MAGNETISM.

motive force' (= approximately that of a Daniell cell, in which the liquids are a saturated solution of nitrate of copper and dilute sulphuric acid, 1 acid to 22 water); the coulomb, that of electric quantity; the farad, that of capacity; and the quadrant, that of self-induction. See UNITS, SCIENTIFIC.

Self-induction.-When a current is suddenly started in a coil of wire, the ultimate result is to set up a magnetic field. But, while this is being set up, energy is being absorbed by the field, and the current falls short of its full intensity. Similarly, when the current ceases this energy is restored, and the current seems piled up as if it had momentum of its own like water in a hydraulic ram. The stronger the magnetic field that will be produced-the more lines of induction will thread the coil-the more marked is this effect; and this exaggeration is brought about by multiplying the turns in the coil (keeping down the resistance, if necessary, by increasing the thickness of the wire used), or by inserting a core of soft iron, or both.

Induction of Currents in Magnetic Field.-Lay two circuits in one another's neighborhood. The sudden production or increase of current in the one will produce a brief current in the other in such a sense that there is mechanical repulsion between the induced current and the originating one; the cessation or diminution of the primary current induces, in the opposite sense, a brief current in the secondary circuit. These are phenomena of the magnetic field of the primary circuit; and the primary circuit can be replaced by a magnet or electro-magnet, whose approach or strengthening induces brief currents in one sense, and whose recession or weakening induces brief currents in the opposite sense. No current passes in the secondary coil so long as the primary current or magnet remains constant or stationary. For the ways in which this production of a secondary current is utilized, see DYNAMOELECTRIC MACHINE: INDUCTION. If we try to move a good conductor-i.e., a knife-in a strong magnetic field the motion is resisted or damped; the production of the induced currents generated by motion in the field absorbs energy.

Rotary Features of Magnetism.-As a simple case, consider the field in the immediate neighborhood of a linear current. The lines of magnetic force run in circles round the wire; a magnet pole tends to be driven in such a sense that, if it be positive or north-seeking, it will travel round an advancing current in the same sense in which the point of a corkscrew travels round the axis of the advancing corkscrew. If a magnet were flexible it would form a coil round the current; and conversely, a flexible currentbearing wire tends to coil round a strong bar-magnet, and currents parallel to bar-magnets tend to rotate round the magnetic axis of the magnet.

Nature of the Magnetic Field.—All the phenomena of the magnetic field are explicable as due to whirlpool currents of electricity in closed vortex-rings, the axes of which are the magnetic lines of induction. The reaction of tendencies to the formation of these vortex-rings from differ.

MAGNETISM.

ent sources results in the production of local variations of stress in the ether which result in attractive and repellent movements between currents or magnets, or between currents and magnets, or in the production of currents, or of magnetic induction; and the resultant forces are along the axes of the whirls which tend to shorten themselves longitudinally and to spread out laterally. The electric displacements in the whirls are therefore at right angles to the lines of magnetic force. With other dispositions of the magnetic field we have other forms of the lines of force; but they are always closed curves which mark the axes of vortex motions or shears, and which lie wholly in air, or partly in air and partly in metal or other substance.

Electro-magnetic Propagation.-When a disturbance is set up in one place which leads to the formation of a magnetic field, the change from the original condition of the ether to the complex condition which is known as 'magnetic field' is marked by a magnetic or electro-magnetic propagation of the disturbance; and the theoretical velocity of this propagation has been shown to be about 300,000 kilometres per second, which is practically exactly the same as the speed of the propagation of light. In a linear current the direction of the current is the direction of propagation; the disturbance is propagated in the ether, not in the conductor; and the magnetic and electric displacements are at right angles both to the direction of propagation and to one another. Without a linear conductor to guide the propagation, the disturbance is propagated equally in all directions; and Clerk-Maxwell adi. vanced the proposition that light is a phenomenon of this order, an electro-magnetic phenomenon involving vortical stresses, rather than the mere vibration of an elastic ether. This proposition was strikingly confirmed by the researches of Hertz 1888. He found that by producing waves of electro-magnetic propagation of periodic disturb ances he could reproduce with long waves, which he found to travel at the predicted rate, the phenomena of reflection at the surface of a conductor, refraction, polarization, interference, etc., which are manifested by those short and frequent ether-waves which give rise to the phenomena of light and radiant heat; and his results have shown that the plane of maguetic disturbance, at right angles to that of electric disturbance, is the analogue of the plane of polarization, which must be at right angles to the plane of vibration. By Hertz's researches the science of light has been made a part of the science of electro-magnetism.

