Magnetism.

of its offering no resistance to such a disposition, becomes more powerfully magnetic under induction than steel, where such resistance exists. Experiment very strongly confirms the truth of this theory. Helices of copper wire, in which a current is made to circulate, manifest all the properties of a magnet. Such are shown, in skeleton, in figs 7 and 8. Each convolution of the spiral may be taken as a substitute for one of the rings above spoken of. In helix fig. 7, the current, after entering, goes from right to left (contrary to the hands of a watch), and it is hence called left-handed; in fig. 8 it goes with the hands of a watch, and is right-handed. The extremities of both helices act on the magnetic needle like the poles of a magnet while the current passes. The poles are shown by the letters N and S, and this can be easily deduced from Ampere's rule (see GALVANISM), for, suppose the little figure of a man to be placed in any part of

the helix fig. 7, so that, while he looks towards the axis of the helix, the current enters by his feet, and leaves by his head, the north pole will be at his left hand, as shown in the figure. In the left-handed helix (fig. 8), the poles are reversed according to the same rule. If either of these helices be hung so as to be capable of horizontal motion, which, by a simple construction, can easily be done, as soon as the current is established, the north and south poles place themselves exactly as those of the magnetic needle would do; or, if they were hung so as to be able to move vertically in the magnetic meridian, they would take up the position of the dipping-needle (q. v.).

These movements can be still further explained by reference to the mutual action of electric currents on each other. It is found that when turo currents are free to more, they endeavor to place themselves parallel to each other, and to move in the same direction, and that currents running in the same direction attract, and those running in opposite directions repel. The apparatus fig. 9 is intended to prove this. The rectangle cdef is movable round the pins a and b, resting on two mercury cups. The arrangement is such that while the rectangle cdef is movable about its axis, a current can continue steadily to flow in it. Further description is unnecessary, the diagram explaining itself. If a wire in which a current passes downwards be placed vertically near cd, cd is attracted by it; but if the current pass upwards, it d is repelled, and ef attracted. Place, now, the wire below and parallel to de. If the current passes in the direction d to e. no change takes place, as the attraction cannot show itself; but if the current moves from e to d, the whole turns round till it stands where e was, and both currents run the same way. If the wire be placed at right angles to de, the rectangle turns round and comes to rest, when both currents are parallel, and in the same direction.

Fig. 9.

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 not come to rest till the currents moving below it place themselves parallel to

and in the direction of the earth's currents. This is shown in fig. 10, where a section of a magnet is represented in its position of rest with reference to the carth-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 magnetic 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 comes to rest when d and e lie east and west.

WEST

Fig. 10.

Electro-magnetism includes all phenomena in which an electric current produces magnetism. The most important result of this power of the current is the electromagnet. This consists (fig. 11) generally of a round bar of soft iron bent into the horseshoe form, with an insulated wire coiled round its extremities. When a current passes

Magnetism. 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 electromagnets 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 magnetism induced is in proportion to the strength of the current and of the number of turns in the 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 must be taken into account, the weight which the magnet sustains is in proportion to the squares of the strengths of the currents, 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-magnet, from the ease with which it is made to assume or lay aside its mag

ances.

netism, or to reverse its poles, is of the utmost value in electrical and mechanical contrivThe action of the electro magnet is quite in keeping with Ampere's theory, as the current of the coil, acting 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.

Electro-magnetic Machines.-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. NS is a fixed permanent magnet (it could be equally well an electro-magnet); the electro-magnet, ns, is fixed to the axis ee, and the ends of the coil are soldered to the ring c, encircling a projection on the axis. The ring has 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 opposite 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 return to the battery. The magnetism induced by the current 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 NS, 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 steam-engine; but such attempts, both in respect of economy and constancy, have proved utter failures.

Magneto-electricity includes all phenomena where magnetism gives rise to electricity. Under Induction of Electric Currents (q.v.), it is stated that when a coil in which a

Magneto.

current circulates is quickly placed within another coil unconnected with it, a contrary induced current in the outer coil marks its entrance, and when it is withdrawn, a direct induced current attends its withdrawal. While the primary coil remains stationary in the secondary coil, though the current continues to flow steadily in the primary, no current is induced in the secondary coil. It is also shown that if, while the primary coil is stationary, the strength of its current be increased or diminished, each increase and diminution induces opposite currents in the secondary coil. Change, in fact, whether in the position or current strength of the primary coil, induces currents in the secondary coil, and the intensity of the induced current is in proportion to the amount and suddenness of the change. In singular confirmation of Ampere's theory, a permanent bar maguet may be substituted for the primary coil in these experiments, and the same results obtained with greater intensity. When a bar magnet is introduced into the secondary coil, a current is indicated, and when it is withdrawn, a current in a contrary direction is observed, and these currents take place in the directions required by Ampere's theory. A change of position of the magnet is marked by a current, as in the former case. If we had the means of increasing or lessening the magnetism of the bar, currents would be induced the same as those obtained by strengthening or weakening the current in the primary coil. It is this inductive power of iron at the moment that a change takes place in its magnetism, that forms the basis of magneto-electric machines. The manner in which this is taken advantage of will be easily understood by reference

