The lines of flow in this case correspond to the lines of force when the soft-iron bar with its great permeability is in the field. 508. Magnets Formed by Induction. When a soft-iron bar is placed in front of a magnet as shown in figure 275, at the end nearest the north pole of the magnet the lines of force are directed toward the end of the bar as toward the south pole of a magnet and at the other end they are directed away from the bar as from a north pole. The bar of iron thus becomes a magnet by induction. If it were of steel it would retain some of this magnetism when taken out of the field. FIG. 276.-Bar of soft iron parallel FIG. 277.-Bar of soft iron across the lines of force. Suppose a long bar of soft iron to be placed in a magnetic field parallel to the direction of the lines of force. The result will be as shown in figure 276, lines of force will be drawn into the bar in consequence of its great permeability entering it at one end and leaving it at the other, so that one end becomes a south pole and one a north pole. On each side of the bar the field is weakened. When, however, the bar is placed across the field of force as in figure 277 it will have only a very slight effect on the field of force since the lines of force can pass through only a small thickness of iron. So also a thin flat sheet of iron placed perpendicular to the lines of force of the field would have practically no effect on the field. 509. Effect of Heat and Jarring in Case of Magnetizing by Induction. The magnetism induced in an iron or steel bar placed in a magnetic field parallel to the lines of force may be increased by striking the bar with a hammer or jarring it while under the influence of the field, also by heating the bar red-hot and allowing it to cool in the magnetic field. These disturbances seem to facilitate the arrangement of the molecules under the influence of the magnetic force and help to overcome the resistance to magnetization which especially characterizes hard steel. 510. Magnetic Induction in the Earth's Field. If a bar of soft iron having no permanent magnetism is placed in the earth's field parallel to the lines of force, that is, in the direction of the dipping needle, its lower end in north latitudes will become a north pole and its upper end a south pole, as may be shown by a magnetic needle. If jarred by the blow of a hammer while in this position it will be found permanently magnetized. If, however, it is placed at right angles to the lines of force of the earth it is scarcely magnetized at all (§508). In consequence of induction iron ships are magnetized by the earth differently when pointing in different directions. In such vessels the standard compass is usually compensated by having soft-iron bars so placed near it that the magnetism induced in them will in every position just balance that induced in the ship, while permanent steel magnets may be used to compensate the permanent magnetism of the ship. 511. Hysteresis.-When the magnetic field in which a mass of iron is placed is varied in strength, the changes in the magnetism of the iron lag behind the changes in the field. This is known as hysteresis and is discussed in connection with the magnetization of iron by electric currents, §683. PERMEABILITY, DIAMAGNETISM, AND INFLUENCE OF MEDIUM. S B N C 512. Magnetic Substances Attracted.-When a fragment of iron is placed in a magnetic field it experiences a force in that direction in which the strength of the field increases most rapidly. If at A (Fig. 278) it is drawn directly toward A the magnet in the direction of the lines of force. If at B it is drawn toward the magnet at right angles to If at C it will be drawn in a direction oblique to the line of force somewhat as shown. If it is in a uniform field, as in the earth's magnetic field, or is at a point in a magnetic field where the force is a maximum or a minimum, it will be in FIG. 278. the lines of force. equilibrium and have no tendency to move in any direction. Such a point of equilibrium would be found midway between two equal and similar poles. If the fragment is long in shape it will turn and point in the direction of the line of force, but it will not always tend to move along that line. Any substance whose permeability is greater than vacuum will act in this way in a vacuum and such are known as paramagnetic or simply magnetic substances. 513. Diamagnetic Substances.-Faraday (1845) experimented on the behavior of a great variety of substances in the intense. field between the poles of a powerful electromagnet. A little oblong of pure copper when suspended by a fine fiber in this field was found to set itself at right angles to the lines of force, as shown in figure 279. So also fragments of wood, paper, aluminum, bismuth, glass, and many other substances. These substances Faraday called diamagnetic. Substances like nickel, cobalt, and manganese which behave like iron, setting themselves in the direction of the lines of force, he called paramagnetic or magnetic. FIG. 279.