MAGNETISM.

370

when the south pole is brought near the end of the bar, that endwhich approaches receives the north polarity, and the other, ofcourse, the south : whence the needle, instead of being repelled,is now attracted, By approaching a small magnetic needle todillerent parts of the bar, it will be found that one-half of it pos-sesses one hind of polarity, and the other the contrary kind; themagnetic.centre, however, or the limit betwixt the two polarities,is not always in the middle of the bar, but is generally nearer thatend which is presented to the magnet. The difference increasesas the bar is lengthened ; and when the latter exceeds a certainlength, it acquires several poles. This depends on the strength ofthe magnet; and when it happens, the first magnetic centre comesvery near to the end of the bar which stands next the magnet, andthe successive centres are formed betwixt every two poles. Thus,supposing the north pole of a magnet to be brought to the end ofsuch a bar, the end it touches becomes a south pole; a few inchesfarther a north polarity takes place, after that a south polarity, andso on. The poles become weaker and weaker as they recedefrom the end which the magnet touches; so that if the bar be ofconsiderable length, they totally vanish long before they come tothe other end. Hence, by applying a magnet to one end of along bar, we shall not thereby give any magnetism to the other;and this shall happen when a magnet capable of lifting two poundsof iron is applied to a bar of about an inch square, and live feetlong.'

4. The Action of Magnetism shewn by the 11f.pui.sion of

two Pieces of IVike.

■ Tie two pieces of soft iron-wire, AB, A15, fig. In and !6, eachto a separate thread, AC, AC, which join at top, and formingthem into a loop, suspend them so as to hang freely. Then bringthe marked end D, fig. 17, which is the north, of a magnetic barjust under them, and tlie wires will immediately repel each other,as shewn in lig. Hi; and this divergency will increase to a certainlimit, according as the magnet is brought nearer, and vice versa.The reason of this phenomenon is, that by the action of the northmagnetic pole D, both the extremities B, 15, of the wires, acquirethe same,' viz. the south polarity; consequently they repel eachother ; and the extremities A, A, acquire the north polarity, inconsequence of vvhch they also repel each other. If instead ofthe north pole D, you present the south pole of the magnetic bar,the repulsion will take place as before ; but now the extremities B, B,acquire the north, and the extremities A, A, acquire the south po-larity. On removing the magnet, the wires, if of soft iron, willsoon collapse, having lost all their magnetic power; but if steel-wires, or common sewing-needles be used, they will continue torepel each other after the removal of the magnet: the magneticpower being retained by steel. The experiment may bo agree-ably diversified by using four or more needles, and presenting aN. pole to one pair, and a S. to another, &c.

5. In what Circumstances a Magnet can Support the

Greatest Weight.

By a crooked wire we may shew that the power of a magnet va-ries according to circumstances. Thus, let a piece of wire about aquarter of an inch in diameter, and four or five inches long, bebent in the form represented by AC 15, fig. 18, with a sharp cornerat C. Tie it fast to a cross-bar, or let it be held by an assistantwith tin,- corner downwards. Then apply either pole of the mag-net DK to one Of its extremities; and if in this situation a smallpiece of iron, as H, be put to the corner C, it will remain sus-pended. On applying the contrary pole of another magnet to theother extremity of the wire, the piece of iron will immecliately falloff; but if a pole of the same kind be applied, it will not only bestill kept suspended, but be more strongly attracted than before.In this experiment, the first magnet is assisted by the action of thesecond, but to strengthen a magnet in this manner, it does not ap-pear necessary to use a magnet at all. Thus, having found hvtrial how much a magnetic bar can lift, procure an oblong pieceof iron about tour inches long, and somewhat heavier than the barcan bear. Apply one end of this ta (lie pole of the bar holding itwith your hand till you place under the otiier end a larger pie -e ofiron. It will then be fotind that the magnet will support the pieceof iron which it could not do before. The lower piece of inm isto he placed between an half and three quarters of an inch below

