MAGNET

, Magnes, the Loadstone; a kind of ferruginous stone, resembling iron ore in weight and colour, though rather harder and heavier; and is endued with divers extraordinary properties, attractive, directive, inclinatory, &c. See Magnetism.

The Magnet is also called Lapis Heraclœus, from Heraclea, a city of Magnesia, a port of the ancient Lydia, where it was said it was first found, and from which it is usually supposed that it took its name. Though some derive the word from a shepherd named Magues, who first discovered it on Mount lda with the iron of his crook. It is also called Lapis Nauticus, from its use in navigation; also Siderites, from its virtue in attracting iron, which the Greeks call si<*>hros.

The Magnet is usually found in iron mines, and sometimes in very large pieces, half magnet, half iron. Its colour is different, as found in different countries. Norman observes, that the best are those brought from China and Bengal, which are of an irony or sanguine colour; those of Arabia are reddish; those of Macedonia, blackish; and those of Hungary, Germany, England, &c, the colour of unwrought iron. Neither its figure nor bulk are constant or determined; being found of all shapes and sizes.

The Ancients reckoned five kinds of Magnets, different in colour and virtue: the Ethiopic, Magnesian, Bœotic, 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 of the eyes.—The Moderns, more happy, take it to conduct them in their voyages.

The most distinguishing properties of the Magnet are, That it attracts iron, and that it points towards the poles of the world; and in other circumstances also dips or inclines to a point beneath the horizon, directly under the pole; it also communicates these properties, by touch, to iron. By means of which, are obtained the mariner's needles, both horizontal, and inclinatory or dipping needles.

The Attractive Power of the Magnet, was known to the Ancients, and is mentioned even by Plato and Euripides, who call it the Herculean stone, because it commands iron, which subdues every thing else: but the knowledge of its directive power, by which it disposes its poles along the meridian of every place, or nearly so, and causes needles, pieces of iron, &c, touched with it, to point nearly north and south, is of a much later date; though the discoverer himself, and the exact time of the discovery, be not now known. The first mention of it is about 1260, when it has been said that Marco Polo, a Venetian, introduced the mariner's compass; though not as an invention of his own, but as derived from the Chinese, who it seems had the use of it long before; though some imagine that the Chinese rather borrowed it from the Europeans.

But Flavio de Gira, a Neapolitan, who lived in the 13th century, is the person usually supposed to have the best title to the discovery; and yet Sir G. Wheeler mentions, that he had seen a book of astronomy much older, which supposed the use of the needle; though not as applied to the purposes of navigation, but of astronomy. And in Guiot de Provins, an old French poet, who wrote about the year 1180, there is an express mention made of the loadstone and the compass; and their use in navigation obliquely hinted at.

The Variation of the Magnet, or needle, or its deviation from the pole, was first discovered by Sebastian Cabot, a Venetian, in 1500; and the variation of that variation, or change in its direction, by Mr. Henry Gellibrand, professor of astronomy in Gresham college, about the year 1625.

Lastly, the Dip or inclination of the needle, when at liberty to play vertically, to a point beneath the horizon, was first discovered by another of our countrymen, Mr. Robert Norman, about the year 1576.