to fig. 3. NS is a permanent horse-shoe magnet, and let us suppose it to be fixed; CD is a bar of soft iron, with coils A and B wound round its extremities, and may be looked upon as the armature of the magnet. CD is capable of rotation round the axis E F. So long as C D remains in the position indicated in the figure, no currents are induced in the surrounding coils, for no change takes place in the magnetism induced in it by the action of NS. The moment that the poles of C D leave N S, the magnetism of the soft iron diminishes as its distance from NS increases; and when it stands at right angles to its former position, the magnetism has disappeared. During the first quarter-revolution, therefore, the magnetism of the soft iron diminishes, and this is attended in the coil (for both coils act, in fact, as one) by an electric current, which becomes manifest when the ends e, e, of the coil are joined by a conductor. During the second quarter-revolution, the magnetism of the armature increases till it reaches a maximum, when its poles are in a line with those of N S. A current also marks this increase. and proceeds in the same direction as before; for though the magnetism increases instead of diminishes, which of itself would reverse the induced current, the poles of the revolving armature, in consequence of their change of position with the poles of the permanent magnet, have also been reversed, and this double reversal leaves the current to move as before. For the second half-revolution the current also proceeds in one direction, but in the opposite way, corresponding to the reversed position of the armature. Thus, in one revolution of a soft iron armature in front of the poles of a permanent magnet, two currents are induced in the coils encircling it, in opposite directions, each lasting half a revolution, starting from the line joining the poles.

Magneto electric Machine.-The general construction of a simple magneto-electric machine is shown in fig. 14. NS is a fixed permanent magnet. B B is a soft iron plate, to which are attached two cylinders of soft iron, round which the coils C and D are

Magneto.

wound. CBBD is thus the revolving armature, corresponding to C D in fig. 13. AA is a brass rod rigidly connected with the armature, and also serving as the rotating axle. F is a cylindrical projection on A A, and is pressed upon by two fork-like springs, H and K, which are also the poles of the machine. The ends, m, n, of the coils are soldered to two metal rings on F, insulated from each other. When the armature revolves, A A and F move with it. F, H, and K are so constructed as to act as a commutator, reversing the current at each semi-revolution. By this arrangement, the opposite currents proceeding from the coil at each semi revolution are transmitted to H and K in the same direction, so that these, which constitute the poles of the battery, so to speak, remain always of the same name. When the armature is made to revolve with sufficient rapidity, a very energetic and steady current is generated. Of late years immense progress has been made in the construction of such machines. In 1866 Wilde of Manchester surprised the scientific world by a machine of unprecedented power; and more recently, Granime of Paris has constructed another still more astonishing. These are driven by steam-engines, and completely eclipse both in power and constancy the largest galvanic battery hitherto put together. See MAGNETO ELECTRIC MACHINE. See also ARMATURE, DECLINATION NEEDLE, DIAMAGNETISM, DIPPING NEEDLE, and ROTATION, MAGNET

ISM OF.

MAGNETISM, ANIMAL. See ANIMAL MAGNETISM, ante.

MAGNET O-ELECTRIC MACHINE (More recent forms of). Of late years, quite a new era has arisen in the construction of magneto-electric machines. The compactness. simplicity of construction, and marvelous power which the new machines possess, give them quite a novel importance in practical electricity. The names chiefly associated with the new improvements are Wilde of Manchester, Siemens and Wheatstone, and Gramme of Paris. Mr. H. Wilde, in 1866, patented a magneto-electric machine, founded on the principle that a current or a magnet indefinitely weak can be made to induce a current or a magnet of indefinite strength. A general description will show how this is proved and applied.

Wilde's original machine is shown in front elevation, fig. 1. It consists of two machines very similar to each other, the upper one M M', and the lower EE'. The upper and smaller machine consists of 16 permanent magnets, placed one behind the other. The front one only is seen. The poles of these are fixed at g, g (fig. 2), to what is termed the magnet cylinder. This consists of a hollow tube, made up of heavy masses of cast-iron, c, c, at each side, separated from each other by brass rods, b, b, the whole being knit firmly together, above and below, by brass bolts at r, r'. The cast-iron side pieces thus form the poles of the magnetic battery.

