MAGNETISM. magnet is disturbed, both poles return to their original positions. Here, then, is a striking dissimilarity in the poles, by means of which we are enabled to distinguish them as north pole and south pole. When thus suspended, let us now try the effect of another magnet upon it, and we shall find that the pole of the suspended magnet that is attracted by one of the poles of the second magnet is repelled by the other, and vice versa; and where the one pole attracts, the other repels. If, now, the second magnet be hung like the first, it will be found that the pole which attracted the north pole of the first magnet is a south pole, and that the pole which repelled it is a north pole. We thus learn, that each magnet has two poles, the one a north, and the other a south pole, alike in their power of attracting soft iron, but differing in their action on the poles of another magnet, like poles repelling, and unlike poles attracting, each other. It might be thought that, by dividing a magnet at its centre, the two poles could be insulated, the one half containing all the north polar magnetism, and the other the south. When this is done, however, both halves become separate magnets, with two poles in each-the original north and south poles stauling in the same relation to the other two poles called into existence by the separation. We can therefore never have one kind of magnetism without having it associated in the same magnet with the same amount of the opposite magnetism. It is this double manifestation of force which constitutes the polarity of the magnet. The fact of the freely suspended magnet taking up a fixed position, has led to the theory, that the earth itself is a huge magnet, having its north and south magnetic poles in the neighbourhood of the poles of the axis of rotation, and that the magnetic needle or suspended magnet turns to them as it does to those of a neighbouring magnet. All the manifestations of terrestrial magnetism give decided confirmation of this theory. It is on this view that the French call the north pole of the magnet the south pole (pole austral), and the south the north pole (pole horéal); for if the earth be taken as the standard, its north magnetic pole must attract the south pole of other magnets, and vice versa. In England and Germany, the north pole of a magnet is the one which, when freely suspended, points to the north, and no reference is made to its relation to the magnetism of the earth. Form of Magnets.-Artificial magnets are either bar-magnets or horseshoe-magnets. When powerful magnets are to be made, several thin bars are placed side by side with their poles lying in the same way. They end in a piece of iron, to which they are bound by a brass screw or frame. Three or four of these may be put up into the bundle, and these again into bundles of three and four (fig. 4). Such a N Fig. 4. collection of magnets is called a magnetic magazine or battery. A magnet of this kind is more powerful than a solid one of the same weight and size, because thin bars can be more strongly and regularly magnetised than thick ones. Fig. 7 is a horseshoemagnetic magazine. The central lamina protrudes slightly beyond the other, and it is to it that the armature is attached, the whole action of the magnet being concentrated on the projection. A natural magnet is shewn in fig. 5. It is a parallelopiped of magnetic iron ore, with pieces of soft iron, NN and magnetic needle; and if its lower end, s, be dipped a regular magnet, as may be seen by using a small into iron filings, it attracts them as a magnet would do. When it is taken away from NS, the filings fall off, and all trace of magnetism disappears. It need not be in actual contact to shew magnetic properties; when it is simply brought near, the same thing is seen, though to a less extent. If the inducing magnet be strong enough, the induced magnet, ns, when in contact, can induce a bar like itself, placed at its extremity, to become a magnet; and this second induced magnet may transmit the magnetism to a third, and so on, the action being, however, weaker each time. If a steel bar be used for this experiment, a singular difference is observed in its action; it is only after some time that it begins to exhibit magnetic properties, and, when exhibited, they are feebler than in the soft iron bar. When the steel bar is removed, it does not part instantly with its magnetism, as the soft iron bar, but retains it permanently. Steel, therefore, has a force which, in the first instance, resists the assumption of magnetism; and, when assumed, resists its withdrawal. This is called the coercitive force. The harder the temper of the steel, the more is the coercitive force developed in it. It is this force also, in the loadstone, which enables it to retain its magnetism. Fig. 7. H Magnetisation. By Single Touch (Fr. simple touche, Ger. einfacher Strich).-The steel bar to be magnetised MAGNETISM. The same is laid on a table, and the pole of a powerful an arch, to the starting-point. A soft iron armature magnet is rubbed a few times along its length, is placed at the poles of the induced magnet. That always in the same direction. If the magnetising the operation may succeed well, it is necessary for pole be north, the end of the bar it first touches both magnets to be of the same width. each time becomes also north, and the one where method may also be followed for magnetising bars. it is lifted south. The same thing may be done by The bars (fig. 10) NS and N'S', with the armatures putting, say, the north magnetising pole first on the ab and cd, are placed so as to form a rectangle; and middle of the bar, then giving it a few passes from the horseshoe-magnet is made to glide along both the middle to the end, returning always in an arch in the way just described. from the end to the middle. After doing the same to the other half with the south pole, the magnetisation is complete. The first end rubbed becomes the south, and the other the north pole of the new magnet.