THE LODESTONE-STEEL MAGNETS

THE LODESTONE.

Ar the ancient city of Magnesia, in Asia Minor, are found considerable quantities of a mineral which has the peculiar property of attracting iron, and of adhering to it with considerable force. This mineral was well known to the Greeks, who were familiar with its most striking properties. The Greeks named the mineral " magnes," from the city of its origin, and the properties it possesses are hence called magnetic. The mineral is an iron ore giving, on analysis, the composition Fe3O4. It probably consists of protoxide of iron FeO (ferrous oxide) and peroxide, Fe,O, (ferric oxide), being possibly a kind of solution of one in the other. It is a lustrous dark grey substance, forming regular octohedral crystals of specific gravity 425. mineral is found in many other places, viz. in Germany, Sweden, Spain, and China; but it does not always exhibit the property of attracting or adhering to iron when found, though the property may be imparted to it. It is hence known as magnetic ore, whether found already possessing the attractive property or not. It is also known as black oxide of iron, and may be prepared in a small way by plunging red hot iron into water, when a black scale of magnetic oxide will be formed. It is better prepared by passing steam over iron turnings which have been placed in a tube and are heated to a red heat.

This

MAGNETISM-ELECTRO

The Greeks do not appear to have been familiar with an important property of the mineral, viz. the tendency of a piece of it to set itself so that its parts always occupy the same relative geographical position. This would only be apparent if it were suspended so as to be free to turn in a horizontal plane. For example, if a spot of black and a spot of white paint be put on two opposite parts of a specimen and the specimen be freely suspended, a line joining the two spots will always have the same geographical direction when the specimen comes to rest. specimen comes to rest. It might happen that the black spot was always due east of the white. This property does appear, however, to have been known to the Norwegians about the end of the eleventh century, and the stone was consequently called the "leidarstein" or guiding stone. The English word corresponding to and having the same meaning as this is "lodestone," and the native magnetised mineral is still known by this name.

A specimen of the stone does not exhibit equally at all parts its property of attracting iron objects, but is generally found to possess two points or regions of greatest activity in this respect.

STEEL MAGNETS.

One of the most striking properties of the lodestone is that in virtue of which it can impart its own properties to a bar of hard

steel. If a bar of steel be stroked along its length with one of those parts of a lodestone which show the strongest magnetic effects, the steel bar itself will thereafter behave as a lodestone. Care must be taken, if the bar is stroked more than once, to stroke the bar in one direction only, beginning the stroke at one extreme end of the steel bar and continuing it uniformly to the other extreme. The steel bar is then known as an artificial magnet, or, more commonly, as a "permanent magnet," since it retains its magnetic properties indefinitely if carefully handled. Bars of soft iron or mild steel may be similarly treated, but they do not exhibit the above magnetic properties so strongly, nor do they retain their magnetism so well as hard steel. A smart jar or blow is usually sufficient to remove all signs of their having been magnetised. A good steel bar, on the contrary, retains its "memory," if we may so call it, of having been touched by the lodestone well, and exhibits the lodestone properties very per fectly. Either end of it will attract and pick up small iron objects, such as nails, nibs, etc.; and, if suspended horizontally, and free to turn, it will oscillate to and fro about its suspension, until it finally comes to rest, with its length lying practically north and south. Since it is difficult to find for, so heavy an object, a suspension which is free from twist, the latter property of the bar magnet may be better observed by placing it on a large cork in a basin of water. It will be found that not only does the bar-magnet, when left to itself, always take up a position with its length running north and south, but that the same end always points south, and the other end, therefore, always north. The two ends are hence commonly known as the "south end" and "north end" respectively, but many text-books refer to them as the "north seeking" or "marked" end, and "south seeking" or "unmarked" end,

for reasons which will appear later on. In this work, however, the end that points north will be called north, and the end which points south will be called south, since this is simpler and confusion is not likely to arise.

