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366. The action exerted by a conducting wire on a magnet (356), is obviously not a direct attractive or repulsive one; but is rather a tangential force, by which the opposite poles of the magnet tend to rotate round the conducting wire in different directions, and assume a state of equilibrium when the opposing actions of the wire on both poles become equally balanced. Reasoning on this fact, Faraday concluded, that if the action of the current could be confined to one pole only of the needle, perpetual rotation, providing no opposing forces interfered, might be produced. After a series of experiments on this subject, he succeeded perfectly, and thus developed one of the most interesting and extraordinary phenomena in electrical science.

The most convenient apparatus for illustrating this rotation of magnets round a conducting wire, consists of two or three slender magnets, NS, NS, fixed equidistant from each other, with their poles in the same position, in the piece of wood A, supported by a pointed wire, so as to move readily on its

S

centre. The middle of the piece of wood A, is excavated and contains a few drops of mercury, communicating by means of a curved wire with the external circular trough of mercury E. A pointed copper wire, supported by a screw at c, dips into the mercury in A; and is furnished with a cup containing mercury, so as to be readily connected with an electromotor, by means of the inversor (360). The cup c and trough E are then connected, the former with the copper, the

latter with the zinc plate of the electro

motor. So that the positive current descends to a, and then reaching E through the curved wire, escapes to z. It thus acts only on the poles NN of the magnets, which if austral poles (214), will immediately begin to rotate round the

conducting wire cc, from left to right, or in a direction like that of the hands of a watch. By turning the bars of the inversor, or otherwise changing the direction of the current, the direction of the rotation will immediately become altered. The same thing also occurs, when the position of the poles of the magnets are reversed. Let the magnets or currents be arranged as they may, the direction of the rotation always corresponds to the formula of Ampere (358). It may here be remarked, that in this as in all other experiments in electromagnetism where wires dip into mercury, their ends should be cleaned and amalgamated, by being dipped into a solution of nitrate of mercury, to ensure perfect contact.

367. In accordance with the law of equality of action and reaction (45), if the magnets be fixed and the conducting

E

wires moveable, the latter will readily rotate round the former. This may be very easily shown by means of the horse-shoe magnets N s, placed in a vertical position with circular troughs, AB, screwed upon its legs, a light wire frame supported by a fine steel point on each pole of the magnet, is so arranged that its vertical branches just touch the surface of the mercury in a B. Each of the wire frames terminates in a cup containing a drop of mercury, into which the ends of the cross wires from E dip. Connect the cup of mercury E, by means of a wire, with the copper plate of the

electromotor (322), either directly or by means of the inversor (360), and let the wires cz, coming from the circular

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troughs AB, be both connected with the zinc plate of the apparatus. Under these circumstances a current of positive electricity will pass from the copper plate to the cup E, and there being divided into two portions will descend the vertical branches of the wire frame, and reach the troughs AB, leaving the apparatus by the wires cz. Directly the current is in motion, the wire frame suspended on the north pole of the magnet begins to rotate rapidly in a direction from left to right, and that round the south pole, in a contrary direction, from the reaction of the fixed magnet on the moveable conducting wires. If the direction of the current be reversed, either by altering the connexions with the electromotor, or by shifting the bars of the inversor, the direction of the rotation will also become reversed.

368. The rotation of a conducting wire may be also conveniently shown, by bending a wire into an heliacal coil like a corkscrew, and allowing it to rest by one extremity on the depression on each pole of the horse-shoe magnet (367), the other end dipping into the mercury in the circular troughs AB. On connecting one of these with the zinc, and the other with the copper plate of the electromotor, the current will ascend through one helix, descend the pole of the magnet which supports it up the other pole, and reaching the second helix. will descend along it; and thus by the mercurial trough into which it dips, reach the zinc plate of the exciting appaIn this variation of the experiment, the heliacal coils of wire will rotate round either pole in the same direction, because whilst the positive current ascends in one, it descends in the other.

ratus.

369. If instead of submitting a conducting wire to the action of one magnetic pole only (367), it be so arranged as to be exposed to the influence of both poles, a vibrating, instead of a rotatory, motion ensues. Let a light wire, w, be suspended from a brass rod connected with the cup mercury, c, so that its lower end just dips into a cavity cut out in the base of the instrument, filled with mercury,

of

and connected by a wire with the cup z. Let a horse-shoe magnet be placed, as shown in the figure, and connect c with the copper, and z with the zinc plate of an electromotor (322); the current of positive electricity will descend w, and being acted upon by both poles of the magnet, the wire will tend to rotate to the right, round the austral pole, n, and to the left, round the boreal pole, s. As it cannot obey both these forces at once, being opposed in direction, it takes an intermediate course, as would be expected, from the law of composition of forces (47), and is thrown forwards out of the mercury, in the direction indicated by the arrow. Connexion being thus broken with the battery, the wire, by its gravity, falls into the mercury, and is again thrown out, keeping up this pendulum-like motion for a long time. Let the direction of the positive current be changed, or reverse the position of the magnet, and a vibrating motion of the wire, in an opposite direction, or backwards, will ensue.

370. If the electric current be made to pass through a spur wheel, w, instead of a wire, a rotatory movement between the poles of the magnet ensues. Thus, if the posi

MAGNETIC PROPERTIES OF CONDUCTING WIRES. 255

tive current passes from the cup c to the axis of the wheel w, it descends through that spoke which happens to dip into

the

mercury, and passes from thence to z, and to the zinc plate of the electromotor. As soon as the current descends the radius of the wheel, the portion dipping into the mercury is thrown out, as in the vibrating wire (369); another spoke of the wheel dips into the mercury, and is thrown out in its turn, and so on, a continual rotatory motion ensuing. If the direction of the poles of the magnet, or of the electric current, be reversed, the wheel will still rotate, but in an opposite direction. The wheel w may be replaced by an entire disc of metal with advantage, as the motion is then more uniform and continued.

371. If a horse-shoe magnet be approached to a suspended rectangle of wire, through whose sides an electric current is passing (365. A,B,C), it will be forcibly attracted, whilst the current is passing, and the poles of the magnet placed in one direction; and repelled, if either of these positions be reversed. This apparent attraction is really owing to the same cause which determines the vibration of a wire suspended freely between the poles of a magnet (369). The rectangle having a tendency to rotate, in common with all conducting wires (367), round the poles of the magnet, in opposite directions, it is compelled, by the law of composition of forces (47), to advance between, or move from, these poles, according to the positions in which they are respectively placed.

372. If the freely suspended rectangle before described (365), through which an electric current is moving, be left to itself, uninfluenced by any opposing cause, it will be acted upon by the magnetism of the earth (220), and will assume a definite position; which it will, if sufficiently mobile, regain when disturbed from it by any applied force. That face of the rectangle through which the positive current is moving in the direction of the hands of a watch, always turning towards the south, whilst the other, or that