For horse-shoe magnets, Hoffer's method is generally followed. The inducina mag met is placed vertically on the inagnet to be formed, and moved from the ends qo the lbend, or in the opposite way, and brought round again, in an arch, to the starting-point. _A soft iron armature is placed at the poles of the induced magnet. That the operation .-may succeed well, it is necessary for both magnets to be of the same width. The same method rnay also be followed for magnetizing bars. The bars with the armatures are placed so as to form a rectangle; and the horseshoe-magnet is made to glide along both in the way just described.
Magnetization by the Earth. —The inductive action of terrestrial magnetism is a strik ing proof of the truth of the theory already referred to, that the earth itself is a mag .net. When a steel rod is held in a position parallel to the dipping-needle (q.v.), it 'becomes in the course of time permanently magnetic. This result is reached sooner when the bar is rubbed with a piece of soft iron. A bar of soft iron held in the same position is more powerfully but only temporarily affected, and when reversed, the poles are not reversed with the bar, but remain as before. If when so _Ad it receive at its end a few sharp blows of a hammer, the rnagnetism is rendered nermanent, and now the poles are reversed when the bar is reversed. The torsion caused by the blows of the hammer appears to communicate to the bar a coercitive force. We may- understand from this how the tools in work-shops are generally magnetic. Whenever large masses of iron are stationary for any length of time they are sure to give evidence of magneti zation, and it is to the inductive action of the earth's poles acting through ages that the magnetism of the loadstone is to be attributed.
Preservation and Power of Magnets.—Magnets, when freshly magnetized, are some 'times more powerful than they afterwards become. In that case they gradually fall off In strength till they reach a point at which their strength remains constant. This is oalled the point of saturation. If a magnet has not been raised to this point, it will lose mottling after magnetization. We may ascertain whether a magnet is at saturation by magnetizing it with a more powerful magnet, and seeing whether it retains more mag netism than before. The saturation point depends on the coercitive force of the magnet, and not on the power of the magnet with which it is rubbed. When a magnet is above .saturation, it is soon reduced to it by repeatedly drawing away the armature from it. After reaching this point, magnets will keep the same strength for years together if not :subjected to rough usage. It is favorable for the preservation of magnets that they be
provided with an armature or keeper. For further information, see article ARMATURE. The power of a horse-shoe magnet is usually tested by the weight its armature can bear without breaking away from the magnet. Hacker gives the following formula for this weight: 117=a Vmg ; 117 is the charge expressed in pounds; a, a constant to be ascertained for a particular quality of steel; and nt, is the weight in pounds of the magnet. He found, in the magnets that he constructed, a to be 12.6. According to this value, a mag net weighing 2 oz. sustains a weight of ,S lbs. 2'oz., or 25 times its own weight; whereas a magnet of 100 lbs. sustains only 271 lbs., or rather less than 3 times its own weight. 'Small magnets, therefore, are stronger for their size than large ones. The reason of this may be thus explained: TWO magnets of the same size and power, acting separately, support twice the weight that one of them does; but if the two be joined, so as to form one magnet, they do not sustain the double, for the two magnets being in close proxim ity, act inductively on each other, and so lessen the conjoint power. Similarly, several magnets made up into a battery have not a force proportionate to their number. Large magnets in the same way may be considered as made up of several laminm, interfering mutually with each other, and rendering the action of the whole very much less than the sum of the powers of each. The best method of ascertaining the strength of bar magnets is to cause a magnetic needle to oscillate at a given distance from one of their poles, the axis of the needle and the pole of the magnet being in the magnetic meridian. These oscillations observe the law of pendulum motion, so that the force tending to 'bring the needle to rest is proportionate to the square of the number of oscillations in a stated time.
Action of Magnets on each °there—Coulomb discovered, by the oscillation of the mag netic needle in the presence of magnets in the way just described, that when magnets are so placed that two adjoining poles may act on each, other without the interference of the opposite poles, that is, when the magnets are large compared with the distance between their -centers, their attractive or repulsive force varies inversely as the square of the distance. ,Gauss proved from this theoretically, and exhibited experimentally, that when the dis tance between the centers of two magnets is large compared with the size of the mag nets, that is, when the action of boa poles comes into play, their action on each, other varies 'inversely as the cube of the distance.