MAGNET, MAGNES, the Loadstone; a sort of ferruginous stone, in weight and colour resembling iron ore, though somewhat harder and more heavy; endowed with divers extraordinary properties, attractive, directive, inclinatory, &c. See MAGNETISM.
The magnet is also called Lapis Heraclæus, from Heraclæa, 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, by reason of its use in navigation; and fiderites, from its attracting iron, which the Greeks call σίδερα.
The magnet is usually found in iron mines, and sometimes in very large pieces, half magnet, half iron. Its colour is different, according to the different countries it is brought from. Norman observes, that the best are those brought from China and Bengal, which are of an iron 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 determined, but it is found of all forms and sizes.
The ancients reckoned five kinds of magnets different in colour and virtue: the Ethiopic, Magnesian, Bæotic, 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 happy, take 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 to a point beneath the horizon, directly under the pole; and that it communicates these properties, by touch, to iron. On which foundation are built the mariner's needles, both horizontal, and inclinatory or dipping needles.
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, whereby it disposes its poles along the meridian of every place, and occasions needles, pieces of iron, &c. touched with it, to point nearly north and south, is of a much later date; though the exact time of its discovery, and the discoverer himself, are yet in the dark. The first tidings we hear of it is in 1260, when Marco Polo the Venetian is said by some to have introduced the mariners compass; tho' not as an invention of his own, but as derived
ved from the Chinese, who are said to have had the use of it long before; tho' some imagine that the Chinese rather borrowed it from the Europeans.
Flavio de Gioa, 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 uses of navigation, but of astronomy. And in Guyot 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 its declination from the pole, was first discovered by Seb. Cabot, a Venetian, in 1500; and the variation of that variation, by Mr Gellibrand, an Englishman, about the year 1625. See VARIATION.
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 R. Norman, about the year 1576. See the article Dipping-Needles.
Phenomena of the MAGNET. 1°. In every magnet there are two poles, one whereof 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 are found by holding a very fine short needle over it; for where the poles are, the needle will stand upright, but nowhere 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 one another, 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 parts or segments of the stone, which before were joined, will now avoid and fly each other. 6°. If the magnet be cut by a section perpendicular to its axis, the two points, which before were conjoined, will become contrary poles; one in one, the other 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 any how applied to the stone, it receives virtue from it only as to its length. 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 touches or joins 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 place. 13°. A loadstone will take up much more iron when armed or capped than it can alone. And though an iron ring or key be suspended by the loadstone, yet the magnetical particles do not hinder that ring or key from turning round any way, either to the right or left. A loadstone is said to be armed, when its poles are surrounded with plates of steel. To determine the quality 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. 14°. The force of a loadstone may be variously increased or lessened by the various application of iron, or another loadstone, to it. 15°. A strong magnet at the least distance from a lesser or a weaker, cannot draw to it a piece of iron adhering actually to such lesser or weaker stone; but if it come to touch it, it can draw it from the other: but a weaker magnet, or even a little piece of iron, can draw away or separate a piece of iron contiguous to a greater or stronger loadstone. 16°. In these northern parts of the world, the south pole of a loadstone will raise up more iron than the 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. Mr Boyle found it true in glasses sealed hermetically; and glass is a body as impervious as most are to any effluvia. 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. 19°. A piece of iron wire well touched, will, upon being bent round in a ring, or coiled round on a stick, &c. generally, quite lose its directive virtue, but always have it much diminished: 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 strait, the virtue will not be destroyed in those parts; though it will in all the rest. This was first observed by Grimaldi and de la Hire; and is confirmed by the experiments of Mr Derham; who adds further, that though coiling or bending the wire as above, would always destroy its virtue by day, yet it would not do it in the evening. 20°. This sphere of the activity of magnets is greater and less at different times: in particular, that reserved in the repository of the royal society will keep a key or other body suspended to another, sometimes, at the height of eight or ten feet; and at others, not above four feet. To which we may add, that the variation of the magnetical needle from the meridian, varies at various times of the day; as appears from some experiments of Mr Graham, and likewise from observations made during one of Capt Cook's voyages. See VARIATION. 21°. By twisting a piece of wire touched with a magnet, its virtue is exceedingly 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, being split, or cleft in two, the poles are sometimes changed, as in a cleft magnet; the north becoming the south, and the south the north: and yet some-
