(Magnes) the LOADSTONE: a species of iron ore. See Magnetism, and Mineralogy Index.

The magnet is also called Lapis Heracleus, from Heraclea, a city of Magnesia, a port of the ancient Lydia, where it is said to have been first found, and from which it is usually supposed to have taken its name. Though others derive the word from a shepherd named Magnes, who first discovered it with the iron of his crook on Mount Ida. It is also called Lapis Nauticus, from its use in navigation; and siderites, from its attracting iron, which the Greeks call eidones.

The ancients reckoned five kinds of magnets, different in colour and virtue; the Ethiopic, Magnesian, Boeotic, Alexandrian, and Natolian. They also took it to be male and female: but the chief use they made of it was in medicine; especially for the cure of burns and defluxions on the eyes.—The moderns, more fortunate in its application, employ it to conduct them in their voyages. See Navigation.

The most distinguishing properties of the magnet are, That it attracts iron, and that it points to the poles of the world; and in other circumstances also dips or inclines Magnetism

Introduction

General Principles.

If the mineral body called magnet or loadstone (an ore of iron which will be described under Mineralogy) is brought within a moderate distance from a piece of iron or steel, or other ferruginous body, such as a small key, a sewing needle, or the like, the ferruginous body will approach the magnet; and if no obstacle intervene, will come in contact with it, and the two bodies will adhere together, so as to require an evident force to separate them from each other.

Again, if a magnet be freely balanced, so that it be left at liberty to assume any direction, as if be suspended by a thread, or made to float on the surface of water by placing it on a piece of cork or wood, it will soon settle itself in one particular direction, so as to turn one part of its surface towards the northern point of the horizon, and the opposite part of course towards the southern point. These two parts of the surface of the magnet are called its north and south poles; this property of the magnet, of assuming this particular direction, is called its polarity, or its directive power; and when a magnet is placed so as to arrange itself in such a direction, it is said to traverse.

The direction in which a suspended magnet finally settles is called the magnetic meridian, and it is different in different places, and at different times. It is generally, however, very different from the real meridian line, so that the north pole of a magnet declines a little to the east or west, and the south pole to the west or east. The difference of the magnetic from the astronomical meridian, is called the declination, or variation of the magnet; and the declination is said to be east or west, according as the north pole of the magnet verges to the one or the other of these points.

If an oblong magnet be suspended on a pivot by its centre of gravity, it does not settle in a perfectly horizontal position, but one of its poles is depressed below the magnet, the horizontal line, and the other elevated as far above it, making an angle with the horizon that is also different on different parts of the earth's surface. This depression of one of the poles is called the dipping of the magnet.

If two magnets that are each freely suspended, be brought within a moderate distance from each other, so that the north pole of the one is opposed to the south pole of the other, they will attract each other; and if no obstacle intervene, will rush together; but if the two north poles, or the two south poles, be mutually opposed, the magnets will repel each other.

Such are the leading properties of what is called the natural magnet; but what is of more importance, as we shall see hereafter, any piece of iron or steel may, by being rubbed with a natural magnet, or by some other processes to be afterwards explained, be made to acquire the same properties, and thus in every useful respect serve the same purposes as the natural magnet. These pieces of iron or steel thus magnetised, are called artificial magnets; and when they are of a slender, oblong form, they are termed magnetic needles. When afterwards we speak of the polarity, the declination, or the dipping of the magnetic needle, we would be understood as alluding to these slender, oblong, artificial magnets.

A straight line joining the two poles of a magnet is Axis and called its axis, and a line drawn transversely on the face of the magnet, perpendicular to the axis, is called a magnet.

The properties of natural and artificial magnets above enumerated, are attributed to the agency of some unknown force or power, either inherent in the magnet, or imparted to it by the processes to which it is subjected. This force is sometimes called magnetism, but we shall for the present denominate it the magnetic power. The most important property of the magnet is its polarity, as it is by means of this that the mariner is enabled to find his way along the trackless ocean, where, before the discovery of this important property, he had no other guide but the stars, and could therefore seldom venture far from the coast. It is by this property too, that the miner is enabled to pursue a direct course through the bowels of the earth, or the traveller direct his steps through immense forests, or over sandy deserts. The uses of the magnet are therefore obvious and important, and the science which places these uses in the best point of view, and thus enables us to turn them to the greatest advantage, is well deserving our attention. Many of the facts to be related under this article are highly curious, and form a pleasing addition to those scientific amusements which are so well calculated to excite the attention of beginners in the study of experimental philosophy.