The Phenomena of the Magnet, are as follow: 1, In every Magnet there are two poles, of which the one points northwards, the other southwards; and if the Magnet be divided into ever so many pieces, the two poles will be found in each piece. The poles of a Magnet may be found by holding a very fine short needle over it; for where the poles are, the needle will stand upright, but no where else.—2, These poles, in different parts of the globe, are differently inclined towards a point under the horizon.—3, These poles, though contrary to each other, do help mutually towards the Magnet's attraction, and suspension of iron. —4, If two Magnets be spherical, one will turn or conform itself to the other, so as either of them would do to the earth; and after they have so conformed or turned themselves, they endeavour to approach or join each other; but if placed in a contrary position, they avoid each other.—5, If a Magnet be cut through the axis, the segments or parts of the stone, which before were joined, will now avoid and fly each other.—6, If the Magnet be cut perpéndicular to its axis, the two points, which before were conjoined, will become contrary poles; one in the one, and one in the other segment.—7, Iron receives virtue from the Magnet by application to it, or barely from an approach near it, though it do not touch it; and the iron receives this virtue variously, according to the parts of the stone it is made to touch, or even approach to.— 8, If an oblong piece of iron be anyhow applied to the stone, it receives virtue from it only lengthways.—9, The Magnet loses none of its own virtue by communicating any to the iron; and this virtue it can communicate to the iron very speedily: though the longer the iron joins or touches the stone, the longer will its communicated virtue hold; and a better Magnet will communicate more of it, and sooner, than one not so good. —10, Steel receives virtue from the Magnet better than iron.—11, A needle touched by a Magnet will turn its ends the same way towards the poles of the world, as the Magnet itself does.—12, Neither loadstone nor needles touched by it do conform their poles exactly to those of the world, but have usually some variation from them: and this variation is different in divers places, and at divers times in the same places.— 13, A loadstone will take up much more iron when armed, or capped, than it can alone. (A loadstone is said to be armed, when its poles are surrounded with | plates of steel: and to determine the quantity of steel to be applied, try the Magnet with several steel bars; and the greatest weight it takes up, with a bar on, is to be the weight of its armour.) And though an iron ring or key be suspended by the loadstone, yet this does not hinder the ring or key from turning round any way, either to the right or left.—14, The force of a loadstone may be variously increased or lessened by variously applying to it, either iron, or another loadstone.—15, A strong Magnet at the least distance from a smaller or a weaker, cannot draw to it a piece of iron adhering actually to such smaller or weaker stone; but if it come to touch it, it can draw it from the other: but a weaker Magnet, or even a small piece of iron, can draw away or separate a piece of iron contiguous to a larger or stronger Magnet.—16, In these northern parts of the world, the south pole of a Magnet will raise up more iron than its north pole.—17, A plate of iron only, but no other body interposed, can impede the operation of the loadstone, either as to its attractive or directive quality.—18, The power or virtue of a loadstone may be impaired by lying long in a wrong position, as also by rust, wet, &c; and may be quite destroyed by fire, lightning, &c.—19, A piece of iron wire well touched, upon being bent round in a ring, or coiled round on a stick, &c, will always have its directive virtue diminished, and often quite destroyed. And yet if the whole length of the wire were not entirely bent, so that the ends of it, though but for the length of one-tenth of an inch, were left straight, the virtue will not be destroyed in those parts; though it will in all the rest.—20, The sphere of activity of Magnets is greater and less at different times. Also, the variation of the needle from the meridian, is various at different times of the day.—21, By twisting a piece of wire touched with a Magnet, its virtue is greatly diminished; and sometimes so disordered and confused, that in some parts it will attract, and in others repel; and even, in some places, one side of the wire seems to be attracted, and the other side repelled, by one and the same pole of the stone.—22, A piece of wire that has been touched, on being split, or cleft in two, the poles are sometimes changed, as in a cleft Magnet; the north pole becoming the south, and the south the north: and yet sometimes one half of the wire will retain its former poles, and the other half will have them changed. —23, A wire being touched from end to end with one pole of a Magnet, the end at which you begin will always turn contrary to the pole that touched it: and if it be again touched the <*> me way with the other pole of the Magnet, it will then be turned the contrary way.—24, If a piece of wire be touched in the middle with only one pole of the Magnet, without moving it backwards or forwards; in that place will be the pole of the wire, and the two ends will be the other pole.—25, If a Magnet be heated red hot, and again cooled either with its south pole towards the north in a horizontal position, or with its south pole downwards in a perpendicular position, its poles will be changed. —26, Mr. Boyle (to whom we are indebted for the following magnetical phenomena) found he could presently change the poles of a small fragment of a loadstone, by applying them to the opposite vigorous poles of a large one.—27, Hard iron tools well tempered, when heated by a brisk attrition, as siling, turning, &c, will attract thin filings or chips of iron, steel, &c; and hence we observe that siles, punches, augres, &c, have a small degree of magnetic virtue.—28, The iron bars of windows, &c, which have stood a long time in an erect position, grow permanently magnetical; the lower ends of such bars being the north pole, and the upper end the south pole.—29, A bar of iron that has not stood long in an erect posture, if it be only held perpendicularly, will become magnetical, and its lower end the north pole, as appears from its attracting the south pole of a needle: but then this virtue is transient, and by inverting the bar, the poles change their places. In order therefore to render the quality permanent in an iron bar, it must continue a long time in a proper position. But fire will produce the effect in a short time: for as it will immediately deprive a loadstone of its attractive virtue; so it soon gives a verticity to a bar of iron; if, being heated red hot, it be cooled in an erect posture, or directly north and south. Even tongs and fireforks, by being often heated, and set to cool again in a posture nearly erect, have gained this magnetic property. Sometimes iron bars, by long standing in a perpendicular position, have acquired the magnetic virtue in a surprising degree. A bar about 10 feet long, and three inches thick, supporting the summer beam of a room, was able to turn the needle at 8 or 10 feet distance, and exceeded a loadstone of 3 1/2 pounds weight: from the middle point upwards it was a north pole, and downwards a south pole. And Mr. Martin mentions a bar, which had been the beam of a large steel-yard that had several poles in it.—30, Mr. Boyle found, that by heating a piece of English oker red-hot, and placing it to cool in a proper posture, it manifestly acquired a magnetic virtue. And an excellent Magnet, belonging to the same ingenious gentleman, having lain near a year in an inconvenient posture, had its virtue greatly impaired, as if it had been by fire.—31, A needle well touched, it is known, will point north and south: if it have one contrary touch of the same stone, it will be deprived of its faculty; and by another such touch, it will have its poles interchanged.—32, If an iron bar have gained a verticity by being heated red-hot and cooled again, north and south, and then hammered at the two ends; its virtue will be destroyed by two or three smart blows on the middle.—33, By drawing the back of a knife, or a long piece of steel-wire, &c, leisurely over the pole of a loadstone, carrying the motion from the middle of the stone to the pole; the knife or wire will attract one end of a needle; but if the knife or wire be passed from the said pole to the middle of the stone, it will repel the same end of the needle.—34, Either a Magnet or a piece of iron being laid on a piece of cork, so as to float freely on water; it will be found, that, whichsoever of the two is held in the hand, the other will be drawn to it: so that iron attracts the Magnet as much as it is attracted by it; action and re-action being always equal. In this experiment, if the Magnet be set afloat, it will direct its two poles to the poles of the world nearly.—35, A knife &c touched with a Magnet, acquires a greater or less degree of virtue, according to the part it is touched on. It receives the strongest virtue, when it is drawn leisurely from the| handle towards the point over one of the poles. And if the same knife thus touched, and thus possessed of a strong attractive power, be retouched in a contrary direction, viz, by drawing it from the point towards the handle over the same pole, it immediately loses all its virtue.—36, A Magnet acts with equal force in vacuo as in the open air.—37, The smallest Magnets have usually the greatest power in proportion to their bulk. A large Magnet will seldom take up above 3 or 4 times its own weight, while a small one will often take up more than ten times its weight. A Magnet worn by Sir Isaac Newton in a ring, and which weighed only 3 grains, would take up 746 grains, or almost 250 times its own weight. A magnetic bar made by Mr. Canton, weighing 10 oz. 12 dwts, took up more than 79 ounces; and a flat semicircular steel Magnet, weighing 1 oz. 13 dwts, took up an iron wedge of 90 ounces.

