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explained by Dr. Roget in some excellent articles on these subjects in the Library of Useful Knowledge.

All experiments tend to prove that the force emanating from the electric current, which produces such effects on the magnetic needle, acts at right angles to the current. The action of an electrical current upon either pole of a magnet has no tendency to cause the pole to approach or recede, but to rotate about it. If the stream of electricity be supposed to pass through the centre of a circle whose plane is perpendicular to the current, the direction of the force exerted by the electricity will always be in the tangent to the circle, or at right angles to its radius (N. 223). Consequently, the tangential force of the electricity has a tendency to make the pole of a magnet move in a circle round the wire of the battery.

Rotatory motion was suggested by Dr. Wollaston. Dr. Faraday was the first who actually succeeded in making the pole of a magnet rotate about a vertical conducting wire. In order to limit the action of the electricity to one pole, about two-thirds of a small magnet were immersed in mercury, the lower end being fastened by a thread to the bottom of the vessel containing the mercury. When the magnet was thus floating almost vertically with its north pole above the surface, a current of positive electricity was made to descend perpendicularly through a wire touching the mercury, and immediately the magnet began to rotate from left to right about the wire. The force being uniform, the rotation was accelerated till the tangential force was balanced by the resistance of the mercury, when it became constant. Under the same circumstances the south pole of the magnet rotates from right to left. It is evident, from this experiment, that the wire may also be made to perform a rotation round the magnet, since the action of the current of electricity on the pole of the magnet must necessarily be accompanied by a corresponding reaction of the pole of the magnet on the electricity in the wire. This experiment has been accomplished by a vast number of contrivances, and even a small battery, consisting of two plates, has performed the rotation. Dr. Faraday produced both motions at the same time in a vessel containing mercury; the wire and the magnet revolved in one direction about a common centre of motion, each following the other.

The next step was to make a magnet, and also a cylinder, revolve about their own axes, which they do with great rapidity,

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Mercury has been made to rotate by means of Voltaic electricity, and Professor Ritchie exhibited in the Royal Institution the singular spectacle of the rotation of water by the same means, while the vessel containing it remained stationary. The water was in a hollow double cylinder of glass, and, on being made the conductor of electricity, was observed to revolve in a regular vortex, changing its direction as the poles of the battery were alternately reversed. Professor Ritchie found that all the different conductors hitherto tried by him, such as water, charcoal, &c., give the same electro-magnetic results when transmitting the same quantity of electricity, and that they deflect the magnetic needle in an equal degree when their respective axes of conduction are at the same distance from it. But one of the most extraordinary effects of this force is exhibited by coiling a copper wire, so as to form a helix or corkscrew, and connecting the extremities of the wire with the poles of a galvanic battery. If a magnetized steel bar or needle be placed within the screw, so as to rest upon the lower part, the instant a current of electricity is sent through the wire of the helix, the steel bar starts up by the influence of this invisible power, and remains suspended in the air in opposition to the force of gravitation (N. 224). The effect of the electro-magnetic power exerted by each turn of the wire is to urge the north pole of the magnet in one direction, and the south pole in the other. The force thus exerted is multiplied in degree and increased in extent by each repetition of the turns of the wire, and in consequence of these opposing forces the bar remains suspended. This helix has all the properties of a magnet while the electrical current is flowing through it, and may be substituted for one in almost every experiment. It acts as if it had a north pole at one extremity and a south pole at the other, and is attracted and repelled by the poles of a magnet exactly as if it were one itself. All these results depend upon the course of the electricity; that is, on the direction of the turns of the screw, according as it is from right to left, or from left to right, being contrary in the two cases.

The action of Voltaic electricity on a magnet is not only precisely the same with the action of two magnets on one another, but its influence in producing temporary magnetism in iron and steel is also the same with magnetic induction. The term induction, when applied to electric currents, expresses the power

which these currents possess of inducing a particular state upon matter in their immediate neighbourhood, otherwise neutral or indifferent. For example, the connecting wire of a galvanic battery holds iron filings suspended like a magnet as long as the current continues to flow through it: the iron becomes magnetic by the induction of the current. The most powerful temporary magnets are obtained by bending a thick cylinder of soft iron into the form of a horseshoe, and surrounding it with a coil of thick copper wire covered with silk to prevent communication between its coils. When this wire forms part of a galvanic circuit the iron becomes so highly magnetic by the induction of the current flowing through the wire that a temporary magnet of this kind made by Professor Henry of the Albany Academy in the United States sustained a weight of nearly a ton. Another by Mr. Gage has been applied with considerable success as a moving power: its spark is a bright flash, and the snap as loud as a pistol. But the most powerful known is that employed by Mr. Joule in his experiments, which sustains a weight of 2080 lbs. The iron loses its magnetism the instant the electricity ceases to flow, and acquires it again as instantaneously when the circuit is renewed.