It is unnecessary for us to attempt giving here a history of the origin and progress of our knowledge in magnetism. To a general reader, it would be uninteresting, and to such as are better informed, superfluous. We shall only mention the most important works that have appeared on the subject.

Few treatises expressly on magnetism have appeared in this country. In the year 1600, Dr Gilbert, a physician of Colchester, and the friend of Lord Bacon, published an excellent work De Magnete et Corporibus Magneticis, which is still perhaps the most valuable that we possess. Mr Cavallo's Treatise on Magnetism, first published in 1787, contains a great variety of facts and experiments; and a neat compendium of it is given in the 3d volume of the same author's Elements of Natural and Experimental Philosophy. Mr Cavallo's Treatise, and Mr Adams's Essay on Magnetism, form the substance of most of the compilations on this subject that have lately appeared.

To those who wish to enter minutely on the study of magnetism, the following list of foreign publications recommended by the late Professor Robison of Edinburgh will be acceptable.

Æpinii Tentamen Theoriae Magn. et Electr. Eberhardi's Tentamen Theoriae Magnetismi, 1720. Dissertatio sur l'Aimant, par du Fay, 1728. Muschenbroek Dissert. Physico-Experimentalis de Magnete. Pieces qui ont emporté la prise de l'Acad. des Sciences à Paris sur la meilleure construction des Bouffées de déclinaison. Recueil des pieces couronnées, tom. v. Euleri Opuscula, tom. iii. continens Theoriam Magnetis. Berlin, 1751. Æpinii Oratio Academica, 1758. Æpinii item Comment. Petrop. nov. tom. x. Anton. Brugmanni Tentamen Phil. de Materia Magnetica. Franquere, 1765.

There is a German translation of this work by Eilenschach, with many valuable additions.

Scarella de Magnete, 2 tom. fol. Van Swinden sur l'Analogie entre les phénomènes Électriques et Magnetiques, 3 tom. 8vo. Dissertation sur les Aimants Artificielles, par Nicholas Fuß, 1782.

Chap. I. Of Magnetical Apparatus.

The principal instruments employed in magnetical experiments and observations, are reducible to three instruments, heads: First, Magnets of various kinds and forms; Secondly, Magnetic needles and compasses; and, Thirdly, the Dipping needle. Of compasses we have nothing to say here, having fully treated of them under Compass.

Magnets, as we have said, are either natural or artificial. The natural magnet may be cut into various forms, according to the experiments that are to be made with it. The most usual shape is oblong, having the poles at the two most distant extremities. Dr Gilbert, whom we shall mention more at large hereafter, made his magnets of a spherical shape, so as to resemble the terrestrial globe. Magnets of this shape are called terrella, or little earths, and are usually marked upon their surface the magnetic poles, meridian, and equator.

Natural magnets of an oblong shape have usually a piece of soft iron attached to each pole, called the core of magnets. Another piece of soft iron placed so as to join two of the extremities of the former pieces, and usually furnished with a hook or hole in the middle. The magnet thus fitted up, as represented at fig. 1, is said to be armed, and the iron pieces CD, CD, are called the armature of the magnet AB. The magnet with its armature is commonly inclosed in a brass box, represented in the figure by the dotted lines DC, CC, CD; and to the upper part of the box is fixed a ring E, for holding the magnet.

One of the most common forms of the artificial magnet is that of an oblong bar, as NS, fig. 2, of which N is the north pole, and S the south, having the north end marked with a transverse notch. These bars are made of hardened steel, and are either sold separately, or what is more common, in sets of six in a box.

Another very common form of the artificial magnet is Experiments that of a horse shoe, such as fig. 3, having the two poles N, S brought near each other, and commonly united by a piece of soft iron or conductor. The horse-shoe magnets sometimes consist only of a single crooked bar; but they are frequently composed of several such bars united together by their flat surfaces, and inclosed in a leathern covering that envelopes all but the poles, and thus preserves the bars from rusting.

