309
h'We asufficient power to impregnate l);ird bars. The six beingPut into a case and disposed as in lig. 7, so that two poles of thesame denomination may not be together, and their irons with themas one bar, they will retain the virtues they have received; but iftheir power should, by making experiments, be ever so far im-paired, it may be restored without any foreign assistance in a few‘'■unites. And if a much larger set of bars should be required,these will communicate to them a sufficient power to proceed'’■ill; and they may, in a short time, by the same method, bebrought to their full strength. To expedite the process of making! ’} a gilets, the bars should be fixed in a groove, or between brass-P II1S , to prevent them from sliding; or they may be kept steadym a weight and ruler, as in fig. 14.
I'iie Construction and the Use or the PrincipalMagnetical Instruments.
, Hie magnetical instruments may be reduced to three principalbeads p viz. 1st, the magnets or magnetic bars, which are neces-9dr y to magnetize needles of compasses, or such pieces of steel,! r °n, &c. as may be necessary for divers experiments; and whichave already been sufficiently explained. 2dly, the compasses,"eh as are used in navigation, and for other purposes, which arebb'y magnetic needles justly suspended in boxes, and which, ac-I urdiug to the purposes for which they are particularly employed,, av e several appendages, or differ in size, and in accuracy of"■'■sions, &c. whence they derive the different names of pocket-uiiipasses, steering-compasses, variation-compasses, and aziinuth-^passes: and 3dly, Ihe dipping-needle. The magnetic needlesj "di are commonly’ used at sea, are between four and six inches011 g'> but those which are used for observing the daily variation,le made a little longer, and their extremities point the variationan arch or circle properly divided and affixed to the box.In. ^ est s ' ia I’ e °f a magnetic needle is represented in figs. 9, and] , ’ die first of which shews the upper side, and the second av ' ew f ' !e ,ie( -‘dle, which is of steel, having a pretty large
e >n the middle, to which a conical piece of agate is adaptedcall*" eans a brass-piece O, into which the agate-cap (as it istli ls fastened. Then the apex of this hollow cap rests upone Point of a pin F, which is fixed in the centre of the box, and"in°!\ ' v b' c h the needle, being properly’ balanced, turns veryh,. f y* For common purposes, 1 hose needles have a conicalfast Qlatlon made in the steel itself, or in a piece of brass which is^^iled i H t| le middle of the needle. For a description of thecompass, see Compass , Mariner’s. The azimuth-S| Kht >aSS ' s n °fhing more than a mariner’s compass, to which twoto (• s a ( e adapted, through which the sun is to be seen, in orderthe, 11 ' az imuth, and from tlience to ascertain the declination ofp ar ^ la S"etic needle at the place of observation ; see fig. 11. Thesi»l 1 j C 'i! ars I" which it differs from the usual compass, are fileape S * ’ ® > hi one of which, G, there is an oblong aperture withipti^b'hcular thread or wire stretched through its middle; and
"'ea
..Pposite.
