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either in the same direction as the hands of a watch, or in the opposite direction, then the two will attract one another; but if the one goes in the one direction, and the other in the other, they will then repel one another.

Attraction and Repulsion of Magnets.

101. Ampère, who discovered this property of currents, has likewise shown us that in very many respects a magnet may be likened to a collection of circular currents all parallel to one another, their direction being such that, if you look towards the north pole of a freely suspended cylindrical magnet facing it, the positive current will descend on the east or left-hand side, and ascend on the west or right-hand side. If we adopt this method of viewing magnets, we can easily account for the attraction between the unlike and the repulsion between the like poles of a magnet, for when unlike poles are placed near each other, the circular currents which face each other are then all going in the same direction, and the two will, therefore, attract one another, but if like poles are placed in this position, the currents that face each other are going in opposite directions, and the poles will, therefore, repel one another.

Induction of Currents.

102. Before closing this short sketch of electrical phenomena, we must allude to the inductive effect of

currents upon each other. Let us suppose (Fig. 10) that

we have two circular

[graphic]

coils of wire, covered

with thread, and placed near each other. Let both the extremities of the right-hand coil be connected with the poles of a battery, so as to make a current of electricity circulate round the coil. On the other hand, let the left-hand coil be connected with

a

galvanometer, thus enabling us to detect the smallest current of electricity which may pass through this coil. Now, it is found that when we first connect the right-hand coil, so as to pass the battery current through it, a momentary current will pass through the lefthand coil, and will deflect the needle of the

galvanometer, but this current will go in an opposite direction to that which circulates round the right-hand coil.

103. Again, as long as the current continues to flow through the right-hand coil there will be no current through the other, but at the moment of breaking the contact between the right-hand coil and the battery there will again be a momentary current in the left-hand coil, but this time in the same direction as that of the righthand coil, instead of being, as before, in the opposite direction. In other words, when contact is made in the right-hand coil, there is a momentary current in the lefthand coil, but in an opposite direction to that in the right, while, when contact is broken in the right-hand coil, there is a momentary current in the left-hand coil in the same direction as that in the right.

104. In order to exemplify this induction of currents, it is not even necessary to make and break the current in the right-hand coil, for we may keep it constantly going and arrange so as to make the right-hand coil (always retaining its connection with the battery) alternately approach and recede from the other; when it approaches the other, the effect produced will be the same as when the contact was made in the above experiment that is to say, we shall have an induced current in an opposite direction to that of the primary, while, when it recedes from the other, we shall have a current in the same direction as that of the primary.

105. Thus we see that whether we keep both coils stationary, and suddenly produce a current in the righthand coil, or whether, keeping this current constantly going, we suddenly bring it near the other coil, the inductive effect will be precisely the same, for in both cases the left-hand coil is suddenly brought into the presence of a current. And again, it is the same, whether we suddenly break the right-hand current, or suddenly remove it from the left-hand coil, for in both cases this coil is virtually removed from the presence of a current.

List of Energies.

106. We are now in a position to enumerate the various kinds of energy which occur in nature; but, before doing so, we must warn our readers that this enumeration has nothing absolute or completé about it, representing, as it does, not so much the present state of our knowledge as of our want of knowledge, or rather profound ignorance, of the ultimate constitution of matter. It is, in truth, only a convenient classification, and nothing more.

107. To begin, then, with visible energy. We have first of all

Energy of Visible Motion.

(A.) Visible energy of actual motion-in the planets, in meteors, in the cannon ball, in the storm, in the running stream, and in other instances of

bodies in actual visible motion, too numerous to be mentioned.

Visible Energy of Position.

(B.) We have also visible energy of position—in a stone on the top of a cliff, in a head of water, in a rain cloud, in a cross-bow bent, in a clock or watch wound up, and in various other instances.

108. Then we have, besides, several cases in which there is an alternation between (A) and (B).

A pendulum, for instance, when at its lowest point, has only the energy (A), or that of actual motion, in virtue of which it ascends a certain distance against the force of gravity. When, however, it has completed its ascent, its energy is then of the variety (B), being due to position, and not to actual motion; and so on it continues to oscillate, alternately changing the nature of its energy from (A) to (B), and from (B) back again to (A).

109. A vibrating body is another instance of this alternation. Each particle of such a body may be compared to an exceedingly small pendulum oscillating backwards and forwards, only very much quicker than an ordinary pendulum; and just as the ordinary pendulum in passing its point of rest has its energy all of one kind, while in passing its upper point it has it all of another, so when a vibrating particle is passing its point of rest, its energy is all of the variety (A), and when it has reached its extreme displacement, it is all of the variety (B).

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