Instead of the very arched form of which horse-shoe magnets are usually made, they are sometimes constructed so as to form nearly a semicircle, and in this shape they are very convenient for several experiments.

Artificial magnets, like the natural, when of an oblong shape, are sometimes armed at each end, so as to enable them to apply both poles to a ferruginous body at the same time. One material advantage of the horse-shoe magnet is, that in such an armature is unnecessary, as the poles are brought so near each other as easily to be applied to the object it is proposed to lift, as a key, &c.

A magnetic needle is an oblong piece of steel, tempered so as commonly to assume the blue tinge that is seen in watch-springs, and supported on a brass point, so as, when left at liberty, to arrange itself in the magnetic meridian, but in a horizontal position. These needles are sometimes made pointed at both extremities; sometimes the northern extremity is made in the form of a cross; but perhaps the best form is that of the oblong, with extremities that are nearly obtuse, such as is represented at fig. 4. To balance the needle on its pivot, it is furnished near its middle with a hollow cap, which is formed of some substance that is not attracted by the magnet. The cap is usually of brass; but for nice experiments it is sometimes made of agate, as this latter does not wear so fast as brass, and consequently the needle will longer retain its original suspension.

The dipping needle, fig. 5, consists of an oblong bar of steel, A.B., balanced between two horizontal slips of brass, C.D., C.D., so as when magnetized to form an angle with the horizon, equal to the dipping of the needle at the place where the instrument is made. The two horizontal slips of brass are either fixed to a graduated semicircle that is supported on a stand of wood, or more commonly they form diameters to a brass ring which is graduated on its circumference, and furnished with a ring H, by which it may be held on the finger.

The construction and uses of these instruments will be fully explained in the next chapter; our only object here being to bring the reader acquainted with the names and general form of the instruments that are made use of in the experiments which we are about to describe, for illustrating the principles of magnetism.

Several smaller articles will be required by the experimentalist; but these are easily procured, and need no particular description. Such are a number of fewing needles of various sizes, soft iron bars, pieces of iron wire, small iron balls, iron filings, &c.

**Chap. II. Experimental Illustrations of the Principles of Magnetism.**

**Sect. I. Of Magnetic-Polarity.**

We have stated (No. 3.) that when a magnet is suspended at perfect freedom, it assumes a certain determinate position with respect to the astronomical meridian. This is but a particular case of a much more general fact, which may be expressed by the following proposition.

If an oblong piece of iron be so adjusted, as to be at liberty to take any position; it will assume a certain determinate direction with respect to the axis of the earth, differing according to the place where the experiment is made.

**Experiment 1.—Take a moderately sized straight iron rod, as a piece of iron wire about the thickness of a goose quill, and about eight or ten inches long; pass it through one extremity of a large wine cork, so that it may be at right angles to the axis of the cork, and adjust it in such a manner that it may swing in water in a horizontal position. Now, provide a pretty large earthen vessel, as a hand basin or round deep dish, nearly filled with water; and when the water is free from agitation, cautiously put in the wire, in such a direction as not to be very far from the north and south line. The iron rod will, after some time, be found to have arranged itself so as, in Britain, to form an angle with the meridian of about 25 degrees.

This experiment requires some nicety, and it will sometimes be long before the iron assumes its proper position; but if due attention be paid to all the particulars above mentioned, it will at length arrange itself in the magnetic line. It is necessary that the rod should be placed not too far from the magnetic line, as if it be laid at right angles to that line, it will never acquire the proper direction. The situation of the rod in this experiment is in the true magnetic line, so far as respects the meridian; but, as it is horizontal, it is not in the position that a magnet would assume, if freely suspended by its centre of gravity. An iron rod may, however, be made to take such a position, as well as a magnet.

