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north and a south pole mutually attract one another, which proves that there are two distinct kinds of magnetic forces, directly opposite in their effects, though similar in their mode of action.
Induction is the power which a magnet possesses of exciting temporary or permanent magnetism in such bodies in its vicinty as are capable of receiving it. By this property the mere approach of a magnet renders iron or steel magnetic, the more powerfully the less the distance. When the north pole of a magnet is brought near to, and in the line with an unmagnetised iron bar, the bar acquires all the properties of a perfect magnet, the end next the north pole of the magnet becomes a south pole, while the remote end becomes a north pole. Exactly the reverse takes place when the south pole is presented to the bar; so that each pole of a magnet induces the opposite polarity in the adjacent end of the bar, and the same polarity in the remote extremity; consequently the nearest extremity of the bar is attracted, and the farther repelled, but as the action is greater on the adjacent than on the distant part, the resulting force is that of attraction. By induction, the iron bar not only acquires polarity, but the power of inducing magnetism in a third body; and although all these properties vanish from the iron as soon as the magnet is removed, a lasting increase of intensity is generally imparted to the magnet itself by the reaction of the temporary magnetism of the iron. Iron acquires magnetism more raidly than steel, yet it loses it as quickly on the removal of the magnet, whereas the steel is impressed with a lasting polarity.
A certain time is requisite for the induction of magnetism, and it may be accelerated by anything that excites a
vibratory motion in the particles of the steel, such as the smart stroke of a hammer, or heat succeeded by sudden cold. A steel bar may be converted into a magnet by the transmission of an electric discharge through it, and as its efficacy is the same in whatever direction the electricity passes, the magnetism arises from its mechanical operation exciting a vibration among the particles of the steel. It has been observed that the particles of iron easily resume their neutral state after induction, but those of steel resist the restoration of magnetic equilibrium, or a return to the neutral state: it is therefore evident, that any cause which removes or diminishes the resistance of the particles will tend to destroy the magnetism of the steel; consequently, the same mechanical means which develope magnetism will also destroy it. On that account, a steel bar may lose its magnetism by any mechanical concussion, such as by falling on a hard substance, a blow with a hammer, and heating to redness, which reduces the steel to the state of soft iron. The circumstances which determine whether it shall gain or lose being its position with respect to the magnetic equator, and the higher or lower intensity of its previous magnetic state.
Polarity of one kind only can exist in any portion of iron or steel, for in whatever manner the intensities of the two kinds of polarity may be diffused through a magnet, they exactly balance or compensate one another. The northern polarity is confined to one half of a magnet, and the southern to the other, and they are generally concentrated in or near the extremities of the bar. When a magnet is broken across its middle, each fragrant is at once converted into a perfect magnet; the part which originally had a north pole, acquires a south pole at the fractured
end, the part that originally had a south pole gets a north pole; and as far as mechanical division can be carried, it is found that each fragment, however small, is a perfect magnet.
A comparison of the number of vibrations accomplish by the same needle, during the same time, at different distances from a magnet, gives the law of magnetic intensity, which, like every known force that emanates from a centre, follows the inverse ratio of the square of the distance, a law that is not affected by the intervention of any substance whatever between the magnet and the needle, provided that substance be not itself susceptible of magnetism. Induction and the reciprocal action of magnets are, therefore, subject to the laws of mechanics, but the composition and resolution of the forces are complicated, in consequence of four forces being constantly in activity, two in each magnet.
The phenomena of magnetism may be explained on the hypothesis of two extremely rare fluids pervading all the particles of iron, and incapable of leaving them. Whether the particles of these fluids are coincident with the molecules of the iron, or that they only fill the interstices between them, is unknown and immaterial; but it is certain that the sum of all the magnetic molecules, added to the sum of all the spaces between them, whether occupied by matter or not, must be equal to the whole volume of the magnetic body. When the two fluids in question are combined they are inert, so that the substances containing them show no signs of magnetism; but when separate they are active, the molecules of each of the fluids attracting those of the opposite kind, and repelling those of the same kind. The decomposition of the united fluids is
accomplished by the inductive influence of either of the separate fluids; that is to say, a ferruginous body acquires polarity by the approach of either the south or north pole of magnet. The electric fluids are confined to the surfaces of bodies, whereas the magnetic fluids pervade each molecule of the mass; besides, the electric fluid has a perpetual tendency to escape, and does escape, when not prevented by the coercive power of the surrounding air and other non-conducting bodies. Such a tendency does not exist in the magnetic fluids, which never quit the substance that contains them under any circumstances whatever; nor is any sensible quantity of either kind of polarity ever transferred from one part to another of the same piece of steel. It appears that the two magnetic fluids, when decomposed by the influence of magnetizing forces, only undergo a displacement to an insensible degree within the body. The action of all particles so displaced upon a particle of the magnetic fluid in any particular situation, compose a resultant force, the intensity and direction of which it is the province of the analyst to determine. In this manner M. Poisson has proved that the result of the action of all the magnetic elements of a magnetised body is a force equivalent to the action of a very thin stratum covering the whole surface of a body, and consisting of the two fluids the austral and the boreal, occupying different parts of it; or, in other words, the attractions and repulsions externally exerted by a magnet are exactly the same as if they proceeded from a very thin stratum of each fluid occupying the surface only, both fluids being in equal quantities, and so distributed that their total action upon all the points in the interior of the body are equal to nothing. Since the resulting force is the difference of the two
polarities, its intensity must be greatly inferior to that of either.
It may be observed that, in addition to the forces already mentioned, there must be some coercive force analogous to friction which arrests the particles of both fluids, so as first to oppose the separation of the fluids, and then to prevent their reuniting. In soft iron the coercive force is either wanting or extremely feeble, since the iron is easily rendered magnetic by induction, and as easily loses its magnetism; whereas in steel the coercive force is extremely energetic, because it prevents the steel from acquiring the magnetic properties rapidly, and entirely hinders it from losing them when acquired. The feebleness of the coercive force in iron, and its energy in steel, with regard to the magnetic fluids, is perfectly analogous to the facility of transmission afforded to the electric fluids by non-electrics, and the resistance they experience in electrics. At every step the analogy between magnetism and electricity becomes more striking. The agency of attraction and repulsion is common to both, the positive and negative electricities are similar to the northern and southern polarities, and are governed by the same laws, namely, that between like powers there is repulsion, and between unlike powers there is attraction; each of these four forces is capable of acting most energetically when alone, but the electric equilibrium is restored by the union of the two electricities, and magnetic neutrality by the combination of the two polarities, thus respectively neutralizing each other when joined. All these forces vary inversely as the square of the distance, and consequently come under the same mechanical laws. A like analogy extends to magnetic and electrical induction. Iron and steel are in a state