Magnet. sometimes one half of the wire will retain its former poles, and the other half will have them changed. To which it may be added, that laying one or other side of the half uppermost, causes a great alteration in its tendency or aversion to the poles of the magnet. 23°. A wire being touched from end to end with the same pole of the magnet, the end whereat you begin will always turn contrary to the pole which touched it: if it be again touched the same 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 ones of a large magnet. 27°. Hard iron tools well tempered, when heated by a brisk attrition, as filing, turning, &c. will attract thin filings or chips of iron, steel, &c. and hence we observe files, punches, augers, &c. to have a small degree of magnetic virtue. 28°. The iron-bars of windows, &c. which have a long time stood in an erect position, grow permanently magnetical; the lower ends of such bars being the north-pole, and the upper the southern. 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 will shift 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 the 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. Nay, tongs and fire-forks, by being often heated and set to cool again in a posture nearly erect, have gained this magnetical 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 eight or ten feet distance, and exceeded a loadstone of three and an half 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 ocher red-hot, and placing it to cool in a proper posture, it manifestly acquired a magnetic virtue. And an excellent magnet of the same ingenious gentleman's having lain near a year in an inconvenient posture, had its virtue surprisingly 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 will have its poles quite changed. 32°. If a bar of iron 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 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 accordingly 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 that end of the needle which in the other case it attracts. 34°. Either a magnet or a piece of iron being laid on a piece of cork, so as to swim freely in water; it will be found, that, whichever 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. 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 touch, 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 in possession 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 and in the open air. 37°. The smallest magnets have generally the greatest power in proportion to their bulk. A large magnet will seldom take up above three or four times its own weight, whereas a small one will frequently take up more than ten times its weight. A magnet worn by Sir Isaac Newton in a ring, and which weighed only three grains, would take up 746 grains, or almost 250 times its own weight. A magnetic bar made by Mr Canton, according to the method described in the next article, and which weighed 10 ounces 12 penny-weights, took up something more than 79 ounces; and a flat femicircular steel magnet that weighed an ounce and 13 penny-weights took up an iron wedge of 90 ounces.
Artificial Magnet, is a term usually applied to steel bars impregnated with the virtues of the natural magnet or loadstone; and are much more common, as well as more convenient for use, than the others.
The late Dr Godwin Knight possessed a surprising skill in magnetism, being able to communicate an extraordinary degree of attractive or repulsive virtue, and to alter or reverse the poles at pleasure; but as he refused to disclose 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 have died with him. In the 69th volume of the Philosophical Transactions, however, Mr Benjamin Wilson hath given a process which at least discovers one of the leading principles of Dr Knight's art, and may perhaps be a means of discovering the whole to those who shall be less reserved. The doctor's process, according to Mr Wilson, was as follows. Having provided himself with a great quantity of clean iron-filings, he put them into a large tub that was more than one third filled with
Magnet. clean water; he then, with great labour, worked the tub to and fro for many hours together, that the friction between the grains of iron by this treatment might break off such smaller parts as would remain suspended in the water for a time. The obtaining of these very small particles in sufficient quantity seemed to him to be one of the principal desiderata in the experiment. The water being by this treatment rendered very muddy, he poured the same 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 in order to dry it; but as he had not obtained a proper quantity thereof by this one step, he was obliged to repeat the process many times. Having at last procured enough of this very fine powder, the next thing was to make a paste of it, and that with some vehicle which would contain a considerable 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 particular 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 upon the several pieces; one of which is in the British museum. This paste was then put upon wood, and sometimes on tiles, in order to bake or dry it before a moderate fire, at about the distance of a foot or thereabouts. He found that a moderate fire was most proper, because a greater degree of heat made the composition frequently crack in many places. The time required for the baking or drying of this paste was generally about five or six 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 artificial magnets for a few seconds or more, as he saw occasion. By this method the virtue they acquired 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.
As to the method of making artificial magnets of steel, none hath succeeded in it better than Mr Canton, whose process is as follows.
Procure a dozen of bars; six of soft steel, each three inches long, one quarter of an inch broad, and one twentieth 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: also six pieces of hard steel, each five inches and a half long, half an inch broad, and three-twentieths 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 round them at one end. Then take an iron poker and tongs, (fig. 1.) 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 down-
wards, 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 was 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 the distance of one fourth of an inch, 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, and the two south poles towards its north end: slide them backward and forward three or four times the whole length of the bar, and removing them from the middle of this, 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 room, 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 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
Magnetism, 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 an 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 is known not to have a sufficient power to impregnate hard bars. The half dozen be-
ing put into a case (fig. 6.), in such a manner as that Magnetism two poles of the same denomination may not 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, out of curiosity, 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.