According to Ampere's theory, the earth, being a magnet, has currents circulating about it, which, according to his rule, must be from east to west, the north pole of the earth being,in our way of speaking, a south pole. A magnet, then, will Dot come to rest till the currents moving below it place themselves parallel to 1 and in the direction of the earth's currents. Tbis is / shown in fig. 10, where a section of a magnet is repre sented in its position of rest with reference to the earth-current. The upper current being further away from the earth-current, is less affected by it, and it is the lower current that determines the position. A mag-netic needle, therefore, turns towards the north to allow the currents moving below it to place themselves parallel to the earth's current. This also is shown by the rectangle in fig. 9, which cornea to rest when d and e lie east and west.
includes all phenomena in which an electric current produces magnetism. The most important result of this power of the current is the electro magnet. This consists (fig. 11) generally of a round bar of soft iron bent into the horse shoe form, with an insulated wire coiled round its extremities. When a current passes through the coil, the soft iron bar becomes instantly magnetic, and attracts the armature -with a sharp click. When the current is stopped, this power disappears as suddenly as it came. Electro-magnets far outrival permanent magnets in strength. Small electro magnets have been made by Joule which support 3,500 times their own weight, a feat immeasurably superior to anything performed by steel magnets. When the current is of moderate strength, and the iron core more than a third of an inch in diameter, the magnet ISM induced is in proportion to the strength of the current and of the number of turns in th,e coil. When the bar is thinner than one-third of an inch, a maximum is soon reached beyond which additional turns of the wire give no additional magnetism; and even when the core is thick, these turns must not be heaped on each ,other, so as to place them 'beyond influencing the core. It follows from the above principle, that, in the horse-shoe magnet, where the inductive action in the armature rnust be taken into account, the weight which. the magnet sustains i-s in proportion to the squares of the strengths of the cur rents, and to the squares of the number of turns of the wire. This maximum is in different magnets proportional to the area of section, or to the square of the diameter of the core. 'The electro-rnagnet, from the ease with which it is made tO assume or lay aside its mag metisin, or to reverse its poles, is of the utmost value in electrical and mechanical contriv ances. The action of the electro-magnet is quite in keeping with Ampere's theory, as
the current of the coil, actin& on the various currents of the individual molecules, places them parallel to itself, in which condition the soft iron bar acts powerfully as a magnet. 'The direction of the current and the nature of the coil being known, the poles are easily determined by Ampere's rule.
ilachines. —These take advantage of the facility with which the poles .of an electro-magnet may be reversed, by which attractions and repulsions may be so arranged with another magnet as to produce a constant rotation. The forms in which they occur are exceedingly various, but the description of the apparatus in fig. 12 will suffice to illustrate their principle of working. N S is a fixed permanent magnet (it could be equally well an electro-magnet); the electro-magnet, ns, is fixed to the axis e e, and the ends of the coil are soldered to the ring c, encircling a projection on the axis. The ring ha,s two slits in it dividing it into two halves, and filled with a non-conducting material, so that the halves are insulated from each other. Pressing on this broken ring, on oppo site sides, are two springs, a and b, which proceed from the two binding-screws into which the wires, ± and —, from the battery are fixed. In the position shown in the figure, the current is supposed to pass along a, to the half of the ring in connection with the end f, of the coil, to go through the coil, to pass by g to the other .half of the ring, and to pass along b, in its ieturn to the battery. The magnetism induced by the curient in the electro-magnet,makes s a south, and n a north pole, by virtue of which N attracts 8, and S attracts n. By this double attraction, ns is brought into a line with N S, where it would remain, did not just then the springs pass to the other halves of the ring, and reverse the current, making s a north, and n a south pole. Repulsion between the like poles instantly ensues, and ns is driven onwards through a quarter-revolution, and then attraction as before between unlike poles takes it through another quarter, to place it once more axially. A perpetual rotation is in this way kept up. The manner in which a constant rotary motion may be obtained by electro-magnetism being understood, it is easy to conceive how it may be adapted to the discharge of regular work. Powerful machines of this kind have been made with a view to supplant the stearn-engine; but such attempts, both in respect of economy and constancy. have proved utter failures.