**Exper. 2.—Instead of passing the iron rod through the extremity of a cylindrical or conical piece of cork, let it be passed through the centre of a spherical piece of cork or wood, so that the centre of gravity may coincide with the centre of the sphere, and let the whole be of such a specific gravity as to remain suspended in any part of the water, without ascending or descending. If the iron rod thus fitted be placed as in the last experiment, it will at length arrange itself in the true magnetic direction, so as to make an angle of about 25 degrees with the meridian, and with one extremity depressed below the horizon at an angle of about 73 degrees.

These experiments were contrived by Dr Gilbert, and fully shew that the property of assuming a determinate direction with respect to the earth's axis is not confined to magnets, or iron rendered magnetic by the usual processes. There is, however, a remarkable difference between the polarity of unmagnetized iron and that of natural and artificial magnets. It is of no consequence in the former which extremity be placed towards the north, or which below the surface of the water, as either will retain the position it first acquired, unless disturbed by agitation, or by the proximity of a magnet; and both extremities may easily be made to change situations. The effect produced on the iron is therefore temporary. But if a magnetic needle be freely freely suspended, the same extremity always points towards the north, and this northern extremity always dips below the horizon, at least in these northern latitudes; and if the position of the needle be disturbed by mechanical motion, or by the application of a magnet, it will be restored when the disturbing cause is removed.

The polarity of magnets therefore is permanent.

We have said that the magnetic line varies at different times, and in different places. The declination of the magnet is so uncertain as to impose great impediments to the art of navigation, as it is necessary, in the course of a long voyage, frequently to ascertain the degree of variation for any particular time or place. The method of doing this is mentioned under Compass. The declination observed in different places at different times, has been laid down in tables; and as such tables are very useful, we shall here subjoin one, given by Mr Cavallo.

| Latitude | Longitude | Declination | Years in which the observations were made | |----------|-----------|-------------|----------------------------------------| | North | Weft. | Easf. | | | 70° 17' | 163° 24' | 30° 21' | 1779 | | 69° 38' | 164° 11' | 31° 0 | 1778 | | 66° 36' | 167° 55' | 27° 50' | | | 65° 43' | 170° 34' | 27° 58' | | | 63° 58' | 165° 48' | 26° 25' | | | 59° 39' | 149° 8 | 22° 54' | | | 58° 14' | 139° 19' | 24° 40' | | | 55° 12' | 135° 0 | 23° 29' | | | 53° 37' | 134° 53' | 20° 32' | | | | Weft. | | | | 50° 8 | 4° 40' | 20° 36' | 1776 | | 48° 44' | 5° 0 | 22° 38' | | | 40° 41' | 11° 10 | 22° 27' | | | 33° 45' | 14° 50' | 18° 7 | | | 31° 8 | 15° 30' | 17° 43' | | | 28° 30' | 17° 0 | 14° 0 | | | 23° 54' | 18° 20' | 15° 4 | | | 20° 30' | 20° 3 | 14° 35' | | | 19° 45' | 20° 39' | 13° 11' | | | 16° 37' | 22° 50' | 10° 33' | | | 15° 25' | 23° 36' | 9° 15' | | | 13° 32' | 23° 45' | 9° 25' | | | 12° 21' | 23° 54' | 9° 48' | | | 11° 51' | 24° 5 | 8° 19' | | | 8° 55' | 22° 50' | 8° 58' | | | 6° 29' | 20° 5 | 9° 44' | | | 4° 23' | 21° 2 | 9° 1 | | | 3° 45' | 22° 34' | 8° 27' | | | 2° 40' | 24° 10' | 7° 42' | | | 1° 14' | 26° 2 | 5° 35' | | | 0° 51' | 27° 10' | 4° 59' | | | 0° 7 | 27° 0 | 4° 27' | | | South | | | | | 1° 13' | 28° 58' | 3° 12' | | | 2° 48' | 29° 37' | 2° 52' | | | 3° 37' | 30° 14' | 2° 14' | | | 4° 22' | 30° 29' | 2° 54' | | | 5° 0 | 31° 40' | 1° 26' | |

It is of still more importance to know the progressive change of the declination at any certain place, and we shall therefore give here the following table of the declination as observed at London in different years, from 1576 to 1800.