In order to obtain powerful magnetic effects starting from a lodestone, it was usual to prepare a number of small steel magnets and to bind them together in a bundle. Having obtained such a "magazine" a magnet much stronger than the individual members of the magazine could be made by stroking a larger bar of hard steel with one end of the magazine, care being taken, as before, always to stroke in one direction, and not to rub to and fro. For a long time it was supposed that the harder the steel, the better it retained its magnetism, no matter what the shape or dimensions of the bar. If, however, a bar of steel of some thickness is hardened glass-hard, it will be found that the glass-hard portion is in reality a sort of crust or envelope, the innermost portions of the steel being of a softer temper. This has the result that a thick bar does not make so strong a magnet as might be expected, and in order, therefore, to produce very strong permanent magnets, the best plan is to build them up of thin strips of steel not exceeding about in. in thickness, preferably not more than § in. These strips, each of which is of the shape which it is desired to give the magnet, are then clamped or fastened together. The best fastening is obtained by clamping all the strips together in their proper positions before they are hardened, and drilling two or more holes right through them in that position. They may be then unclamped, hardened and magnetised separately and reassembled; brass bolts or screws may be then passed through the holes, thus firmly clamping together the strips or "laminæ." Such a magnet is usually called a compound

permanent magnet, or a laminated permanent magnet. The The ends are usually provided with soft iron shoes of various shapes, called pole-pieces.

POLARITY.

So far, in connection with the effect on iron or steel, we have spoken only of the attractive property of the lodestone or permanent magnet. Forces of repulsion may, however, be equally exhibited. If, for example, two bars of steel be both converted into permanent magnets by being stroked with the lodestone, or with a steel magnet, and the end which points north be marked N on each magnet, it will be found, if one be floated on a cork, that the N-end of the one held in the hand will repel the N-end of the floating one, and can be made to chase it all round the basin. Similarly, repulsion takes place between the two S-ends, while attraction occurs between a S-end and a N-end. If the magnet that is held in the hand be placed alongside the floating one so that the two N-ends and the two S-ends are close together, the floating one will be driven away from the other broadside on. The regions of greatest activity of the lodestone, already referred to, exhibit similar effects and correspond to the ends of the bar magnet. Owing to the irregular shape of most lodestones one cannot generally speak of their "ends." It might have been inferred from the tendency of a magnet to point N and S, that the two ends of a magnet exhibit different and apparently opposite qualities; the above experiments fully confirm this. To account for this the earlier theories of magnetism regarded the effects as being due to two subtle fluids, one of which was to be found on the N pole of a magnet and the other on the S pole. The fluids had to be conceived as possessing no weight, or as being what was called "imponderable," and as occupying no space. It was, no

doubt, to these views of the phenomena of magnetism that the two-fluid theory of electricity owed its origin. So long as the study of electricity was confined to the effects and phenomena of what is called static electricity, that is, of charges of electricity as distinct from currents, the two sciences of magnetism and electricity advanced pretty much hand in hand. When, however, further discoveries directed men's attention more particularly to the effects of electric currents, the study of magnetism fell behind, and great progress was made in electrical knowledge,

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FIG. 104.-DIAGRAM ILLUSTRATING
MAGNETIC FORCES DUE TO A
PERMANENT MAGNET.

new and better theories of electricity being constantly advanced. Hence, as we shall shortly see, magnetic theory, which at one time seemed to mould electric theory, ultimately came so much under the influence of electrical ideas as to be very largely modelled on the same lines as those which govern the theory of electricity.

The ends of the magnet are commonly referred to as its poles, but there are two points a short distance from each end which are properly speaking the true poles of the magnet. These poles are imaginary points at which all the magnetism of the magnet may be supposed to be collected. To make this clear, the following experiment may be referred to. If the magnetism of the magnet be tested or explored by means of a very small iron ball, and the force which is necessary to pull the iron ball from the magnet be taken as a measure of

the strength of the magnetism*, it will be found that the magnetism is strongest at or near each end, but that it gradually diminishes in strength from each end to a region about the middle of its length, where the strength is zero. The diagram, Fig. The diagram, Fig. 104, represents graphically these facts, the height of the curve at every point along the length of the magnet indicating at that point the strength of the magnetism measured in the manner described.

Next, if the magnet be pivoted at P, and placed where it can be acted upon by other magnets, or by the earth's magnetism, the total "torque," or twist, tending to swing it round into its position of rest may be regarded as being made up of a number of little twists. Each of these may be represented by the height of the curve (Fig. 104) at any point along the magnet multiplied by the distance of the point from the pivot. If now we take every possible point and multiply these two quantities together at each point, we get the total torque by adding all the so-obtained products together. We could get the same result if all the magnetism of the magnet could be collected at two certain points equidistant from the pivot, half the magnetism at each point, instead of being distributed in varying degree all along the length. The position of these points is given by the distance from the pivot, measured along the magnet, of the centres of the triangular areas under the curves.

If, then, the two points c1, c2 are the centres of the two areas, the two points, , give the positions of the so-called true poles. In physics the length of the magnet proper is considered to be the distance between these two points p, and 2, and the steel is regarded merely as a fortuitous envelope to the magnetism, the dimensions.

