MAGNETISM. 
If a magnet be cut in a direction perpendicular to the 
axis paffiug through its two poles A and B, fig. 7, there 
will be formed by the feCtion two new poles, fuch as F 
and E ; fo that, if A be the fouth pole of the whole ftone, 
E will be a north pole, and F a fouth pole. By this bi- 
feCtion, therefore, *he north fide of the ltone will acquire 
a fouth pole, and the foqth fide a north pole. 
A magnet, however good it may be, unlefs it be very 
large, will fcarcely fupport a few pounds of iron ; and, 
in general, the weight which a magnet can carry is al¬ 
ways much«below its own weight. Biit means have been 
found out, by employing what is called arming , to make 
it produce a much more conliderable effect. We (hall 
therefore defcribe the method of arming a magnet. Firft 
give the magnet a figure nearly regular, and fquare its 
tides where the two poles are fituated, lo that thefe ttvo 
fides may form two parallel planes. Then make, of foft 
iron, for iieel is not fo good, two pieces, fuch as fig. 8, 
the long and fiat lide of which may be of the fame length 
and breadth as the faces of the magnet where the poles 
are fituated. The proper thicknefs however of this fide, 
as well as the projection of the foot, and its thicknefs, 
can be found only by repeated trials. Thefe two pieces 
mult embrace the magnet on the two faces where the 
poles are fituated, the feet palling below as if to fupport 
it; and they mult be fattened in that fituation by tranf- 
verfe bands of copper, furrounding the magnet, and com- 
prefiing the long branches of thefe pieces againlt the faces 
eft' the poles. Then provide a piece of foft iron, of the 
form feen at fig. 9, a little longer than the diftance be¬ 
tween the bands of iron applied to the poles of the mag¬ 
net, and in thicknefs fomew-hat more than the flat faces 
of the lower part of the feet of the arming. In regard 
to the height, it mull: be regulated by what may appear 
molt convenient. Pierce a hole in it towards the middle, 
to receive a hook for the purpofe of fufpending from it 
the weight to be fupported by the magnet. Fig. 10 re- 
prefents an armed magnet, which will be fufficient to give 
an idea of the whole arrangement, without any farther 
explanation. A magnet armed in this manner will fup¬ 
port a much greater weight, than one not armed. A 
Hone, for example, of two or three, ounces, will by thefe 
means fupport fifty or fixty ounces of iron 5 that is, twenty 
or thirty times its own weight. 
It has alio been obferved, that, amongft the natural 
magnets, the fmalleft generally poflefs a greater attractive 
power, in proportion to their fize, than thofe which are 
larger. There have been often feen natural magnets not 
exceeding the weight of 20 or 30 grains, which could lift 
a piece of iron that weighed 40 or 50 times more than 
themfelves. Mention is made of a (mail magnet worn in 
a ring, which weighed about three grains, and was capa¬ 
ble of taking up 746 grains, or nearly 250 times its own 
weight. But magnets of above two pounds weight fel- 
dom lift up ten times their own weight of iron. Le- 
mery fays he faw a magnet of the fize of a moderate ap¬ 
ple, which fupported 22 pounds. One has been feen, 
of 11 ounces, which could fupport 28 pounds. The fum 
of 200I. fterling was alked for it. But the largeft magnet 
we have read of was a prefent from the emperor of China 
to the king of Portugal, John V. Its volume is very 
nearly 265 cubic inches, its weight 38 pounds j\ ounces, 
and it fultained, on the 21ft of February, 1781, the weight 
of 202 pounds 7 ounces. 
It often happens, that a natural magnet, cut off from 
a larger loadftone, will itfelf be capable of lifting a greater 
weight of iron than the original large loadilone from which 
it was cut off. This muft be imputed to the heterogeneous 
nature of the large loadftone; for, fuppofe that one part 
of it contains a good quantity of pure metal ftrongly mag¬ 
netic al, the reft of it being impure or mixed with other 
fubftances, it is plain that the impure part can only ob- 
ftruCt the aCtiou of the purer part; hence this latter, being 
Separated from the reft, multaCt more powerfully than the 
whole together did. 