| Years | Declination | Observers | |-------|-------------|-----------| | 1576 | 11° 15' | Burrowes. | | 1580 | 11° 11' | | | 1612 | 6° 10' | Gunter. | | 1622 | 6° 0 | | | 1633 | 4° 6 | Gellibrand. | | 1634 | 4° 5 | | | 1656 | 0° 0 | Bond. | | 1665 | 1° 22° 1/2 | Gellibrand. | | 1666 | 1° 35° 1/2 | | | 1672 | 2° 30' | Halley. | ### Table of the Mean Monthly Variation of the Magnetic Needle for 20 Years at London

| Years | January | February | March | April | May | June | July | August | September | October | November | December | |-------|---------|----------|-------|-------|-----|------|------|--------|-----------|---------|-----------|----------| | | | | | | | | | | | | | | | 1786 | - | - | - | - | - | - | - | - | - | - | - | - | | 1787 | 23 | 19.2 | 23 | 19.8 | 23 | 20.3 | 23 | 18.5 | 23 | 17.0 | 23 | 18.3 | | 1788 | 23 | 25.6 | - | - | - | - | - | - | - | - | - | - | | 1789 | - | - | - | - | - | - | - | - | - | - | - | - | | 1790 | 23 | 38.9 | - | - | - | - | - | - | - | - | - | - | | 1791 | 23 | 35.6 | - | - | - | - | - | - | - | - | - | - | | 1792 | 23 | 41.1 | - | - | - | - | - | - | - | - | - | - | | 1793 | 23 | 46.9 | 23 | 48.3 | 23 | 48.8 | 23 | 46.2 | 23 | 47.3 | 23 | 48.5 | | 1794 | 23 | 54.2 | - | - | - | - | - | - | - | - | - | - | | 1795 | - | - | - | - | - | - | - | - | - | - | - | - | | 1796 | - | - | - | - | - | - | - | - | - | - | - | - | | 1797 | - | - | - | - | - | - | - | - | - | - | - | - | | 1798 | - | - | - | - | - | - | - | - | - | - | - | - | | 1799 | - | - | - | - | - | - | - | - | - | - | - | - | | 1800 | - | - | - | - | - | - | - | - | - | - | - | - | | 1801 | - | - | - | - | - | - | - | - | - | - | - | - | | 1802 | - | - | - | - | - | - | - | - | - | - | - | - | | 1803 | - | - | - | - | - | - | - | - | - | - | - | - | | 1804 | - | - | - | - | - | - | - | - | - | - | - | - | | 1805 | - | - | - | - | - | - | - | - | - | - | - | - |

The declination of the magnetic needle has been found to be different, even at different hours of the day. The following table contains the result of some observations made by Mr Canton on the daily variation, and, on the mean variation of each month.

The declination observed at different hours of the same day. June 27, 1759:

| H. Min. | Decl. W. | Degrees of the Therm. | The mean Variation for each Month in the Year | |---------|----------|-----------------------|-----------------------------------------------| | 0 18 | 12° 2 | 62° | January, 7° 8' | | 6 4 | 18 5 | 62 | February, 8° 52 | | 8 30 | 18 5 | 65 | March, 11° 17 | | 9 2 | 18 54 | 67 | April, 12° 26 | | 10 20 | 18 57 | 69 | May, 13° | | 11 40 | 19 4 | 68½ | June, 13° 21 | | 0 50 | 19 9 | 70 | July, 13° 14 | | 1 38 | 19 8 | 70 | August, 12° 19 | | 3 10 | 19 8 | 68 | September, 11° 43 | | 7 20 | 18 59 | 61 | October, 10° 36 | | 9 12 | 19 6 | 59 | November, 8° 9 | | 11 40 | 18 51 | 57½ | December, 6° 58 |

*Phil. Trans. 1806. p. 416.* Charts have been constructed for shewing the declination of the needle in various parts of the earth by means of curve lines. Referring these charts and several other circumstances with regard to this subject, see Variation of the Compass.

It may not be improper here to point out the general method of applying the polarity of the magnet to the useful purposes of navigation, mining, &c.