Armed Magnet, denotes one that is capped, cased, or set in iron or steel, to make it take up a greater weight, and also more readily to distinguish its poles. For the methods of doing this, see Mr. Michell's book on this subject.

Artificial Magnet, is a bar of iron or steel, impregnated with the magnetic virtue, so as to possess all the properties of the natural loadstone, and be used instead of it. How to make Magnets of this kind, by means of a natural Magnet, and even without the assistance of any Magnet, was suggested many years since by Mr. Savary, and particularly described in the Philos. Trans. number 414. See also Abridgment, vol. 6, pa. 260. But as his method was tedious and operose, though capable of communicating a very considerable virtue, it was little practised. Dr. Gowin Knight first brought this kind of Magnets to their present state of perfection, so as to be even of much greater efficacy than the natural ones. But as he refused to discover his methods upon any terms whatever (even, as he said, though he should receive in return as many guineas as he could carry), these curious and valuable secrets in a great measure died with him. The result of his method however was first published in the Philos. Trans. for 1744, art. 8, and for 1745, art. 3. See also the vol. for 1747, art. 2. And in the 69th vol. Mr. Benjamin Wilson has given a process, which at least discovers one of the leading principles of Dr. Knight's art. The method, according to Mr. Wilson, was as follows. Having provided a great quantity of clean iron filings, he put them into a large tub that was more than one-third filled with clean water; he then, with great labour, shook the tub to and fro for many hours together, that the friction between the grains of iron, by this treatment, might break or rub off such small parts as would remain suspended in the water for some time. The water being thus rendered very muddy, he poured it into a clean iron vessel, leaving the filings behind; and when the water had stood long enough to become clear, he poured it out carefully, without disturbing such of the sediment as still remained, which now appeared reduced almost to impalpable powder. This powder was afterwards removed into another vessel, to dry it. Having, by several repetitions of this process, procured a sufficient quantity of this very fine powder, the next thing was to make a paste of it, and that with some vehicle containing a good quantity of the phlogistic principle; for this purpose, he had recourse to linseed oil, in preference to all other fluids. With these two ingredients only, he made a stiff paste, and took great care to knead it well before he moulded it into convenient shapes. Sometimes, while the paste continued in its soft state, he would put the impression of a seal; one of which is in the British Museum. This paste so moulded was then set upon wood, or a tile, to dry or bake it before a moderate fire, being placed at about one foot distance. He found that a moderate fire was most proper, because a greater degree of heat would make the composition crack in many places. The time requisite for the baking or drying of this paste, was usually about 5 or 6 hours, before it attained a sufficient degree of hardness. When that was done, and the several baked pieces were become cold, he gave them their magnetic virtue in any direction he pleased, by placing them between the extreme ends of his large magazine of artisicial magnets, for a few seconds. The virtue they acquired by this method was such, that, when any of those pieces were held between two of his best ten guinea bars, with its poles purposely inverted, it immediately of itself turned about to recover its natural direction, which the force of those very powerful bars was not sufficient to counteract. Philos. Trans. vol. 65, for 1779.