11.5 
The force of a magnet may be varioufly increafed or 
leflened by the various application of iron, or another 
magnet, to it. Thus, the holding of a piece of iron of 
iome magnitude to one pole of a magnet, increafes the 
attraction of the other pole, fo as to enable it to lift a 
greater weight. Alfo, the attractive power of a magnet 
may be increafed considerably by gradually adding more 
and more weight to it; fcrr by this means it will be found 
that the magnet will keep fufpended on one day a little 
more weight than it did the preceding day; which addi¬ 
tional weight being added to it on the following day, or 
fome .time after, it will be found that the magnet can 
keep fufpended a weight ftill greater, and fo on as far as 
a certain limit. On the contrary, by an improper fitua¬ 
tion, or by putting a very final! weight of iron to it, the 
magnet may gradually lole much of its ftrength. Heat 
weakens the power of a magnet; and a white heat de¬ 
stroys it entirely, or at lead in a great meafure. Hence 
it appears, that from this caufe alone, befides others which 
tnay concur, the power of a magnet muft be continually 
varying. 
A ftrong magnet at the lead diftance from a fmaller, or 
a weaker, cannot draw to it a piece of iron adhering actu¬ 
ally to fuch weaker ftone; bur, 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 feparate a piece of 
iron contiguous to a greater or ftronger loadftone. 
In thefe northern parts of the world, as various authors 
have laid, the north pole of a magnet generally has an 
attractive power fomewhat ftronger than the fouth pole:- 
but in the fouthern parts of the earth, the fouth pole of 
the magnet is faid to pofiefs the greateft attractive power. 
See the article Compass, vol. iv. p. 889. It is proper alfo 
to remark, that, though we call that pole of a magnet 
which inclines towards the earth in the northern latitude 
a north, pole, it is properly fpeaking a fouth pole; for, as we 
muft call that pole of the great interior magnet the north 
pole which is in the north, and as this pole produces the 
contrary polarity in the proximate end of a needle, that 
end muft be pofiefled of fouth polarity. 
Neither the attraction nor the repulfion of magnetifm is 
fenfibly affeCted by the interpofition of bodies of any forr, 
except iron or ferruginous bodies in general. Thus, fup¬ 
pofe that when a magnet is placed at an inch diftance 
from a piece of iron, there is required an ounce of force 
to remove it; or, which is the fame thing, fuppofe that 
the attraction towards each other is equal to one ounce,3 
it will be found that the fame degree of attraction remains 
conltantly unaltered, viz. always equal to one ounce, 
though a plate of other metal, or of glafs, or paper, or 
other body, be interpofed between the magnet and the 
iron ; or though they be inclofed in feparate boxes of glafs 
or other matter. Neither the abfence nor prefence of air 
has any effect upon them. In Ihort, no other fubftance 
befides iron, or thofe bodies which contain that metal in 
any of its metallic ftates, does fenfibly aft'eCt the attraction 
or repulfion of magnetifm. Accordingly, Mr. Boyle 
found this true in glafies fealed hermetically ; and glafs is 
a body as impervious as molt are to any effluvia. The 
magnetic virtue is fenfibly continued through the fub¬ 
ftance of feveral contiguous bodies or pieces of iron, as 
keys, &c. It pervades the pores «ff„the hardelt bodies; 
and equally attracts the iron in vacuo as in open air. 
It is by the attractive power of the magnet that we 
ufually afeertain whether any fubftance be magnetic. If 
the body contain any confiderable quantity of iron in its 
compofition, its magnetifm is eafily afeertained, by au- 
proaching it with the pole of a pretty ftrong magnetic 
bar. The ores of iron are attracted more or ltfs readily, 
according as they contain a greater or a fmaller quantity 
of metal, and as that metal is in a more or lefs perfect 
metallic ftate. By the aCtion of fire, iron ores are gene 
rally put into a ftate of being much more readily attracted. 
Dr. Morighim, of Gotha, has afeertained, by repeated 
experiments, thatnon-niagn§tifed needles, \vheu they have 
been. 
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