Permanent magnet

“ The permanent steel magnet serves a double purpose. When the telephone was first used commercially, the instrument now used as a receiver was also used as a transmitter. As a transmitter it is a dynamo-electric machine. Every time the iron diaphragm is moved in the magnetic field of the pole of the permanent magnet, which in this case is the free end of the spool (the iron of the spool being magnetic by contact with the permanent magnet) , there is a current set up in the wire wound on the spool; a short impulse, lasting only as long as the movement lasts. ”

“ When it begins to turn around, principally, but also before, it is affected by a magnet, and at a certain stage it is susceptible to magnetic influence to an astonishing degree. A small permanent magnet, with its poles at a distance of no more than two centimetres will affect it visibly at a distance of two metres, slowing down or accelerating the rotation according to how it is held relatively to the brush. ”

“ The Horseshoe Magnet, H M (Fig. 2) , is the most popular form of small magnets. The red paint has nothing to do with the magnetism. The piece, A, is called its armature, and is made of soft iron, while the magnet itself should be made of the best steel, properly hardened. The armature should always be in place when the magnet is not in use, and care should be taken to thoroughly clean the ends of the magnet before replacing the armature. The horseshoe magnet is artificial, and it is called a permanent magnet, because it retains its strength for a long time, if properly cared for. ”

“ If an insulated wire is wrapped round a cylinder of soft or unmagnetic iron, and a current passed through the wire, the cylinder is converted into a magnet and becomes able to sustain weights. If the current ceases, the cylinder is no longer a magnet, and drops the weight. A magnet is therefore evidently a substance in which electric currents are circulating at right angles to its axis, and a permanent magnet is one in which such currents permanently circulate from the constitution of the body without being supplied from without. ”

“ To imitate with an electro magnet as nearly as possible a permanent magnet, so that the former can be used to replace the latter, it is necessary that the magnetism in the iron core should remain constant. This could, of course, be done by exciting the electro magnet with a constant current from a separate source. ”

“ Permanent Magnets.—A permanent magnet is a piece of steel in which an electric force is exerted at all times. An electro-magnet is a piece of iron which is magnetized by a winding of wire, and the magnet is energized only while a current of electricity is passing through the wire. ”

“ 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. ”

“ Subsequently Henry and Ten Eyk, in America, constructed some electro-magnets on a large scale. One horse-shoe magnet made by them, weighing 60 lbs., would support more than 2000 lbs.
In September 1858, there were constructed for the Atlantic-telegraph cable at Valentia two permanent magnets, from which the electric induction is obtained: each is composed of 30 horse-shoe magnets, 21⁄2 feet long and from 4 to 5 inches broad; the induction coils attached to these each contain six miles of wire, and a shock from them, if passed through the human body, would be sufficient to destroy life. ”

“ They were affected by the variations which occur in the intensity of the earth’s magnetism; and no matter how carefully a needle was made, it varied in strength from week to week, from year to year; again, a current might be so strong as to create magnetism overwhelming in comparison with that of the earth, and quite beyond the measuring power of a compass needle. A galvanometer on a plan due to Professor James Clerk Maxwell, employs a permanent magnet, or an electro-magnet, which is stationary, between the poles of which may freely turn a coil bearing the current to be measured. ”

“ 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. ”

“ Now when either feeble oscillations, which have been rectified by a detector, or small currents from a B battery, flow through the magnet coils the permanent steel magnet is energized to a greater extent than when no current is flowing through it. This added magnetic energy makes the magnet attract the diaphragm more than it would do by its own force. ”

“ 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. ”

“ The depression of a second key causes the first key to be raised. The currents actuate a ratchet-wheel mechanism at the receiving station, whereby the hand on a small dial is moved on letter by letter. A noticeable feature in the modern A B C indicator, as well as in all modern forms of telegraph instruments, is the adoption of “induced” magnets in the moving portion of the apparatus. A small permanent magnet is always liable to become demagnetized, or have its polarity reversed by the action of lightning. ”

“ It is evident that when we use permanent magnets or springs as directing forces in measuring instruments, we cannot count upon an absolute constancy in the indications, as the magnetism of the magnetized pieces, or the tension of the springs, modifies in time. The apparatus require to be regulated from time to time, and hence the idea of substituting electro-magnets for permanent ones.
Fig. 1. ”

“ It is evident, from the last 3 experiments, that lines of force will pass through iron whenever possible, on their way from the N to the S pole of a magnet. When the armature of a horseshoe magnet is in place, most of the lines of magnetic induction crowd together and pass through it rather than push their way through the air. Air is not a good conductor of lines of force; and the magnet has to do work to overcome the resistance of the air, when the armature is removed, in order to complete the magnetic circuit. This work causes a magnet to become gradually weaker. ”

“ In the section devoted to “Miscellaneous Observations,” he remarks that the magnetic power may almost be said to be created by friction, rather than communicated by it; for a magnet acquires strength by giving magnetism to iron; so that, if all the magnets in the world were lost, magnetism might be revived by rubbing the end of one steel bar against the side of another. ”

“ If the brushes be given positive lead, that is, placed in advance of the neutral plane in the direction of rotation, the cross magnetizing force is converted into one that tends to increase that of the field magnet, while if they be given negative lead, it tends to demagnetize the field magnet. ”

“ Therein he shows, as the result of his customary careful experimental explorations that the magnetism of every known substance (even tissues of the human frame) is manifested in one of two ways. Either the body is, like iron, attracted by the magnet, taking a position coincident with the magnetic forces which he calls paramagnetic (para beside or near, magnetes, magnes, magnet) or bodies—like bismuth, for instance—are repelled by the poles and should therefore be called diamagnetic (dia, across) for they set themselves across, equatorially, or at right angles to the magnetic lines. ”

“ It is found that if a body be moved before the poles of a magnet in such a direction as to cut the lines of force, electricity is excited in that body, and also around the magnet. The ordinary magneto-electric machines of the shops are illustrations of the application of this property of magnets. They consist essentially in a horse-shoe [Pg 15] magnet, in front of which is caused to rotate, by means of appropriate gearing, or wheel and band, an iron bobbin, or pair of bobbins, coiled with wire. ”

“ There are numberless experiments which might be given to further exhibit the relation of mass motion to magnetism, but a single one more must suffice. No rotation of a magnet upon its own axis can produce any effects upon a current that is exterior to it; but if a loop of wire be kept stationary adjacent to a magnet, as in Fig. 5, while the magnet revolves, a current of electricity is produced; and if the magnet be kept stationary, and the loop revolves, a current will also be produced, but in the opposite direction. ”

“ It will be remembered that if a bar magnet is suddenly plunged into a hollow coil of wire, a momentary electric current will be generated in the coil. The current is easily detected by means of an instrument called a galvanometer. The space in the vicinity of a magnet is filled with a peculiar invisible force called magnetism. The magnetism flows along a certain path, passing through the magnet itself and then spreading out in curved lines. ”

“ If the apparatus is powerful, and the distance small, the declination will amount to an angle of forty-five degrees or more; but this deviation does not give an exact idea of the real effect which may be produced by galvanism; for the motion of the needle is counteracted by the magnetism of the Earth. When the influence of this latter power is destroyed by means of another magnet, the needle will place itself directly across the connecting wire: so that the real tendency of a magnet is to stand at right angles to an electric current. ”