The presence of the ball disturbs the normal distribution of magnetism, and the method is therefore inaccurate. It is, however, very instructive, and the results are of the right order.

of the steel being unimporant except so far as the moment of inertia of the bar is affected by them. The strength of one true pole multiplied by its distance from the other gives what is called the "magnetic moment" of a magnet. This quantity is very important in magnetics, as it represents the power of the magnet to control the position of a pivoted compass needle, or the degree of its own tendency to be controlled by the earth.

The units, in terms of which magnetism is measured, are given in the section on Units and Testing.

An imaginary straight line joining the poles of a magnet is called its magnetic axis.

TERRESTRIAL MAGNETISM

The reason why a suspended lodestone or magnet always comes to rest in one definite position is that the earth behaves as though it were itself a huge magnet having two magnetic poles not very far from the two ordinary geographic poles.

In view of what has been said on p. 168 regarding the mutual repulsion of similar, and the mutual attraction of dissimilar poles, it is clear that the magnetic polarity at the "Earth's north magnetic pole," must be opposite in character to that at the north end of a compass needle, and similar to that at the end which points south. Hence if we call that kind of polarity "north" which points north because it is attracted by the northern part of the earth, the magnetic polarity of the earth in the northern geographic region must be called "south." It was in order to avoid confusion arising from this, that the practice of calling the poles of a needle "north-seeking," and "south-seeking," or "marked,” and "unmarked," respectively, arose. In technical work, at least, it is now the universal custom to call that kind of magnetic polarity "north," which the earth has at its southern geographic region, or which in a compass needle will point north. In such

terms then we may rightly say that the earth's "north" magnetic pole is in the south geographic, or antarctic, region. The study of the magnetic properties of the earth is extremely interesting, but is of little value to the electrical engineer. The engineer who has some accurate research work to do may need to know or to measure the strength of the earth's magnetic field in his laboratory, and a method of determining it is given in Section V. A detailed knowledge, however, of the daily and yearly variations of that field are of value rather to the meteorologist than to the engineer, however interested the latter may be in them from a scientific standpoint. Not much more need be said, therefore, in a work of this scope than that the earth's northern magnetic pole is situated somewhere in Boothia Felix, that its position varies, and that therefore the indications of the compass vary from time to time. A knowledge of these variations is, of course, of very great importance to navigators.

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The present position is such that a compass needle in England points about 16 degrees west of true north. At any place due south of the northern magnetic pole, the compass points true north, of course. magnet which is perfectly free to turn in a vertical plane as well as in a horizontal one will not only point a little west of north in England, but will tilt with its north end. pointing down towards the ground. The angle which the magnetic axis of the needle makes with the horizontal is called the angle of dip, and in London is now about 67°. The true direction here of the earth's directive action on the needle is thus along a line sloping down towards the earth, like the direction of raindrops when a strong wind is blowing from the S.S.E. Mention is made of this here because in certain galvanometer work and in some methods of iron testing the "horizontal component" and the "vertical

component" of the earth's directive force are referred to and utilised. Methods of measurement of these are given in the section on Testing. The earth's action on a needle is purely directive and not translational. This is seen when a magnet is floated on a cork; it is swung round but is not drawn bodily by the earth's magnetism in any one direction. It is easy to show that the force of gravity is distinct from that of magnetism by weighing a piece of hard steel before and after magnetisation. Magnetisation does not alter the weight.

ELECTRO-MAGNETISM.

The magnetic effects of an electric current have already been referred to on page 7. They are readily obtained of great strength, and are hence of far more value to the engineer than the magnetism of steel magnets; whilst that of the lodestone is useless.

It has already been pointed out (page 7) that a current flowing in a conductor is capable of affecting a compass needle. This effect varies with the strength and with the direction of the current. If the current be reversed, its action on a compass needle is also reversed, and the effect also increases or decreases with an increase or decrease of the current strength.

The nature of these magnetic effects may be exemplified as follows.

In Fig. 105 (a) w represents a wire lying N and S, beneath which is a compass needle c at rest, ie. pointing N and S. By the side of the wire and to the east of it is a second magnetised needle d mounted on a horizontal axis, balanced so as to lie horizontal, but otherwise free to turn in a plane parallel to the length of the wire. If now a current be sent through the wire in the direction of the arrow the two needles will move to some such positions as are shown in dotted lines, or if the current be strong enough, they will take up the positions indicated in Fig. 105 (6). If the magnet needles