-By Divided Touch (Fr. touche séparée, Ger. getrennter Strich). This method is shewn in fig. 8. The bar ns to be magnetised is placed on a piece of wood W, with its ends abutting on the extremities of two powerful magnets NS and SN. Two rubbing magnets are placed with their poles together on the middle of ns, inclined at an angle rather less than Fig. 9. Magnetisation by the Earth.-The inductive action of terrestrial magnetism is a striking proof of the truth of the theory already referred to, that the earth itself is a magnet. 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 held it receive at its end a few sharp blows of a hammer, the magnetism is rendered permanent, 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 workshops are generally magnetic. Whenever large masses of iron are stationary for any length of time, they are sure to give evidence of magnetisation, 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. For The Preservation and Power of Magnets.-Magnets, when freshly magnetised, are sometimes 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 called the point of saturation. If a magnet has not been raised to this point, it will lose nothing after magnetisation. We may ascertain whether a magnet is at saturation by magnetising it with a more powerful magnet, and seeing whether it retains more magnetism 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, 30 with it. They are then simultaneously moved magnets will keep the same strength for years away from each other to the ends of ns, and brought together if not subjected to rough usage. It is back in an arch again to the middle. After this favourable for the preservation of magnets that they is repeated a few times, the bar ns is fully magnet- be provided with an armature or keeper. ised. The disposition of the poles is shewn in the further information, see article ARMATURE. figure by the letters N or n, power of a horseshoe-magnet is usually tested by meaning a north, and S or 8, the weight its armature can bear without breaking a south pole. This method away from the magnet. Häcker gives the following communicates a very regular formula for this weight: W = a/m2; W is the magnetism, and is employed charge expressed in pounds; u, a constant to be for magnetic needles, or ascertained for a particular quality of steel; and where accuracy is needed.- m is the weight in pounds of the magnet. He The magnetisation by Double found, in the magnets that he constructed, a to be Touch is of less practical 126. According to this value, a magnet weighing importance, and need not 2 oz. sustains a weight of 3 lbs. 2 oz., or 25 times here be described. It com- its own weight; whereas a magnet of 100 lbs. municates a powerful, but sustains only 271 lbs., or rather less than 3 times sometimes irregular mag- its own weight. Small magnets, therefore, are netism, giving rise to con- stronger for their size than large ones. The reason secutive poles that is, to of this may be thus explained. Two magnets of more poles than two in the the same size and power, acting separately, support magnet. 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 proximity, 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 For horseshoe magnets, Hoffer's method is generally Fig. 10. followed. The inducing magnet (fig. 9) is placed vertically on the magnet to be formed, and moved from the ends to the bend, or in the opposite way, and brought round again, in MAGNETISM. same way may be considered as made up of several lanne, 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. S Fig. 13. Artion of Magnets on each other.-Coulomb may be taken as a substitute for one of the rings above spoken of. In helix (fig. 13), the current, discovered, by the oscillation of the magnetic after entering, goes from right to left (contrary to needle in the presence of magnets in the way the hands of a watch), and it is hence called leftjust described, that when magnets are so placed that two adjoining poles may act on each other without handed; in fig. 14, it goes with the hands of a the interference of the opposite poles, that is, when the magnets are large compared with the distance between their centres, their attractive or repulsive force varies inversely as the square of the distance. Gauss proved from this theoretically, and exhibited experimentally, that when the distance between the centres of two magnets is large compared with the size of the magnets, that is, when the action of both poles comes into play, their action on each other varies inversely as the cube of the distance. Effect of Heat on Magnets. -When a magnet is heated to redness, it loses permanently every trace of magnetism; iron, also, at a red heat, ceases to be attracted by the magnet. At temperatures below red heat, the magnet parts with some of its power, the loss increasing with the temperature. The temperatures at which other substances affected by the magnet lose their magnetism differ from that of iron. Cobalt remains magnetic at the highest temperatures, and nickel loses this property at 662 F. Ampere's Theory of Magnetism.-This theory forms the link between magnetism and galvanic electricity, and gives a simple explanation of the phenomena of electro-magnetism and magnetoelectricity. We shall therefore preface the short discussion of these two subjects by a reference to it. Ampere considers that every particle of a magnet has closed currents circulating about it in the same direction. A section of a magnet according to this theory is shewn in fig. 11. All the separate currents in the various particles may, however, be considered Fig. 14. 