A mariner's compass, or magnetic needle in a case, is so placed as to be as little as possible disturbed by the motion of the vessel, person, &c. In a ship, it is placed in the binnacle (see Binnacle), or suspended from the upper deck in the cabin. Then the head of the vessel is kept by the helm in such a direction as to make any required angle with the line of the needle, or the person (in mining or travelling) advances in a similar manner. Thus, supposing that a vessel gets out from a certain part, in order to go to another place that is exactly westward of the former; as for example, from Land's End in Cornwall to Newfoundland on the coast of North America. The vessel must be directed in such a way, as that its course may be always at right angles with the direction of the magnetic needle, or so that the part of the needle or compass card, which points to the northward, (allowing for the variation) may be always kept to the right hand of the man at the helm, or to the starboard side of the vessel. The reason of this is evident; for, supposing the needle to point duly north and south, the direction of east and west being perpendicular to it, this must be the true course of the vessel. From this example, a little reflection will easily point out how a vessel may be steered in any other course.

The declaration of the magnetic needle is disturbed by the near approach of a ferruginous body, especially if this be of considerable size.

On holding the extremity of a pretty large iron rod, such as a poker, near one end of a magnetic needle properly suspended, the needle will be found to turn considerably from its usual direction. This circumstance, though proper to be mentioned here, will be better understood when we have considered the attractive power of the magnet. The fact is useful, as it teaches us to keep magnetic needles in such a situation as not to be acted on by any considerable body of iron.

A magnet, whether natural or artificial, has a great effect in disturbing the polarity of a magnetic needle than is produced by iron.

Magnetic polarity seems also to be affected by changes in the state of the atmosphere; and the following axioms respecting this effect on the declination of the needle, collected by M. la Cotte, are deserving of attention.

1. The greatest declination of the needle from the north towards the west, takes place about two in the afternoon; and the greatest approximation of it towards the north, about eight in the morning; so that from the last mentioned hour till about two in the afternoon, it endeavours to remove from the north, and between two in the afternoon and the next morning, to approach it.

2. The annual progress of the magnetic needle is as follows:—Between January and March, it removes from the north; between March and May it approaches it; in June it is stationary; in July it removes from it; in August, September, and October it approaches it; its declination in October is the same as in May; in November and December it removes from the north; its greatest western declination is at the vernal equinox, and its greatest approximation to the north, at the autumnal equinox.

3. The declination of the magnetic needle is different, according to the latitude; among us, (i.e., in France) it has always increased since 1657; before that period it was easterly.

4. Before volcanic eruptions and earthquakes, the magnetic needle is often subject to very extraordinary movements.

5. The magnetic needle is agitated before and after the appearance of the northern lights: its declination on these occasions is about noon greater than usual.

So much has already been said respecting the phenomena, &c. of the dipping needle, under the article Dipping Needle, that it is unnecessary here to add much more on the subject. It was there noticed, that at the equator the dipping needle lies quite horizontal, and that one of its extremities inclines more towards the earth, according as the instrument is carried farther from the equator. We may here add, that from some late observations made by experimentalists with balloons, it appears that the higher we ascend above the surface,

(A) In reply to some inquiries respecting the mode of employing the compass in mining, we were favoured by an ingenious friend, who is manager of one of the most extensive coalworks in this island, with the following remarks: "The compass is used in all mines where great accuracy is required. In some coal-mines the cleats or faces of the coal are the guides to the miners in excavating the mine, and the compass is used to ascertain the situation and extent of the excavations. In other coal-mines the courses of the excavations are at first directed by the compass. In doing this, the compass is placed in a given situation, and is made to point the desired course. Then from the centre of one flight a perpendicular line is conveyed to the roof of the mine, and a small mark is there made with chalk; then a person looks at a candle (placed so as nearly to touch the roof), through the lower part of the flight of the compass nearest to him, and through the upper part of the opposite flight. The candle at the roof is moved in any direction until he sees it through both flights of the compass. It is then in a proper place, and a chalk mark is made in the roof immediately above it. A line struck with a chalked cord, between these two marks upon the roof, marks the proper course, by which the workmen are directed in making the excavation. By applying one part of a chalked cord along part of the course or white line thus begun on the roof, and extending the other part of the cord past it to any required distance, and then striking the cord, the course may be continued from time to time as the excavation advances." Chap. II.