tension; and then, when a conducting communication is made between them, the two principles rush together with great violence. If there is a gap in the communication, a spark passes through the intervening air; and if there is a bad conductor in such a gap, it may be shivered in pieces. A fine metal wire will be made red hot, or even melted, by having a strong charge passed through it; and if the human body is made part of the circuit, a peculiar shock is felt. The velocity of such a current is at least equal to that of light, or about 200,000 miles in a second. We have here the electrical excitement converted into mechanical energy, and into light and heat.

But it is in the thunder-storm that this energy of nature is seen on the greatest scale. For, as has been long known, the spark of the electric machine is, on a small scale, the very same thing as the lightning of the clouds (see par. 276).

CURRENT ELECTRICITY.

65. The Galvanic Current.-If a plate of copper, C (fig. 12), and a plate of pure zinc, Z, are put into a vessel of water, mixed with a little sulphuric acid,

and they be kept from touching, no chemical action But the moment they touch at any part,

goes on.

Fig. 12.

either within or out of the liquid, bubbles of gas appear at the copper plate, and they continue to form till we separate them again. We have the same result if we connect the plates, not directly, but by wires. fastened to the plates. All that is visible is the production of the gas bubbles; but a hidden and most curious electric action is going on along the wire, and indeed, through the whole

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arrangement. For if we bring the connecting wire near a magnetic needle, the latter instantly recognises its presence and turns aside from its north and south posi

tion. Or if we break the wire, and put the two ends on the tongue, we perceive a saltish taste, which continues until we lift one of the plates out of the acid water.

On breaking the connecting wire, we may find by a delicate electroscope that the two ends have feeble charges of opposite electricities, the wire leading from the copper being positive. There the electricities remain shut up, and the chemical action suspended, till we bring the ends together. Then the opposite electricities combine and neutralise each other; and the whole, plates and wires, become discharged or reduced to electric rest. But only for the instant; the action of the

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