368

MAGNETISM.

touch the bars; the bars may then be taken away, and the sup-port slid to its place. Magnetism is best communicated to com-pass-needles by the two following methods: Procure a pair ofmagnetic bars, not less than six inches in length. Fasten theneedle down on a board, and with a magnet in each hand drawthem from the centre upon the needle outwards; then raise thebars to a considerable distance from the needle, and bring themperpendicularly down upon the centre, and draw them over again.Phis operation repeated about twenty times will magnetize theneedle, and its ends will point to the poles contrary to those thattouched them. Over one cud of a combined horse-shoe magnet,of at least two in number, and six inches in length, draw from itscentre that half of the needle which is to have the contrary pole tothe end of the magnet: raise the needle to a considerable distance,and draw it over the magnet again ; this repeated about twentytimes at least, and the same for the other half, will sufficientlycommunicate the power. A set of bars are exceedingly useful formagnetizing other bars, or needles of compasses, &c. their powermay also be increased when lost or impaired by mismanagement,&c. A set of such bars, viz. six bars and the two iron-conductors,may be preserved in a box; taking care to place the north pole ofone contiguous to the south pole of the next, and that contiguousto the north pole of the third, &c. as shewn in fig. 7. After whathas been said above, we need not describe how a knife, or anypiece of steel, &c. may be rendered magnetic, or in what man-ner a weak magnet may be rendered more powerfid. But it mayperhaps be necessary to say something concerning the communi-cation of magnetism to the horse-shoe magnet or crooked barslike A B C, fig-8- Place the crooked bar flat upon a table, andto its extremities apply the magnetic bars D F, EG; joining theirextremities F G, with the conductor or piece of soft iron F G ;then to its middle apply the magnetic bars placed at an angle : oryou mav use two bars only, placed as shewn in tig. 8, and strokethe crooked bar with them from end to end, following thedirection of that bent bar; so that on one side of it the magneticbars may stand in the direction of the dotted representation L K.In this manner, when the piece of steel ABC has been rubbed asufficient number of times ou one side, it must be turned with theother side upwards, &c. In communicating magnetism, it is bestto use weak magnets first, and those that are stronger afterwards;butyou must be very careful not to use weak after strong magnets.A magnet loses nothing of its own power by communicating toother substances, but is rather improved. Every kind of violentpercussion weakens the power of a magnet. A strong magnet hasbeen entirely deprived of its virtue, by receiving several smartstrokes of a hammer; indeed, whatever deranges or disturbs theinternal pores of a magnet will injure its magnetic force. Fill asmall dry glass-tube with iron-filings ; press them in rather close,and then touch the tube as if it was a steel-bar, and the tube willattract a light needle ; shake the tube, so that the situation of thefilings may be disturbed, and the magnetic virtue will vanish.Magnets should never be left with two north or two south poles to-gether ; for when they are thus placed, they diminish and destroyeach other’s power. Magnetic bars should therefore be alwaysleft with the opposite poles laid against each other, or by connect-ing their opposite poles by a bar of iron. The power of a magnetis increased by letting a piece of iron remain attached to one orboth of its poles. A single magnet should therefore be alwaysthus left. The difference of steel in receiving magnetism is verygreat, as is easily proved by touching in the same manner, andwith the same bars, two pieces of steel of equal size, but of diff er-ent kinds. With some sorts of steel, a few strokes are sufficientto impart to them all the power they are capable of receiving;other sorts require a longer operation ; sometimes it is impossibleto give them more than a small degree of magnetism. A piece ofspring-tempered steel will not retain as much magnetism as hardsteel; soft steel still less, and iron retains scarcely any. Iron,when oxydated, loses its magnetism. Mr. Cavallo gives the fol-lowing receipt for imitating the natural magnets. “ Take somemartial aethiops, or which is more easily procured, reduce intovery fine powder the scales which fall from red hot iron whenhammered, and abound in smiths’shops. Mix this pewder withdrying linseed-oil, so as to form it into a very stiff paste, and shapeit in a mould so as to give it any form you require; whether of aterrella, a human head, or ayy other. This done, put it into a .

