ROTATION OF ELECTRIC CURRENTS.

261

magnetic meridian, and allow the electric current to traverse the wire coiled round it, in such a direction, that the poles of the temporary magnet may be such as will be repelled by the hemisphere of the globe, to which they are opposite.

381. It has been proved that a conducting wire and a magnet, by their mutual reaction, tend to arrange themselves in a direction at right angles to each other (356, 363), and that if the action of the current, or what comes to the same thing, of the wire conveying it, be limited to one pole only of a magnet at a time, they will tend to rotate round each other in a given and constant direction (366, 367). Wires conveying currents, it has been shown, also possess the properties of mutual attraction, or repulsion, according to the direction of the electric fluid (365), and of being acted upon by the magnetism of the earth, or of a steel permanent magnet, arranging themselves in a constant direction, with regard to the poles of either (371-5). Ampere has extended these facts still further, by showing that two electric currents properly arranged will ever tend to rotate around one another, providing their direction be at right angles to each other. Thus, if a fixed current of electricity be supposed to be moving from A to B, as in fig. 1, p. 262, and a moveable current be supposed to be placed as in CD, whilst the positive fluid in AB and CD is moving in the direction shown by the arrows, attraction will take place between the currents EB and CD, in the angle CEB; for if CD be inclined towards EB, the currents in each will be moving in the same position (365). Repulsion will be exerted in the angle AEC, between CD and AE; for if CD be supposed to be inclined towards AE, the currents in each will move in opposite directions. If then the current AEB be circular, the moveable current CD will tend to revolve round it.

382. This may be proved by surrounding the circular copper trough vv, fig. 2, with some thick insulated copper wire, connected with the binding screws zc. The metallic support s

Fig. 1.

D
E

Fig. 2.

v

is connected, by a wire, with the screw or cup c, and the trough v itself with the screw or cup z. A light wire frame, ABD, furnished with a hoop or circle of thin copper, is provided with a pivot at B, by which it may rest with as little friction as possible on the support s. Fill with dilute sulphuric acid, place ABD on s, so that its hoop may just dip in the dilute acid in v, connect cz and cz to the copper and zinc plates of an electromotor, respectively. Under these circumstances, currents of positive electricity will traverse the wire wound round the trough v, and along the frame ABD, in the direction pointed out by the arrows; and the horizontal circular current in the wire acting on the descending vertical currents in ABD, will cause the latter to revolve in a direction varying with the course of the current in the wire surrounding the vessel v.

383. From the phenomena detailed in this chapter, a highly ingenious theory of magnetism has been proposed by Ampere, differing altogether from the conventional hypothesis already explained (212), in denying the existence of any magnetic matter as distinct from electricity, and considering that all magnetic phenomena are but the visible effects of invisible electric currents, permeating the iron bars or other substances in which they exist. According to this

MAGNETIC HYPOTHESIS OF AMPERE.

236

theory, every molecule of a magnet must be regarded as being surrounded by a current of electricity, constantly and perpetually circulating around it; and that the only difference existing between a magnet and a mere bar of iron, is simply, that in the latter, the electricity present is in a latent and quiescent state; whereas, in the former, it is in a state of rapid rotation around each ultimate atom or particle of iron. All the effects produced by these elementary currents, may be theoretically represented by a set of resultant currents surrounding the mass, as shown in the following figure. The end N of such a bar will be the austral pole, and point

towards the northern hemisphere of the globe, because there the currents of positive electricity represented by the arrows, are moving in a direction from right to left, or opposed to those of the hands of a watch (372). The opposite end will, consequently, be the boreal pole (214); for, on looking at the face s, as shown at s', the currents will appear to be moving from left to right; for the same reason that a word is seen backwards, on looking at it through the paper on which it is written, by holding the latter between the eye and the light.

384. The attraction between the magnetic poles of different names, and repulsion between those of the same kind (209), is on this theory explained, by supposing that, in the former case, the elementary currents are moving in the same, and, in the latter, in different, directions (365). The rotation of a conducting wire round a magnet (367) becomes also reduced to the simple case of the rotation of a vertical round an horizontal current (382); for all magnets, it must be recollected, are, on this hypothesis, supposed to have myriads of

currents traversing them, in a direction at right angles to a line connecting their poles.

On this theory, also, the magnetism of the earth is explained, by supposing the existence of currents of electricity constantly traversing it in a direction, of the positive, from east to west, and of the negative fluid, from west to east. It must be confessed that, opposed as this view is to the generally received theories, it has received much support from the recent discoveries in electro-magnetic induction (397, 402).

CHAPTER XVII.

ELECTRO-DYNAMIC INDUCTION.

General Statement, 385. Secondary Currents induced, by Electricity, 386 -by Magnets, 387—by Electro-magnets, 388—in the same Conductor with the Primary Current, 389-Calorific effects of, 390. Shock from Secondary Currents, 391. Currents excited by Revolving Disc, 392. Electro-magnetic Machines, 393—without Iron, 394-5 -with a Permanent Magnet, 396-8-with an Electro-magnet, 400-1. Theory of Ampere, 402.

385. Of all the numerous and successful researches made by Faraday, in the different departments of electrical science, none are of greater importance, or more worthy of deep attention and study, than the discovery of electro-dynamic induction, which was made by that philosopher in 1831. As a brief generalization of this discovery, it may be stated that, whenever an electric current traverses a wire, it excites a current in an opposite direction in a second wire held parallel to it; and, on suddenly stopping the primary current, the induced one reappears, but in an opposite direction to that which it first followed. Whenever, also, a magnet is moved before a conducting wire in any manner, but especially when the long axes of both magnet and wire are at right angles to each other, similar electric currents are excited or induced in the wire. These induced or secondary currents are but of