Part 36

Magnetism a Dual Power—Antithetic Character of Paramagnetism and Diamagnetism—The Earth Paramagnetic—Properties of Paramagnetic Bodies—Polarity—Induction—Lines of Magnetic Force—Currents of Electricity induced by them—Proved to be Closed Curves—Analogy and Identity of Electricity and Magnetism—Terrestrial Magnetism—Mean Values of the Three Magnetic Elements—Their Variations in Double Progression proved to consist of Two Superposed Variations—Discovery of the Periodicity of the Magnetic Storms—The Decennial Period of the Magnetic Elements the same with that of the Solar Spots—Magnetism of the Atmosphere—Diamagnetism—Action of Electro-Magnetism on Paramagnetic, Diamagnetic Bodies, and on Copper, very different—Proof of Diamagnetic Polarity and Induction—Magnecrystallic Action—Effects of Compression, Heat, and Cleavage on Magnetic Bodies—Mutual Dependence of Light, Heat, Electricity, &c. &c.—The Conservation of Force and the Permanency of Matter Primary Laws of Nature—Definition of Gravity not according to that Law—Gravity only the Residual Force of a Universal Power—Magnetism of the Ethereal Medium.

MAGNETISM may be regarded as a new science in consequence of the profound researches and admirable discoveries of Dr. Faraday. Since the magnetism of matter is only known by the action of a magnet or of electricity upon it, by using an extremely energetic magnet or electro-magnet he has proved that all known substances, whether solid, liquid, or aëriform, are more or less magnetic, but that the magnetism is very different in different substances. For example, if a bar of iron be freely suspended between the poles of a very powerful magnet or electro-magnet, it will be attracted by both poles, and will set or rest in the direction of a straight line joining them; but if a similar bar of bismuth be freely suspended in the same manner, it will rest in a direction at right angles to that which the iron bar assumed. Thus the direction in which the iron sets is axial or in the line of force, while that which the bismuth assumes is equatorial or perpendicular to the line of force. Substances that are magnetic after the manner of iron are said to be paramagnetic, those that are magnetic after the manner of bismuth are diamagnetic. As far as we know, all matter comes under one or other of these laws. Many bodies are paramagnetic besides iron, as the loadstone, which consists of the peroxide and protoxide of iron mixed with small portions of silica and alumina; also some of the gems and metals, as cobalt, nickel, &c. A substance is often paramagnetic if it contains only the 130,000th part of its weight of iron; but by far the greater number are diamagnetic, as all animal and vegetable matter, acids, oils, sugar, starch, bread, &c., and all the gases except oxygen, which is highly paramagnetic; and its force increases with its density: but notwithstanding the predominance of diamagnetic matter at the surface, the terrestrial globe is paramagnetic—in fact it is a powerful magnet.

Besides the substances which are paramagnetic naturally, that property may be imparted by a variety of methods, as by friction with magnets or even juxtaposition with them; and a bar of hard steel held at the angle of the dip will become a magnet on receiving a few strokes with a hammer on its upper end.

Polarity is one of the most distinguishing characters of magnetism: it is the property which a magnet possesses when freely suspended of resting spontaneously in the magnetic meridian, or nearly north and south, and always returning to that position when disturbed in consequence of the mean magnetic attraction of the earth; yet the magnet has no tendency to move to the north or south even when floating on water, because the same pole that attracts one end repels the other. Both poles of a magnet attract iron, which in return attracts either pole of the magnet with an equal and contrary force. The action of a magnet on unmagnetised iron is confined to attraction, whereas the reciprocal agency of magnets is characterised by a repulsive as well as by an attractive force; for a north pole repels a north pole, and a south pole repels a south pole; but a north and south pole mutually attract one another—which proves that paramagnetism is a dual power in which the conservation of force is perfectly maintained, for the force of attraction is exactly equal to the force of repulsion. One kind of polarity cannot exist without the other: they are absolutely simultaneous, dependent, and of equal intensity.