4 I

warm place for some weeks, and it will dry so as to become veryhard ; then render it magnetic by the application of powerful mag"nets, and it will acquire a considerable power.” In making arti-ficial magnets of steel, without the use of another magnet, nonehas succeeded better than Mr. Canton, whose process is as fol-lows: Take 12 bars; six of soft steel; each three inches long, onequarter of an inch broad, and one-twentieth of an inch thick ; withtwo pieces of iron, each half the length of one of the bars, but otthe same breadth and thickness: also six piece', of hard steel, eachfive inches and a half long, half an inch broad, and three-twen-tieths of an inch thick ; with two pieces of iron half the length, butthe whole breadth and thickness of one of the hard bars; aud fitall the bars be marked with a line quite round them at one end-Then take an iron-poker and tongs, or two bars of iron, (the lar-ger and the longer used, the better) and filing the poker uprightbetween 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 notslide: then grasping the tongs with the right hand, a little belowthe middle, and holding them nearly in a vertical position, let thebar be stroked by the lower end from the bottom to the lop, aboutten times on each side, which will give it a magnetic power suffi-cient to lift a small key at the marked end: which end, if the barwas suspended on a point, would turn towards the N. and is there-fore called the north pole ; and the unmarked end is, for the samereason, called the south pole. Four of the soft bars being impreg-nated after this manner, lay the two fig. 13, parallel to each other,at the distance of one-fourth of an inch, between the two piecesof iron belonging to them, aN. and S. pole against each piece otiron: then take two of the four bars already made magnetical andplace them together so as to make a double bar in thickness, lh eN. pole of one even with the S. pole of the other: and the re-maining two being put to these, one on each side, so as to have2N. and 2 S. poles together; separate the N. from the S. pole atone end by a large pin, and place them perpendicularly with thatend downward on the middle of one of the parallel bars, the twoN. poles towards its S. and the two S. poles towards its N. end -slide them backward and forward three or four times the wholelength of the bar, and removing them from the middle of tln s jplace them on the middle of the other bar as before directed, andgo over that in the same manner; then turn both the bars the oth erside upwards, and repeat the former operation: this done, tak ethe two from between the pieces of iron ; and, placing the tw°outermost of the touching bars in the room, let the other two b ethe outermost of the four to touch these with; and this procc 5 *being repeated till each pair of bars have been touched three 0four times over, put the six together after the manner of the four! ifig. 13, and touch them with two pair of the hard bars placed h c "tween their irons, about halfan inch from each other; then faysoft bars aside; and with the four hard ones let the other two nimpregnated, fig. 14, holding the touching bars apart at the loW e ^end near two-tenths of an inch; to which distance let them beparated after they are set on the parallel bar, and brought togetfi®*again before they are taken off: then proceed according to the m e 'thod described above, till eacli pair have been touched two othree times over. But as this vertical way of touching a bar _Wnot give it quite so much of the magnetic virtue as it will recel ^\let each pair be now touched once or twice over in their para* ,position between the irons, with two of the bars held horizontal!.'’or nearly so, by drawing at the same time the N. pole of one fr?Vthe middle over the S. end, and the S. of the other from the m< gdie over the N. end of a parallel bar: then bringing them tomiddle again, without touching the parallel bar, give three .four of those horizontal strokes to each side. I lie horizontouch, after the vertical, will make the bars as strong as they P 0 ^siblv can be made, as appears by their not receiving any addition £strength, when the vertical touch is given by a great numberbars, and the horizontal by those of a superior magnetic po"

The whole process may he gone through in about half an h 011 .^and each of the large bars, if well hardened, may.be made W

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28 Troy ounces, and sometimes - more,thus impregnated, they will give to anits full virtue in less than two minutes; and therefore wiall the purposes of magnetism in navigation and experi

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losophy ynich better than the loadstone, which is known «0' aVe