-Bismuth in magnetic field. Diamagnetic substances when placed in a magnetic field are driven from a stronger field toward a weaker, the force acting on a fragment of such a substance being in the direction in which the strength of the field diminishes most rapidly. This may be well shown in the following way. A ball of bismuth, which is the most strongly diamagnetic substance known, is suspended between the poles of a powerful electromagnet, being hung from one end of a light arm of wood which is itself supported in horizontal position by a delicate bifilar suspension, so that the slightest force will cause the arm to swing around carrying the ball out of the magnetic field. If while the ball hangs between the two poles the current is applied to the electromagnet, the bismuth ball will at once be driven aside out of the intense field. The setting of the diamagnetic bars across the lines of force described at the beginning of this section finds its explanation in the preceding experiment; for the field of force between the magnet poles is most intense next the poles as is shown by the crowding together of the lines of force, and so the ends of the bar are in a much less intense field when the bar stands across the lines of force than if it were to be directed along them; it therefore assumes the former position. 514. Influence of the Medium.-By the following interesting experiment Faraday showed that the medium surrounding a body in a magnetic field plays an important part in determining the magnetic force upon it. When a thin-walled glass capsule, long in shape, is filled with a weak solution of ferric chloride and suspended between the poles of a magnet, it sets itself along the lines of force showing that the ferric chloride is magnetic. This happens whether the capsule is hung in air or water. If, however, it is surrounded by a solution of ferric chloride stronger than that within the capsule it will act as if diamagnetic, placing its length across the lines of force. 515. Permeability of Magnetic and Diamagnetic Substance.When the permeability of a substance is greater than that of FIG. 280.-Permeability of ball greater than that of medium. FIG. 281.-Permeability of ball less than that of medium. the surrounding medium, the lines of force are drawn in toward the substance, as already discussed in §508 and as shown in figure 280 which represents the disturbing effect of a ball of substance whose permeability is greater than that of the medium around it. If, however, the permeability of the ball is less than that of the medium, the lines of force will be spread, as shown in figure 281. A magnetic needle placed near the ball will point aside instead of toward it. In the first case if the ball is in a field that is not uriform, as near the pole of a magnet, it will be attracted or drawn toward the stronger field. If, however, the ball has a permeability less than the surrounding medium, it will be driven away from the pole toward a weaker field. Magnetic or paramagnetic substances may then be defined as those whose permeability is greater than that of vacuum, while those whose permeability is less than vacuum are diamagnetic. 516. Magnetism of Gases.-Faraday also studied the magnetic qualities of different gases. Oxygen gas was found to be attracted toward the poles, while hydrogen was repelled. Oxygen was thus shown to be more permeable than air. Later experiments have shown liquid oxygen to be decidedly magnetic. 517. Magnetic Alloy. In 1903 Heusler made the very interesting discovery that an alloy of 25 parts manganese, 14 aluminum, and 61 copper, had decided magnetic properties, although none of the substances of which it is made is magnetic except in the very slightest degree. It seems to indicate that magnetism depends upon molecular rather than atomic structure. The permeability of this alloy has been found to be nearly 33. 518. Effects of Heat on the Magnetism of Metals and Magnets. The permeability of iron and nickel diminishes as the temperature rises. At 737° C. iron ceases to be magnetic. A small piece of iron heated to a bright red heat is not attracted even by a powerful magnet, but as it cools to 700° it again becomes strongly magnetic. A steel magnet when heated to bright red heat loses all trace of magnetism, and if cooled while away from magnetic influence will be found completely demagnetized. Even when a magnet is slightly heated, say to 100° C., it is not as strong as at lower temperatures. 519. Force with which a Magnet Attracts its Armature.— The force with which a magnet attracts its armature evidently depends on the fact that the permeability of the armature is greater than that of the surrounding medium. If there were no difference between them there would be no change in the lines of force on withdrawing the armature and consequently no attractive force. When the armature is of such a size that most of the lines of force from one pole to the other pass through it, the force of attraction is given very nearly by the formula |