positions of the conducting wire, and every direction of the Voltaic current, together with all the other phenomena of electro-magnetism, are explained by Dr. Roget in some excellent articles on these subjects in the Library of Useful Knowledge.

All the 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, and is therefore unlike any force hitherto known. The action of all the forces in nature is directed in straight lines, as far as we know; for the curves described by the heavenly bodies result from the composition of two forces; whereas that which is exerted by an electrical current upon either pole of a magnetic 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 center 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. 217). 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. Mr. Barlow has proved that the action of each particle of the electric fluid in the wire, on each particle of the magnetic fluid in the needle, varies inversely as the squares of the distances.

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 be

came 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 center 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. Mercury has been made to rotate by means of Voltaic electricity, and Professor Ritchie has 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 the new force is exhibited by coiling a copper wire, so as to form a helix or corkscrew, and connecting the extremities of the wires 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. 218). 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 any particular state upon matter in their immediate neighborhood, otherwise neutral or indifferent. For example, the connecting wire of a galvanic battery holds iron filings suspended like an artificial magnet, as long as the current continues to flow through it; and the most powerful temporary magnets that have ever been made 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 parts. When this wire forms part of a galvanic circuit, the iron becomes so highly magnetic, that a temporary magnet of this kind, made by Professor Henry, of the Albany Academy, in the United States, sustained nearly a ton weight. The iron loses its magnetic power the instant the electricity ceases to circulate, and acquires it again as instantaneously when the circuit is renewed. Temporary magnets have been made by Professor Moll of Utrecht, upon the same principle, capable of supporting 200 pounds' weight, by means of a battery of one plate less than half an inch square, consisting of

two metals soldered together. It is truly wonderful that an agent, evolved by so small an instrument, and diffused through a large mass of iron, should communicate a force which seems so disproportionate. Steel needles are rendered permanently magnetic by electrical induction; the effect is produced in a moment, and as readily by juxtaposition as by contact; the nature of the poles depends upon the direction of the current, and the intensity is proportional to the quantity of electricity.

It appears that the principle and characteristic phenomena of the electro-magnetic science are, the evolution of a tangential and rotatory force exerted between a conducting body and a magnet; and the transverse induction of magnetism by the conducting body in such substances as are susceptible of it.

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 magnetized needle so as to counteract the action of the earth on the first-a precaution requisite in all delicate magnetical experiments.

Electro-magnetic induction has been elegantly and usefully employed by Professor Wheatstone as a moving power in a telegraph, by which intelligence is conveyed in a time quite inappreciable, since the electricity would make the circuit of the globe in the tenth of a second.

SECTION XXXII.

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.

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 toward 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 circular, 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. 219). And as the hypothesis of a force varying inversely as the squares of the distances 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