s
Hil,
fog,
sun,
sight F, there is a narrow perpendicular slit. Theopp 4 or wire H i is stretched from one edge of the box to thethe s s,te . - . The ring A B of the gimhalds rests with its pivots on' v hil s t^ circle T' D> the foot E of which turns in a socket, so thatrog n( i , e box K, M, is kept steady, the compass may he turnedsu n> A order to place the siglits F, G, in the direction of the* 0r t of ' e P lvo t s °f the gimhalds of this, as well as ot the common•u$p e . ? ° n 'passes, should lie in the same plane with the point of
' rre Klit' 0 ’" °^ t be needle, in order to avoid as much as possible thebox , ar 'ty of the vibrations. There are, on the inside of therawn perpendicularly along the sides of the box,box. points where the thread II l touches the edge of the
s Otith - • Sc b" es serve to shew how manv decrees
lines serve to shew how many degrees the north or
sun ; for
'vliicj, 1 0 e of the needle is distant from the azimuth of the s>
^ r ead u ^ ose > the middle of the apertures of. the sights I, G, theticalp,.,, mid tile said lines, must be exactly in the same ver-'[i-.tru In * e .' The use of the thread II I, which is often omitted inb>e man-il.r ^‘ ls ? ort , * s likewise to shew the degrees between*erve r ,3 meridian and the azimuth, when the eye of the ob--Qi't of c „ s P ci ‘pendicularly over it. On the side of the box of thisP. Us hed ; n n 'P asses > there generally is a nut or stop, whicl), whenVisions of ,i ears a g a 'nst the card and stops it, in order that the•hay he m . e car d which coincide with the lines in the box,Vol, 1H _^ e c °mmodiously read off. The dipping-needle,
though of late much improved, is however still far from perfection.The general mode of constructing it is to pass an axis quite throughthe needle, to let the extremities of this axis, like those of the beamof a balance, rest upon its supports, so that the needle may move it-self vertically round, and when situated in the magnetic meridian, itmay place itself in the magnetic line. The degrees of inclinationare shewn upon a divided circle, in the centre of which the needleis suspended. Fig. 12, represents a dipping-needle of the simplestconstruction ; A B is the needle, the axis of which F E rests uponthe middle of two lateral bars C D, C D, which are made fast tothe frame that contains the divided circle A I B K. This machineis fixed oil a stand G ; hut, when used at sea, it is suspended by aring II, so as to hang perpendicularly. When the instrument isfurnished with a stand, a spirit-level O is generally annexed to it,and the stand has three screws,, by which the instrument is situatedso that the centre of motion of the needle, and the division of 90°on the lower part of the divided circle, maybe exactly in thesame line, perpendicular to the horizon. See Level. Weshall here add a few experiments further to elucidate the magnetic:phenomena.
1. To DETERMINE WHETHER ANY SUBSTANCE TS ATTRACTED
by the Magnet or not.
If the substance to he examined contain iron, the attraction willevidently shew itself on bringing near it a magnetic bar. Thequantity of attraction will always be known by the force requisiteto separate them, and its proportion is estimated by the degree ofthat lorce. If the attraction be so small that it cannot be thusperceived, it must be put to swim upon water in an earthen orwooden vessel, by means of a piece of wood or cork. In thisway the attraction will he much more easily manifested by thebody coming towards the magnet when approached to it. It willsometimes be necessary to bring the magnet within one-tenth ofan inch of the body to be attracted ; and as the latter advances,care must be taken to withdraw the magnet; for if they be suffer-ed to strike against each other, the body, if hard, will generallyrecede; and it will likewise he proper to present the magnet tothe body when the latter is at rest. If it be suspected that the givenbody have some magnetism already, the same process is required;only observing to present a piece of soft and clean iron to thebody when swimming upon water or quicksilver., A piece ofiron about half an ounce weight, and an inch in length, will bevery proper for this purpose.
2. To find the Poles of a Magnetic Body.
Present the various parts of the body successively to one of thepoles of a magnetic needle, and it will soon he discovered whichparts of the given body are possessed of a contrary polarity bythe needle’s standing perpendicularly towards them. One of thepoles being thus discovered, turn the opposite pole of the mag-netic needle towards the body, a».d it will soon find out its otherpole. When the magnetism of the body to be examined is veryweak, there will be danger of reversing the polarity by bringingthe needle too near; and as the distance at which this elfect willtake place cannot be determined, it will always be proper to keepit so far distant, that it can only sensibly elfect the needle. Wherethere are only two poles, they may he found out merely bysprinkling some iron-filings upon the body; for these will si anderect upon the polar points.
3. Effects of the Magnet on soft Iron.
Having placed a magnetic needle upon the table, bring a bapof soft iron about eight inches long, and a quarter oi an inchthick, so near that it may draw one end of the needle a little outof the way. In this situation approach gradually the north poleof a magnet to the other extremity of the bar, and the north endof tlie needle will recede from the bar more and more, in propor-tion as the magnet is brought nearer the bar. It the experimentbe repeated with the other pole of the magnet, the north end ofthe needle will then be attracted by the bar. The reason of thisis, that when we bring the north pole of the magnet towards oneend of tlie bar, the latter acquires a south polarity, and the otherone, of course, a north polarity. Hence the needle is repelled,because magnetic poles of tlie same kind repel one another; but
5 B when
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