axes. 329. M. Ampère assumes the mutual attractions and repulsions of electric currents as the fundamental fact to which, by the help of a particular hypothesis respecting the constitution of magnets, all the other facts, not only of electro-magnetism, but of magnetism also, are reducible. He supposes that all bodies possessing magnetic properties, including the earth itself, derive those properties from currents of electricity continually circulating among the particles of which they are composed, and having, with relation to the axes of these bodies, a uniform direction of revolution, in planes perpendicular to those We have already seen (319), that the action of conducting wires, rolled into the form of a flat spiral, produces on one side the effects of a marked pole, and on the other of an unmarked pole of a magnet. The actions of a helix at its two extremities, and at some distance beyond them, is similar; and if the two ends of the wire which has formed the helix be bent back so as to return in a straight course along the axis till they arrive at the middle point, where they are again bent at right angles, so as to pass out between the coils, it may be freely suspended in various ways, and will completely imitate a magnetic cylinder. Such an electro-dynamic cylinder, as M. Ampère has named it, may be substituted in almost every form of experiment for a magnet, whose properties it will possess only so long as a current of voltaic electricity is transmitted through it.(82) 330. We formerly (§ 312) referred to the hypothesis of magnetic elements, which was adopted to ac'count for the phenomena attending the fracture of a magnet, and which considered each particle of magnetized iron as a separate magnet; in like man (82) Such an electro-dynamic cylinder is represented opposite, its ends, after returning through the coil, n s, are attached to the wires, N P, of a small voltaic circuit, consisting of a plate of ner, M. Ampère's hypothesis regards the electric currents, to which a magnet owes its properties, as circulating round each constituent particle, all of which, moving in the same direction, will produce a combined effect which is equivalent to that of a circular current flowing uniformly round the circumference. If we regard a needle with its marked end pointing to the north, then the voltaic currents would ascend on the western side, pass from west to east in the upper surface, and descend on the eastern side: they would circulate, that is, in a direction contrary to that of the currents which are supposed to produce the magnetism of the earth. §331. All the principal effects of terrestrial magnetism may be imitated by distributing a wire round the surface of an artificial globe, in a spiral direction, from the equator to the poles, the two extremties being turned inward, and brought out at the two axes, by which the connexion may be made with zinc, surrounded by a plate of copper, and floating in a basin of dilute acid, in which it is free to move. Such a spiral will 290 AMPERE'S HYPOTHESIS OF MAGNETISM. the battery. A magnetic needle, properly suspended in different situations near such a globe, will arrange itself in positions perfectly analogous to those actually assumed by the dipping needle in corresponding regions of the earth. The origin of the currents which must, upon Ampère's theory, circulate in the globe from east to west, in planes parallel to the magnetic equator, may possibly be found in the action of the solar rays on successive parts of the torrid zone, producing thermo-electric currents in that direction. 332. The tendency which a magnet and conducting wire have to place themselves at right angles to one another, follows from the same hypothe. sis, and may be ascribed to the tendency of the transverse currents in the magnet itself and in the conductor to establish a parallelism between them. (83) The phenomena of revolving motions produced by the tangential force are also easily explained upon the same principles; but we must content ourselves with this brief indication of a theory which has conferred immortal honour upon its inventor. 333. A continued rotation, of a different kind from those which we have been hitherto considering, may be produced in a spur-wheel, suspended perpendicularly between the two poles of a horse shoe magnet, when a voltaic current is transmitted across its diameter by means of some mercury in a groove, into which the wheel is made to dip to allow of freedom of motion. If a mere wire, conducting a current, be suspended in the same situation, it will be attracted or repelled, according to the direction of the current; the opposite poles, acting upon opposite sides of the wire, concurring to give it the same direction. In either case it will be thrown out of the mercury, and the circuit being thereby broken, the effect will cease till the wire falls back by its own weight and restores the connexion. The current will then be re-established, and the same influence again exerted. If the wire terminate in a spur-wheel, dipping by its radii into the mercury, as one radius is thrown out of the mercury, another will enter it, and the wheel will revolve with great rapidity. It is not even necessary to divide the wheel into rays in order to produce the effect; for a circular metallic disc will revolve equally well when it is traversed by an electrical current passing into mercury between the poles of the magnet. § 334. The voltaic current may be influenced by the earth's magnetism precisely in the same way as by a permanent magnet, or an electro-dynamic cylinder; and wires delicately suspended have been made to exhibit all the phenomena of attraction, repulsion, direction, and rotation, by its action, although but feeble. MAGNETO-ELECTRICITY. § 335. The phenomena of electro-magnetism are produced by electricity in motion; accumulated electricity, when not in motion, exerts no magnetic effects. Dr. Faraday early felt convinced that, 66 as every electric current is accompanied by a corresponding intensity of magnetic action at right angles to the current, good conductors of electrici ty, when placed within the sphere of this action, should have a current induced through them, or some sensible effect produced equivalent in force to such a current." These considerations, with their consequence, the hope of obtaining electricity from ordinary magnetism, stimulated him to investigate the subject experimentally, and he was rewarded by an affirmative answer to the question proposed. He thus became, like Ersted, the founder of an entirely new branch of natural philosophy. 336. If a wire connecting the two ends of a delicate galvanometer be placed parallel and close to the wire connecting the poles of a voltaic battery, no effect will be produced upon the needle, however powerful the current may be. If the points opposed in the two wires be multiplied by coiling the one, as a helix, within the convolutions of the other coiled in the same way, both being covered with silk to prevent metallic contact, still no effect will be discernible so long as the current is uninterrupted. When, however, the current of the battery is stopped by breaking the circuit, the needle is momentarily deflected, as by a wave of electricity pass. ing in the same direction as that of the main current. Upon allowing the needle to come to a state of rest, and then renewing the contact, a similar impulse will be given to it in the contrary direction. While the current continues, the needle returns to its state of rest, again to be deflected in the first direction by stopping the current. Motion may be accumulated to a considerable amount in the needle, by making and breaking the contacts with the battery in correspondence with its swing. 337. The same effects are produced when, the current being uninterrupted, the conducting wire is made suddenly to approach or recede from the wire of the galvanometer. As the wires approximate, there will be a momentary current induced in the direction contrary to the inducing current; and as |