The

armature, which revolves within the tube of the magnet cylinder, is a long piece of soft iron, a a, and in section resembles an "H." In the hollows of the "H" the wire is turned longitudinally. This armature is shown separately in fig 3, part of the wooden tops which cover in the wire being removed to show how the wire is turned. This form of armature was first constructed by Siemens. The ends of the armature wire are soldered to two insulated iron rings, n, n' (fig. 3), against which the springs, 8,8 (fig. 1), press, which convey the current from the revolving armature: m is the pulley of the driving-belt. If the crossbar of the "H" stand upright (it lies horizontally in the figure), and the armature be turned round, while wires leading from the binding-screws, r, r' (fig. 1), are connected with a galvanometer, it will be found that the current induced by the motion is in the same direction till the cross is again upright, but inverted. If the motion be continued beyond that point, a current in the opposite direction will ensue, lasting till the cross-bar is in its first position. The right half of

Fig. 1.

Magneto.

the armature gives off always one kind of electricity, and the left the other. The right

Fig. 2.

and left springs, 8, s, are thus always Ike poles, for they change from n to n' (fig. 3) when the current in the armature changes. We come now to describe the singular peculiarity and merit of Wilde's machine. The current got from the magneto-electric machine is not directly made use of, but is employed to generate an electromagnet some hundreds of times more powerful than the magnetic battery originally employed, by means of which a corresponding increase of electricity may be obtained. This electro-magnet, E E' (fig. 1), forms the lower part of the figure, and by far the most bulky portion of the entire machine. It is of the horse-shoe form, E and E' forming the two limbs of it. The core of each of these, shown by the dotted lines, is formed by a plate of rolled iron, 36 in. in height, 26 in. in length, and 1 in. in thickness. Each is surrounded by a coil of insulated copper wire (No. 10), 1650 ft. long, wound round lengthwise in 7 layers. The current has thus, in passing from the insulated binding-screw r to the similar screw ', to make a circuit of 3,300 feet. Each limb of the electro-magnet is thus a flat reel of covered wire wrapped round a sheet of iron, the rounded ends alone of which are seen in the figure. The upright iron plates are joined above by a bridge, P, built up also of iron-plate, aud are fixed below the whole way along with the iron bars v, v to the sides of a magnet-cylinder of precisely the same construction as the one already described. The iron frame-work of the electro-magnet is shown by the dotted lines. The depth of the bridge is the same as the breadth of the bars v', v', which are of the same size as the bars v, v. The various surfaces of juncture in the frame-work are planed, so as to insure perfect metallic contact. The upper and lower machine are in action precisely alike, only the upper magnet is a permanent magnet, and the lower one an electro-magnet. We have the same magnet-cylinder, I, I, the same armature, A, and springs, S, S', and the same poles, Z, Z'; the size is, however, different; the caliber of the magnet-cylinder is 7 inches. The diameter of the lower armature gives the name to the machine-viz., a 7-inch machine. Figs. 3 and 3 are on the scale of the lower machine (fig. 1). The length of wire on the lower armature is 350 feet. It is 35 in. in length, and is made to rotate 1800 times a minute. The cross frame-work attached at g g to the magnet-cylinder, in which the front journal, f. of the armature rotates (at Q), is shown in the lower machine (fig. 1). When the machine is in action, both armatures are driven simultaneously by belts from the same countershaft. For the electric light, the currents conveyed to the springs, S and S', need not be sent in the same direction. In that case, the separation between n and n' is vertical; and each spring presses against only one ring during the whole revolu tion, receiving and transmitting each revolution two opposite currents. Oil for the journal and commutator is supplied from the cup C.

A Wilde's machine 1 ton in weight, measuring about 5 ft. in length and height, and 20 in. wide, driven by a steam engine produces a most brilliant electric light, and exhibits the most astonishing heating powers.

Wheatstone and Siemens gave a new interpretation to Wilde's principle. Their important discovery is of the following nature: Suppose the upper machine in fig. 1 removed, and that we have nothing but the electro-magnet and armature left. If the wires proceeding from the binding screws of the armature be joined up with the electro-magnet, we might fancy that, there being no permanent magnetismi, no result would follow on the armature being moved.

Such, however, is not the case. If the armature be moved at any velocity, it will soon be brought to a halt by the mutual action ensuing. In the electro-magnet there is always some magnetism left. This induces a feeble current in the coil, but this is sufficient to make the magnet stronger and able to induce a stronger current, and this reciprocal action continues until it grows to an enormous intensity. So great, indeed, would it become, that if we had sufficient mechanical energy at our disposal to persist in the motion, the coils of armature and electro-magnet would be melted, and the machine destroyed. This startling discovery may, however, be thought of little value, as a machine that consumes its own electricity is of no external use. All machines now

Fig. 3.