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 shewn 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. 13), 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. 14), 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 estab lished, 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 d Fig. 11. Fig. 12. to be equivalent to one strong current circulating round the whole (fig. 12). We are to look upon a magnet, then, as a system, so to speak, of rings or rectangles, placed side by side, so as to form a cylinder or prism, in each of which a current in the same direction is circulating. Before magnetisation, the currents run in different directions, so that their effect as a system is lost, and the effect of induction is to bring them to run in the same direction. The perfection of magnetisation is to render the various currents parallel to each other. Soft iron, in consequence of its offering no resistance to such a disposition, becomes more powerfully magnetic under Fig. 15. currents on each other. It is found that when two currents are free to move, they endeavour 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. 15 is intended to prove this. The rectangle cdef is movable round the pins, a and b, resting on two mercury cups. MAGNETISM. The arrangement is such that while the rectangle edef 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 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 shew 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. 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 shewn in fig. 16, where a section of a magnet is represented in its position of rest with reference to the earth-current. The upper current being further away from the earthcurrent, 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 shewn by the rectangle in fig. 15, which comes to rest when d and e lie east and west. Electro-magnetism includes all phenomena in which an electric current produces magnetism. The most important result (fig. 17) generally of a is soon reached beyond which additional turns of the wire give no additional magnetismn; 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 horseshoe-magnet, where the inductive action in the armature must be taken into account, that 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 magnetism, or to reverse its poles, is of the utmost value in electrical and mechanical contrivances. The 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 electromagnet 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. 18 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 Fig. 18. ture with a sharp to the ring c, encircling a projection on the axis. click. When the cur- The ring has two slits in it, dividing it into two rent is stopped, this halves, and filled with a non-conducting material, Fig. 17. power disappears as so that the halves are insulated from each other. suddenly as it came. Pressing on this broken ring, on opposite sides, are Electro-magnets far outrival permanent magnets in two springs, a and b, which proceed from the two strength. Small electro-magnets have been made binding-screws into which the wires, + and by Joule which support 3500 times their own from the battery are fixed. In the position shewn weight, a feat immeasurably superior to anything in the figure, the current is supposed to pass along performed by steel magnets. When the current a, to the half of the ring in connection with the end is of moderate strength, and the iron core more f, of the coil, to go through the coil, to pass by g to 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 264 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, MAGNETISM. 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 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 ariaily. 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 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 shewn, that if, while the primary coil is stationary, the strength of its current be increased or diminished, each increase and diminution induce 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-magnet 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 strengthen ing 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 magnetoelectric machines. The manner in which this is taken advantage of, will be easily understood by reference to fig. 19. 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 CD leave NS, 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 NS. 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 construc tion of a simple magneto-electric machine is shewn in fig. 20. NS is a fixed permanent magnet. BB is a soft iron plate, to which are attached two cylinders of soft iron, round which the coils C and NS is a permanent D are wound. CBBD is thus the revolving armahorseshoe magnet, ture, corresponding to CD in fig. 19. AA is a and let us suppose brass rod rigidly connected with the armature, and it to be fixed; CD also serving as the rotating axle. F is a cylinis a bar of soft iron, drical projection on AA, and is pressed upon by two with coils A and B fork-like springs, H and K, which are also the poles wound round its of the machine. The ends, m, n, of the coil are extremities, and may soldered to two metal rings on F, insulated from be looked upon as the armature of the magnet. CD each other. When the armature revolves, AA and F is capable of rotation round the axis EF. So long move with it. F, H, and K are so constructed as to as CD remains in the position indicated in the act as a commutator, reversing the current at each Fig. 19. |