See DECLINATION NEEDLE: DIAMAGNETISM: DIPPINGNEEDLE: DYNAMO-ELECTRIC MACHINE. For literature, see ELECTRICITY; and refer to Sir William Thomson's Reprint of Papers on Electrostatics and Magnetism (1872); Von Helmholtz's Wissenschaftliche Abhandlungen (I. 1882); and O. J. Lodge, Modern Views of Electricity (1889). For instruments, etc., refer to W. E. Ayrton's Practical Electricity 1886); Jamieson's Magnetism and Electricity (1890). MAGNETISM, ANIMAL: see HYPNOTISM.

MAGNETISM.

MAGNETISM, TERRESTRIAL: magnetic properties of the earth as a whole. The globe itself is a magnet: see MAGNETISM. In studying the magnetic field associated with the earth we are confined to its surface, and are unable to trace the lines of force throughout their whole

Fig. 1.-Lines of Equal Magnetic Dip, 1885.

length. We believe, however, that these lines of force have the properties of all lines of force associated with magnets. In general they pass by continuously curved paths from regions in the southern hemisphere to regions in the northern hemisphere. The southern hemisphere, therefore, is the seat of what is called northern or positive magnetism.

MAGNETISM.

The direction of the line of force at any point is given by the direction in which a perfectly free magnet placed there will point (see MAGNETISM). To obtain the direction of the earth's magnetic force we must suspend the magnet accurately by its centre of mass, as in the apparatus known as the Dipping-needle (q.v.). With such an apparatus, let us, beginning at the extreme s. point of Africa, move northward, and study at each successive stage the behavior of the magnet. At first it will be found to make an angle of about 57° with the horizontal, pointing up toward the northwest. This angle of 57° is called the dip, and will steadily diminish as we pass northward, until, a little s.e. of Lake Chad, the magnet will be found to rest perfectly horizontal. Proceeding still northward we shall find the magnet beginning to tilt again, but this time with the north-pointing end downward. As we leave the n. coast of Africa in 20 e. long, the dip will be nearly 45°; it will be 55 as we enter Turkey, gradually increasing to nearly 77° as we leave the n. coast of Norway. Very similar changes in dip will occur as we pass along any longitude line. The general features are shown in fig. 1, reduced from Neumayer's chart for 1885, as given in the new edition of Berghaus's Physikalischer Atlas. Each line is drawn through all places at which the dip has the value indicated by the number attached. The only points requiring particular remark are the position of the line of zero dip, and the position of the point of maximum dip. The line of zero dip is called the magnetic equator. Its noncoincidence with the geographical equator indicates a marked departure of the earth's magnetic condition from the magnetic condition of a uniformly magnetized sphere, whose magnetic axis coincides with the polar axis. The position of maximum dip shown is where the needle points vertical with its north end downward. It is called the magnetic n. pole, and is situated in the n. of Canada, 97° w. long., 704 n. lat. There is also a magnetic s. pole, believed to lie somewhere near 150° e. long, and 73 s. lat. The magnetic poles do not, therefore, lie exactly at the

extremities of a diameter. It should be noted that the dip is the angle between the line of force at a given locality and the horizontal plane there; that is, the dips in different latitudes are referred to different planes. Fig. 2, which represents the section of the earth along the great circle passing through the geographical and magnetic n. poles, will serve to indicate the approximately relative positions of the lines of force. The directions of these at latitudes 0°, 30°, and 60° are indicated

B

Fig. 2.

MAGNETISM.

by arrows, the dotted lines giving the directions of the true vertical at the various points. AB is the geographical polar axis, S the magnetic north pole'-really analogous to the so-called s. pole of a magnet. 00' are the points of zero dip, where the lines of force will be roughly parallel to the magnetic axis.

Fig. 3.-Lines of Equal Magnetic Declination, 1885.

Returning again to the southern extremity of Africa, let us consider more fully the position of the magnet hanging freely by its centre of mass. To fix this position we require