Methods for artificial Magnets were also discovered and published by the Rev. Mr. John Michell, in a Treatise on Artificial Magnets, printed in 1750, and by Mr. John Canton, in the Philos. Trans. for 1751. The process for the same purpose was also found out by other persons, particularly by Du Hamel, Hist. Acad. Roy. 1745 and 1750, and by Marul Uitgeleeze Natuurkund. Verhand. tom. 2, p. 261.

Mr. Canton's method is as follows: Procure a dozen of bars; 6 of soft steel, and 6 of hard; the former to be each 3 inches long, a quarter of an inch broad, and 1-20th of an inch thick; with two pieces of iron, each half the length of one of the bars, but of the same breadth and thickness, and the 6 hard bars to be each 5 1/2 inches long, half an inch broad, and 3-20ths of an inch thick, with two pieces of iron of half the length, but the whole breadth and thickness of one of the hard bars; and let all the bars be marked with a line quite around them at one end. Then take an iron poker and tongs (fig. 1, plate 16), or two bars of iron, the larger they are, and the longer they have been used, the better; and fixing the poker upright between the knees, hold to it, near the top, one of the soft bars, having its marked end downwards by a piece of sewing silk, which must be pulled tight by the left hand, that the bar may not slide: then grasping the tongs with the right hand, a little below the middle, and holding them nearly in a vertical position, let the bar be stroked by the lower end, from the bottom to the top, about ten times on each side, which will give it a magnetic power sufficient to lift a small key at the marked end: which end, if the bar were suspended on a point, would turn towards the north, and is therefore called the north pole; and the unmarked end is, for the same reason,| called the south pole. Four of the soft bars being impregnated after this manner, lay the two (fig. 2) parallel to each other, at a quarter of an inch distance, between the two pieces of iron belonging to them, a north and a south pole against each piece of iron: then take two of the four bars already made magnetical, and place them together so as to make a double bar in thickness, the north pole of one even with the south pole of the other; and the remaining two being put to these, one on each side, so as to have two north and two south poles together, separate the north from the south poles at one end by a large pin, and place them perpendicularly with that end downward on the middle of one of the parallel bars, the two north poles towards its south end, and the two south poles towards its north end: slide them three or four times backward and forward the whole length of the bar; then removing them from the middle of this bar, place them on the middle of the other bar as before directed, and go over that in the same manner; then turn both the bars the other side upwards, and repeat the former operation: this being done, take the two from between the pieces of iron; and, placing the two outermost of the touching bars in their stead, let the other two be the outermost of the four to touch these with; and this process being repeated till each pair of bars have been touched three or four times over, which will give them a considerable magnetic power. Put the half-dozen together after the manner of the four (fig. 3), and touch them with two pair of the hard bars placed between their irons, at the distance of about half an inch from each other; then lay the soft bars aside, and with the four hard ones let the other two be impregnated (fig. 4), holding the touching bars apart at the lower end near two-tenths of an inch; to which distance let them be separated after they are set on the parallel bar, and brought together again before they are taken off: this being observed, proceed according to the method described above, till each pair have been touched two or three times over. But as this vertical way of touching a bar, will not give it quite so much of the magnetic virtue as it will receive, let each pair be now touched once or twice over in their parallel position between the irons (fig. 5), with two of the bars held horizontally, or nearly so, by drawing at the same time the north end of one from the middle over the south end, and the south of the other from the middle over the north end of a parallel bar; then bringing them to the middle again, without touching the parallel bar, give three or four of these horizontal strokes to each side. The horizontal touch, after the vertical, will make the bars as strong as they possibly can be made, as appears by their not receiving any additional strength, when the vertical touch is given by a great number of bars, and the horizontal by those of a superior magnetic power.

This whole process may be gone through in about half an hour; and each of the large bars, if well hardened, may be made to lift 28 Troy ounces, and sometimes more. And when these bars are thus impregnated, they will give to a hard bar of the same size its full virtue in less than two minutes; and therefore will answer all the purposes of Magnetism in navigation and experimental philosophy, much better than the loadstone, which has not a power sufficient to impregnate hard bars. The half dozen being put into a case (fig. 6), in such a manner as that no two poles of the same name may be together, and their irons with them as one bar, they will retain the virtues they have received; but if their power should, by making experiments, be ever so far impaired, it may be restored without any foreign assistance in a few minutes. And if, perchance, a much larger set of bars should be required, these will communicate to them a sufficient power to proceed with; and they may, in a short time, by the same method, be brought to their full strength.