7

the under-part of the oblong piece which hangs at the magnet’The same elfect will be produced by the opposite pole of anothermagnet; but a pole of the same kind would weaken the attraction'Place a magnetic bar A15, lig. 1<1, so (hat one of its poles mayfproject a short way beyond the table, and apply an iron-v eight l-to it; then take another magnetic bar, 1)1’,, like the former, am 1bring it parallel to, and just over, the other, at a little distance’-and with the contrary poles towards each other ; in consequencewhich the attraction of 15 will be diminished, and the iron C, !tsufficiently heavy, will drop off, the magnet AB br ing then onlyable to support a smaller piece of iron. By bringing the magnetsstill nearer to each other, the attraction of B w ill be diminished >tiHfarther; and, when the two magnets come quite into contact (pro"vided they are equal in power), the attraction between 15 and Lwill vanish entirely; but if the experiment be repeated with th |Sdifference, viz. that the homologous poles of the magnets b#brought towards each other, then the attraction between Band k >instead of being diminished, will be increased.

6. The Generation of Boi.es, and of Magnetic Centres/in the Barts of a Broken Magnet.

Take a magnetic bar about six or eight inches long, and a qtiaf'ter of an inch diameter, whose magnetic centre will be in the mid'die, or near it. Break off about one-third pad by a smart strokeof a hammer, and it will be found lhat the broken part, though l 11tile magnet it had but one polarity, will now have acquired a !'>•and S. pole, with a magnetic centre, as if it wi ve a distinct int'S'net. The experiment may he diversified as follows: having mad®a steel-bar about six inches long, and one-fourth of an inch thickquite hard, break it into two unequal parts. Join these, and pH ’’ 5them hard together, giving it the magnetic virtue at the same tin’®by two powerful magnets; while the parts remain in this position/so (hat the bar looks as if it had not been broke, it will have onlytwo poles; but as soon as they are separated, each part will b efound to become a distinct magnet, having a fx. and S. pole pro*per to itself.

7. 'Bo 11 F. MOVE THE MAGNETIC CliNTRE IN A MaCNKT.

This may be done in various ways; as, by striking a magi'H 1 ®bar repeatedly, beating it, bard rubbing, &c.; but in aUthe 33methods the magnetism of the bar is diminished at the same tin 1 .®that the centre is removed ; so that they ought not to be con 1 '"luted beyond what is necessary to produce a sensible removal 0the magnetic centre.

8. Thf. Disadvantages of using Magnets of UneqV* 1,

Bower, and of Steel not i'ropkrly Hardened.

Having communicated the magnetic virtue to a steel-bar by 3magnet of any given power, then rub it with a weaker mag« ct Jand it will be found, that the power of the bar, instead of bei' 1 !?augmented, will be diminished; being uo stronger than if it h 3been rubbed only with the weak magnet. Tim impropriety’ 0 !using soft steel in making artificial magnets mav be understo 0 ®from the following example: take two wires about"fourteen ind 14 *long, and one-eighth of an inch in thickness; let one be of vi’ l 7hard steel, the other of soft steel or iron, though not of the soft 1 ’’sort; then, by means of magnetic bars, give the virtue to the*®wires, treating them both in the same manner, and it will be gf.“®'rally found that the hard wire will luive only two poles, butother a greater number.

9. To Exhibit the Direction in which Iron -filings ^range themselves by Means of the Magnetic Virtu 15,

Lay a sheet of naper fiat upon a table, strew some iron-fd" 1 ^

upon the paper, place a small magnet among them; thengi'’®^few gentle knocks to the table, so as to shake the filings, and y°will find that they dispose themselves about the magnet N^z jshewn in fig. 2, the particles of iron clinging to one another) a° gforming themselves into lines, which at the very poles N, t>. 3 ’,in the same direction with the axis of the magnet; a little side"' 3 ),of the poles they begin to bend, and then they form conip"‘ •arches, reaching from some point in the nor!hern"half of the nn'Enet, to some other point in the southern half. In this exper} 1 ’ 1 ®the filings may he sprinkled on a piece of paste-board wide**placed on a magnetic bar, q-*.