Magnetizer

“ Thus, although several facts seem to show that magnetizers (or mesmerizers) have frequently a strong power over the tables, yet one cannot admit the identity of magnetic power and power over the tables; the one is not the measure of the other. ”

“ The force in the interior is uniform, opposite to the direction of magnetization, and equal to 4/3πI. When it is desired to have a uniform magnet with definitely situated poles, it it usual to employ one having the form of an ovoid, or elongated ellipsoid of revolution, instead of a rectangular or cylindrical bar. If the magnetization is parallel to the major axis, and the lengths of the major and minor axes are 2a and 2c, the poles are situated at a distance equal to 2/3a from the centre, and the magnet will behave externally like a simple solenoid of length 4/3a. ”

“ There are very few boys who have not seen and played with the ordinary magnets, shaped like a horseshoe, which are sold in all toy-stores as well as by those who sell electrical goods.
Well, you know that these magnets will attract and hold fast anything that is made of iron or steel, but they have no effect on brass, copper, zinc, gold, or silver, yet there is nothing that you can see which should cause any such effect. You will notice, then, that magnetism is like electricity; we cannot see it, but we can tell that it exists, because it produces certain effects. ”

“ If the wire consists of a ferromagnetic metal, it will become “circularly” magnetized by the field, the lines of magnetization being, like the lines of force, concentric circles. So long as the wire (supposed isotropic) is free from torsional stress, there will be no external evidence of magnetism.
Magnetic Shielding.—The action of a hollow magnetized shell on a point inside it is always opposed to that of the external magnetizing force, [14] the resultant interior field being therefore weaker than the field outside. ”

“ That they oppose the magnetization is perfectly true, but if you carefully laminate the iron so as to eliminate eddy currents, you will find, strangely enough, that the magnetism rises still more slowly to its final value. For by laminating the iron you have virtually increased the self-inductive action, and increased the time constant of the circuit, so that the currents rise more slowly than before. The lag is not in the iron, but in the magnetizing current, and the current being retarded, the magnetization is of course retarded also. ”

“ Let a magnetic pole be drawn several times around a uniform steel ring, so that every part of the ring may be successively subjected to the magnetic force. If the operation has been skilfully performed the ring will have no poles and will not attract iron filings. Yet it will be magnetized; for if it is cut through and the cut ends are drawn apart, each end will be found to exhibit polarity. Again, a steel wire through which an electric current has been passed will be magnetized, but so long as it is free from stress it will give no evidence of magnetization ”

“ With suitable arrangements of iron and coil and a sufficiently strong current, the intensity of the temporary magnetization may be very high, and electromagnets capable of lifting weights of several tons are in daily use in engineering works (see Electromagnetism) . If the bar inserted into the coil is of hardened steel instead of iron, the magnetism will be less intense, but a larger proportion of it will be retained after the current has been cut off. Steel magnets of great strength and of any convenient form may be prepared either in this manner or by treatment with an electromagnet ”

“ In dynamo-machines the magnetization of the field is due to the currents generated by the machine itself. The soft-iron cores, after they have once been magnetized, always retain some residual magnetism which serves to induce a feeble current in the armature. A portion of this is sent through their coils, increasing their magnetization, which in turn augments the strength of the induced currents, and thus, by this successive action and reaction between the field and the armature, a very powerful magnetization of both is shortly produced. ”

“ The softer and more homogeneous is the iron, the more instantaneously will it acquire and lose its magnetism, and the greater strength of magnetism it is able to acquire. An iron bar, round which are wound a large number of turns of insulated or covered wire, constitutes an electro-magnet. Where the magneto and dynamo machines differ. The difference then between a magneto-electric and a dynamo-electric machine is, that in the former permanent magnets are used, and in the latter electro-magnets take their place. ”

“ The ordinary, permanent magnet, natural or artificial, has little place in the arts. It cannot be controlled. In common phrase, it cannot be made to "let go" at will. The greatest value of magnetism, as connected with electricity, consists in the fact of the intimate relationship of the two. A magnet may be made at will with the electric current, as described above. A little later we shall see how the process may be reversed, and the magnet be made to produce the most powerful current known, and yet owe its magnetism to the same current. ”

“ However, the iron in the earth is affected magnetically, as we have evidence in the loadstone. The earth has the power also to magnetize iron through the medium of its magnetic field, that reaches out in lines of force from pole to pole like those of the artificial magnet. If we hold a bar of iron in line with the magnetic axis of the earth and dip it in line with the dipping needle and then strike it a few blows on the end, it will be found to be feebly magnetic. ”

“ Now, although magnetism can not be collected, imprisoned, or discharged, like electricity, or collected at all, but by its adherence to some substance capable of magnetization, it is obvious there is an intimate association, at least, between it and electricity. They are never found alone. All electricity will magnetize. All magnetism will evolve electricity. All currents of electricity have encircling currents of magnetism, and all deflect the magnetic needle. All magnetic currents give out to intersecting wires, currents of electricity, and all magnets induce them. ”

“ Brass, copper, zinc, etc., are not visibly affected by a magnet. Magnetism will act through paper, glass, copper, lead, etc.
Fig. 24.
30. Making Magnets. One of the strangest properties that a magnet has is its power to give magnetism to another piece of steel. If a sewing-needle be properly rubbed upon one of the poles of a magnet, it will become strongly magnetized and will retain its magnetism for years. Strong permanent magnets are made with the aid of electromagnets. Any number of little magnets may be made from a horseshoe magnet without injuring it. ”