The action of an electric current causes a deviation of the compass from the plane of the magnetic meridian. In proportion as the needle recedes from the meridian, the intensity of the force of terrestrial magnetism increases, while at the same time the electro-magnetic force diminishes; the number of degrees at which the needle stops, showing where the equilibrium between these two forces takes place, will indicate the intensity of the galvanic current. The galvanometer, constructed upon this principle, is employed to measure the intensity of galvanic currents collected and conveyed to it by wires. This instrument is rendered much more sensible by neutralizing the effects of the earth's magnetism on the needle, which is accomplished by placing a second magnetised needle so as to counteract the action of the earth on the first-a precaution requisite in all delicate magnetical experiments.

It has been ascertained by means of this instrument that the action of an electrical current upon a magnet is inversely as the square of the distance, and the energy with which an electro magnet acts is directly as the power of the galvanic battery and the number of coils round the core, and inversely as the resistance of the wire.

SECTION XXXI.

Electro-Dynamics Reciprocal Action of Electric Currents- Identity of Electro-Dynamic Cylinders and Magnets - Differences between the Action of Voltaic Electricity and Electricity of Tension Effects of a Voltaic Current Ampère's Theory - Dr. Faraday's Experiment of Electrifying and Magnetising a Ray of Light.

THE science of electro-magnetism, which must render the name of M. Oersted ever memorable, relates to the reciprocal action of electrical and magnetic currents: M. Ampère, by discovering the mutual action of electrical currents on one another, has added a new branch to the subject, to which he has given the name of electro-dynamics.

When electric currents are passing through two conducting wires, so suspended or supported as to be capable of moving both towards and from one another, they show mutual attraction or repulsion, according as the currents are flowing in the same or in contrary directions; the phenomena varying with the relative inclinations and positions of the streams of electricity. The mutual action of such currents, whether they flow in the same or in contrary directions, whether they be parallel, perpendicular, diverging, converging, circular, or heliacal, all produce different kinds of motion in a conducting wire, both rectilineal and circu-lar, and also the rotation of a wire helix, such as that described, now called an electro-dynamic cylinder on account of some improvements in its construction (N. 225). And, as the hypothesis of a force varying inversely as the square of the distance accords perfectly with all the observed phenomena, these motions come under the same laws of dynamics and analysis as any other branch of physics.

Electro-dynamic cylinders act on each other precisely as if they were magnets during the time the electricity is flowing through them. All the experiments that can be performed with the cylinder might be accomplished with a magnet. That end of the cylinder in which the current of positive electricity is moving in a direction similar to the motion of the hands of a watch, acts

as the south pole of a magnet, and the other end, in which the current is flowing in a contrary direction, exhibits northern polarity.

The phenomena mark a very decided difference between the action of electricity in motion or at rest, that is, between Voltaic and static electricity; the laws they follow are in many respects of an entirely different nature, though the electricities themselves are identical. Since Voltaic electricity flows perpetually, it cannot be accumulated, and consequently has no tension, or tendency to escape from the wires which conduct it. Nor do these wires either attract or repel light bodies in their vicinity, whereas static or ordinary electricity can be accumulated in insulated bodies to a great degree, and in that state of rest the tendency to escape is proportional to the quantity accumulated and the resistance it meets with. In ordinary electricity, the law of action is, that dissimilar electricities attract and similar electricities repel one another. In Voltaic electricity, on the contrary, similar currents, or such as are moving in the same direction, attract one another, while a mutual repulsion is exerted between dissimilar currents, or such as flow in opposite directions. Common electricity escapes when the pressure of the atmosphere is removed, but the electro-dynamical effects are the same whether the conductors be in air or in vacuo.

The effects produced by a current of electricity depend upon the celerity of its motion through a conducting wire. Yet we are ignorant whether the motion be uniform or varied, but the method of transmission has a marked influence on the results; for, when it flows without intermission, it occasions a deviation in the magnetic needle, but it has no effect whatever when its motion is discontinuous or interrupted, like the current produced by the common electrical machine when a communication is made between the positive and negative conductors.

M. Ampère has established a theory of electro-magnetism suggested by the analogy between electro-dynamic cylinders and magnets, founded upon the reciprocal attraction of electro-currents, to which he reduces all the phenomena of magnetism and electro-magnetism, by assuming that the magnetic properties which bodies possess derive these properties from currents of electricity, circulating about every part in one uniform direction. Although every particle of a magnet possesses like properties

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