Page images

the more powerfully the less the distance, but the induced force is always exactly equal to the force which produces it. When the north end 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 end 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 paramagnetism 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 paramagnetism of the iron. Iron acquires the inductive force more rapidly 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 induction, 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 effect 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, while those of steel resist the restoration of 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 paramagnetism of the steel; consequently the same mechanical means which develop the power will also destroy it. On that account a steel bar may lose its paramagnetism by any mechanical concussion, such as by falling on a hard substance, a blow with a hammer, and heating to redness, which makes the steel soft. The circumstances which determine whether it shall gain or lose are its position with

respect to the magnetic equator, and the higher or lower intensity of its previous magnetic state.

A comparison of the number of vibrations accomplished by the same magnetised needle during the same time at different distances from a magnet gives the law of paramagnetic intensity, which 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 the 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. Mr. Were Fox discovered that the law of the paramagnetic force changes from the inverse square of the distance to the simple inverse ratio when the distance between two magnets is as small as from the fourth to the eighth of an inch, or even as much as half an inch when the magnets are large; and in the case of repulsion, that the change takes place at a still greater distance, especially when the two magnets differ materially in intensity.

Without assuming any hypothesis of what magnetism is, or how that force is originated or sustained, Dr. Faraday regards a magnet as a source of power surrounded by curved lines of force which are not only representants of the magnetic power in quality and direction, but also in quantity-an hypothesis which accords perfectly with experiment, and with the action both of electricity and magnetism. The nature and form of these lines may be seen by placing a bar magnet upon a table, spreading a sheet of stiff paper over it so as to be perfectly level and free from creases, and then sifting very clean iron filings through a fine sieve equably over it. The filings will instantly assume the form of the curved lines represented by fig. 1, plate 7, in consequence of the action of the magnet. These lines are the true representatives of the magnetic forces, and being related to a polar power, they have opposite qualities in opposite directions. When a magnet is broken across the middle, each part is at once converted into a perfect magnet; the part that originally had a south pole acquires a north pole at the fractured end; the part that had originally a north pole gets a south pole; and as far as mechanical division can be carried, it is found that each fragment

is a perfect magnet. Fig. 2, plate 7, shows the lines of force in a fractured magnet when the ends are not yet separated; fig. 3 shows them when they are.

Currents of electricity are produced in conducting bodies moved across these lines of magnetic force. If a copper wire at a little distance above the north pole of a bar magnet be moved from left to right, at any angle across the lines of magnetic force, they will induce a current of electricity in the wire flowing from right to left; if the wire be moved with the same velocity in the contrary direction, the induced current will be of equal intensity, but it will flow from left to right. Similar results are obtained from the south pole, and the phenomena are the same when the magnet is moved and the wire is at rest; in both cases the intensity is greater the swifter the motion. It appears that the quantity of electricity induced is directly as the amount of the magnetic curves intersected, and when a wire is moving uniformly in a field of equal magnetic force, the current of electricity generated is proportional to the time, and also to the velocity of motion; for when a metallic disc is made to revolve through the lines of force, the current induced is strongest near the edge where the velocity is greatest; and in different substances moving across the lines of force the intensity of the induced current is directly as the conducting power of the substance. Thus bodies moved near a magnet have an electrical current developed in them, and conversely bodies affected by an electric current are definitely moved by a magnet near them.

By the preceding experiments it appears that magnetic polarity is manifested in two ways; in the magnetised needle, by attraction and repulsion, and in a wire moving across lines of magnetic force it is shown by the opposite directions in which the induced current flows according as the body is moved from the right to the left, or left to right. Hence polarity consists in the opposite and antithetical actions manifested at the opposite ends or opposite sides of a limited or unlimited line of force. Antithesis is the true and most general character of magnetism, whatever may be its mode of action.

It was by the induction of electric currents in copper wires moving across the lines of magnetic force that Dr. Faraday proved that the lines of force issuing from a magnet are closed curves which return again and pass through the interior of the

magnet. He placed two bar magnets of the same length, size, and intensity with their similar poles together, so that they might act as one magnet. A copper wire was then passed between their axes, which after extending through half their length was bent up equatorially and turned back along the outside, so that the whole wire formed a loop, the two ends being connected with a galvanometer. When the whole wire was made to revolve, no effect was produced, although it crossed the lines of magnetic force; but when it was cut in two, so as to separate the external from the internal part, electrical currents of equal intensity, but in contrary directions, were induced in each portion of the wire as they were made separately to cross the lines of force, for the apparatus was so constructed that that could be done. The exterior wire crossed the lines of force which issued from the magnets at right angles to their axes, while the equatorial part of the interior wire traversed the returning lines of force. It is evident that these forces neutralized each other when the whole wire revolved: consequently the internal and external lines of force must have been of equal intensity and opposite in direction, so as to balance one another. By this and a very great number of other experiments Dr. Faraday has proved that the magnetic lines of force are continuous closed curves alike in shape, size, and power. They extend indefinitely beyond the magnet, and undergo no change by distance.

Thus the magnetic force pervades the interior of the mass; if electricity does the same, a compensation must either take place, or it also must move in lines of force, sensible only at the surface. Electricity has a perpetual tendency to escape, and does escape, when not prevented by the coercive power of the air, and other non-conducting substances. Such a tendency does not exist in magnetism, which never leaves the substance containing it under any circumstances whatever. There must be some coercive force, analogous to friction, which arrests the magnetic forces, so as first to oppose their separation, and then to prevent their reunion. In soft iron the coercive force is either wanting or extremely feeble, since iron is easily rendered paramagnetic by induction, and as easily loses that quality; whereas in steel the coercive force is extremely energetic, because it prevents the steel from acquiring the paramagnetic 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 paramagnetic force, is perfectly analogous to the facility of transmission afforded to electricity by non-electrics, and the resistance it experiences in electrics. At every step the analogy between electricity and magnetism 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 as the electric equilibrium is restored by the union of the two electric states, and magnetic neutrality by the combination of the two polarities, they respectively neutralise 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 electric induction. Iron and steel are in a state of equilibrium when neutral; but this equilibrium is immediately disturbed on the approach of the pole of a magnet, which by induction transfers one kind of polarity to one end of an iron or steel bar, and the opposite kind to the other-effects exactly similar to electrical induction. There is even a correspondence between the fracture of a magnet and that of an electric conductor; for if an oblong conductor be electrified by induction, its two extremities will have opposite electricities; and if in that state it be divided across the middle, the two portions, when removed to a distance from one another, will each retain the electricity that has been induced upon it. The analogy, however, does not extend to transference. A body may transfer a redundant quantity of positive electricity to another, or deprive another of its electricity—the one gaining at the expense of the other; but a body cannot possess only one kind of polarity. With that exception, there is such perfect correspondence between the theories of magnetic attractions and repulsions, and electric forces in conducting bodies, that they not only are the same in principle, but are determined by the same formulæ. Experiment concurs with theory in proving the identity of these two influences. Hence, if the electrical phenomena be due to a modification of the ethereal medium, the magnetic phenomena must be owing to an analogous cause.

Curved lines of magnetic force issue from every point of the

« PreviousContinue »