“ The poles of the magnet display the same phenomena as regards attraction and repulsion, as do the opposite kinds of electricity. If we suspend a magnet and bring the north pole of another to the north pole of the suspended magnet, the latter will turn away; but if we apply the north pole of one to the south pole of the other they will be attracted just as opposite electricities attract each other.
Magnetization is the term applied to the making of artificial magnets, which act is accomplished by bringing the needle in contact with other magnets, or sometimes by means of the electric current. ”

“ In all substances except iron, nickel, and cobalt, the coefficient of magnetization is so small that the induced magnetization of the body produces only a very slight alteration of the forces in the magnetic field. We may therefore assume, as a first approximation, that the actual magnetic force within the body is the same as if the body had not been there. The superficial magnetization of the body is therefore, as a first approximation, , where is the rate of increase of the magnetic potential due to the external magnet along a normal to the surface drawn inwards. ”

“ The effects of pulling stress may be observed either when the wire is stretched by a constant load while the magnetizing force is varied, or when the magnetizing force is kept constant while the load is varied. In the latter case the first application of stress is always attended by an increase—often a very great one—of the magnetization, whether the field is weak or strong, but after a load has been put on and taken off several times the changes of magnetization become cyclic. ”

“ We may remark that other bodies besides iron and steel are capable of magnetization; nickel and cobalt have the like property. Magnetism, properly so called, treats of certain bodies known as Magnets, describes the properties they possess, and the influence of magnetic force upon other substances. Electricity and magnetism are always associated, but practically the force is the same, electricity being the current or motive power, so to speak; and it is to Faraday that the world is indebted for the discovery of magneto-electricity. ”

“ Horse-shoe magnets should have a short bar of soft iron adapted to connect the two poles, and should never be laid by without such a piece of iron adhering to them. Bar magnets should be kept in pairs with their poles turned in contrary directions, and they should be kept from rust. Both kinds of magnets have their power not only preserved but increased, by keeping them surrounded with a mass of dry filings of soft iron, each particle of which will re-act by its induced magnetism upon the point of the magnet to which it adheres, and maintain in that point its primitive magnetic state. ”

“ On the application of a small magnetizing force to a bar of soft annealed iron, a certain intensity of magnetization is instantly produced; this, however, does not remain constant, but slowly increases for some seconds or even minutes, and may ultimately attain a value nearly twice as great as that observed immediately after the force was applied.30 When the magnetizing current is broken, the magnetization at once undergoes considerable diminution, then gradually falls to zero, and a similar sudden change followed by a slow one is observed when a feeble current is reversed. ”

“ The magnetization of a bar of steel or iron, according to this theory consists in establishing within the metal by induction a series of electrical currents, all revolving in the same direction at right angles to the axis or length of the bar.
Fig. 3
It was this theory which led Arago, as he states, to adopt the method of magnetizing sewing needles and pieces of steel wire, shown in Fig. 3. This method consists in transmitting a current of electricity through a helix surrounding the needle or wire to be magnetised. ”

“ Then the curve bends over, forming what is often called a “knee,” and a third stage is entered upon, during which a considerable increase of magnetizing force has little further effect upon the magnetization. When in this condition the metal is popularly said to be “saturated.” Under increasing magnetizing forces, greatly exceeding those comprised within the limits of the diagram, the magnetization does practically reach a limit, the maximum value being attained with a magnetizing force of less than 2000 for wrought iron and nickel, and less than 4000 for cast iron and cobalt. ”

“ Electromagnets with iron armatures in contact across their poles will retain, after the current has been cut off, a very large part of their magnetism, even if the cores be of the softest of iron. But so soon as the armature is wrenched off, the magnetism disappears. An air gap in a magnetic circuit always tends to hasten demagnetizing. A magnetic circuit composed of a long air path and a short iron path demagnetizes itself much more rapidly than one composed of a short air path and a long iron path. ”

“ When the load on a hardened wire is gradually increased, the maximum value of I is found to correspond with a greater stress than when the load is gradually diminished, this being an effect of hysteresis. Analogous changes are observed in the residual magnetization which remains after the wire has been subjected to fields of different strength. The effects of longitudinal pressure are opposite to those of traction; when the cyclic condition has been reached, pressure reduces the magnetization of iron in weak fields and increases it in strong fields (Ewing, Magnetic Induction, 1900, 223) . ”

“ If we use a piece of hardened steel instead of the soft iron it will become magnetic and remain so when the circuit is opened, because the natural tendency of the molecules to turn back to the neutral position is not great enough to overcome the coercive force, or molecular friction, of hardened steel, as has been also described in a previous chapter. To make the best electromagnet we need qualities of iron just the opposite from those of the permanent magnet. ”

“ Though soft iron takes the magnetic influence more readily than steel, it does not retain it as steel does, and the latter is therefore used in making artificial magnets. When a magnet imparts its magnetic influence it loses none of its own power, whether it be an original loadstone or an artificial magnet. There, are many ways of imparting magnetism permanently to steel, but I will notice only two of them. If you wish to magnetize a bar or needle pass one pole of a magnet from one end of it to the other a considerable number of times, always in the same direction. ”

“ If, on the other hand, the molecules of iron are not magnets, but are only capable of magnetization, the magnetization of the deposited filament will depend on the magnetizing force in the same way in which that of soft iron in general depends on it. ”