Induction is the power which a magnet possesses of exciting temporary or permanent paramagnetism in such bodies in its vicinity as are capable of receiving it. By this property the mere approach of a magnet renders iron and steel paramagnetic, 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 earth’s surface where there is sensible dip, and bending round enter the earth again at the magnetic equator. They induce electric currents in conducting-wires, moving across them exactly the same as in artificial magnets; and when a hollow helix, or coil of copper wire, whose extremities are connected with a galvanometer, is placed in the magnetic dip, and suddenly moved across the lines of force, the needle of the galvanometer will vibrate through an arc of 80° or 90°, in consequence of the electric current induced by these lines of magnetic force in the wire, and the action is greater when a core of soft iron is placed in the helix, which becomes a temporary magnet by induction. Again, if a copper plate be connected with a galvanometer by two copper wires, one from the centre, and another from the circumference, in order to collect and convey the electricity, it is found that, when the plate is made to revolve in a plane passing through the line of the dip, the galvanometer is not affected. But as soon as the plate is inclined to that plane, electricity begins to be developed by its motion across the lines of magnetic force; it becomes more powerful as the inclination increases, and arrives at a maximum when the plate revolves at right angles to the line of dip. When the revolution is in the same direction with that of the hands of a watch, the current of electricity flows from its centre to the circumference; and when the rotation is in a contrary direction, the current sets the opposite way. Thus a copper plate, revolving at right angles to the line of the dip, becomes a new electrical machine, differing from the common plate-glass machine by the copper being the most perfect conductor, whereas glass is the most perfect non-conductor; besides insulation, which is essential to the glass machine, is fatal to the copper one. The quantity of electricity evolved by the metal does not appear to be inferior to that devolved by the glass, though very different in intensity. Even a ship crossing the lines of force must have electric currents running through her. Dr. Faraday observes that such is the facility with which electricity is generated by the magnetic lines of force, that scarcely any piece of metal can be moved without a development of it; consequently, among the arrangements of steam-engines and metallic machinery, curious electro-magnetic combinations probably exist which have never yet been noticed. Thus magnetic lines of force certainly issue from the surface of the globe.

No doubt the earth is a magnet on a vast scale, but it differs from all others in having four poles of maximum magnetic force of different intensities, the two in the northern hemisphere having a secular motion in a contrary direction from the two in the southern. They are not even symmetrically placed; hence the magnetic intensity varies so much in the different points on the earth’s surface, that the dynamic equator, or line passing through all the points of least intensity, is a very irregular curve surrounding the globe, but by no means coinciding with the terrestrial equator. In consequence of the mean action of these four forces, the north end of a magnetised needle, arranged so as to revolve in a vertical plane, dips or inclines beneath the horizon in the northern hemisphere, and the south end in the southern. The two hemispheres are separated by a line encircling the earth, called the magnetic equator, or line of no dip, in which the dipping or inclination needle is horizontal. On each side of this line the inclination increases till at last the needle becomes perpendicular to the horizon in two points, or rather small spaces, in each hemisphere, known as the magnetic poles, which are quite different from the poles of the earth’s rotation. The mean action of the four poles of magnetic intensity causes the mariner’s compass, or a magnetic needle suspended so as to revolve in a horizontal plane, to remain at rest when pointing to the two magnetic poles. It is then in the magnetic meridian of the place of observation, which is thus determined by the mean action of all the four magnetic forces.

These mean values of the three magnetic elements, namely, the declination, inclination or dip, and magnetic intensity, are well known to be subject to secular, annual, and diurnal variations. The secular only become sensible after some years, but the annual and diurnal variations have a double progression—that is to say, two maximum and two minimum values in their respective periods of a year and twenty-four hours; for example, the declination needle makes two deviations to the west and two to the east in the course of twenty-four hours, and that with great regularity. Now General Sabine discovered that the double progression arises from two combined or superposed variations having different hours of maxima and minima, and that they are due to two distinctly different causes—the one being the difference in the sun’s position relatively to the place of observation at the different seasons of the year, and hours of the day and night; the other being a mean annual and diurnal variation proved by General Sabine to exist in those great magnetic storms or casual disturbances which affect the magnetic elements simultaneously over enormously extensive tracts of the globe.

Moreover the General discovered that, besides these annual and diurnal variations, the magnetic storms have a variation which accomplishes its vicissitudes in ten or more nearly eleven years, the increase from year to year being gradual, till its maximum becomes twice as great as its minimum value. In consequence of this inequality in the storms or casual disturbances, each of the magnetic elements has a variation of similar period and similar maxima and minima. Now the number and magnitude of the spots on the sun had been observed by M. Schwabe, of Dessau, to increase to a maximum, and decrease again to a minimum, regularly in the very same period of between ten and eleven years; and General Sabine found that this variation in the solar spots, and that in the magnetic elements, not only have the same periods of maxima and minima, but that they correspond in all their minutest vicissitudes. Thus a very remarkable and unexpected connexion exists between terrestrial and solar magnetism. The dual and antagonist principle is perfectly maintained in the earth’s magnetism, all the phenomena and their variations being in opposite directions in the two hemispheres. (N. 226.)