MAGNETISM. 
piece of iron magnetic without the magnet; and thefe 
artificial magnets are even fnfceptible of acquiring a 
ftrength rarely found in natural magnets. We fhall give 
an account of the different methods of communicating 
the magnetic virtue ; but we muff premife a few obferva- 
tions on what may be called magnetifm by induElion. 
The whole of this part of the fubject may be laid to de¬ 
pend on one general faff, viz. that “any piece of iron, 
■when in the neighbourhood of a magnet, is itfelf a mag¬ 
net, and poffeffes all the material properties of that body." 
1. Let there be a large and ftrong magnet properly fup- 
ported in the horizontal direction, at a diftance from iron 
or other fimilar bodies, and with its poles perfectly free. 
Take alfo any fmall piece of common iron, not more than 
two or three inches long, fuch as a common fmall key ; and 
take another pieceof iron, as a fmaller key, or fhort piece 
of wire about the fize of a goofe-quill. In the firlt place, 
hold the key in a horizontal pofition, with one end op- 
pofite one of the poles of the magnet, but fo as not to 
be in contaCt with it. Then bring the other piece of iron 
to the other end of the key, and it will hang by the key, 
and will fo continue to hang, though we withdraw the 
key from the magnet horizontally, till there is a certain 
interval between the key and the magnet, when the former 
will be no longer able to fupport the piece of iron. Even 
at this dillance the key will, however, be found capable 
of fupporting a piece of iron confiderably fmaller than the 
former, till its diltance from the magnet be increafed. 
Again, hold the key with one extremity below one of the 
poles of the magnet, and touch'the other extremity with 
the fmall piece of iron, the latter will adhere till the key 
be removed too far below the magnet. Thirdly, hold the 
key with one of its extremities above one of the poles of 
the magnet, but at fuch a diftance that there is room for 
the fmall piece of iron to go betweeen the key and the 
magnet, without touching the latter. The piece of iron 
will be fupported by the key, as in the two former in- 
ftances. Fourthly, let the magnet be placed in a vertical 
pofition, and hold the key with one extremity immediately 
below or above one of the poles. The piece of iron will 
be fnpported in a fimilar manner ; in the former cafe by the 
extremity of the key that is mod remote from the mag¬ 
net, and in the latter by that which is adjacent. If, in- 
ftead of approaching the magnet with the key, we reverfe 
the circumftances, the effeCt of the magnet in rendering 
the key magnetical will be (till more evident. Suppofe 
the piece of iron to he lying on the table; let one end of 
it he touched with the key, and there will be found no 
attraction between them ; but if, while we hold the key 
very near one extremity of the wire, we bring the pole of 
the magnet near the other end of the key, we (hall fee the 
wire rife from the table, and adhere to the key. In all 
thefe cafes the attractive power of the key, that is, its 
magnetifm, is evidently derived from its juxtapofition to 
the magnet. 
2. Let two pieces of iron wire be fufpended by fe- 
parate ends of a piece of thread, fo that they may be 
liung from a pin in the wall in afituation parallel to each 
other, or in contaCt. Now bring one end of a bar-magnet 
a little below the wires, and they will repel each other. If 
thefe wires are foft iron, they will collapl'e immediately 
on the magnet being withdrawn ; but, if they are formed 
of hard iron or of Iteel, they will continue apart for a con- 
fiderable time. 
Here the two wires are, by the proximity of the mag¬ 
net, become magnets, and the extremities next the bar 
have each acquired a fimilar polarity, i. e. both contrary 
to that of the adjacent pole of the bar. They therefore 
repel each other. 
3. Let a bar-magnet, fuch as N S, fig. 14. be laid in a 
horizontal pofition ; and let a fmall key, as B C, be held 
near the north pole,of the magnet, in the direction of its 
axis. Let a very fmall magnetic needle, fupported on a 
fliarp pivot, be brought near that end of the key, C, 
which is molt remote from N. The needle will imme¬ 
119 
diately turn its fouth pole towards C, as is indicated by 
the feathered part of the arrow c. Hence it appears that 
the key has acquired a directive power like a magnet, and 
that its remote extremity performs the office of a north 
pole, as it attracts the fouth pole of the needle, and repels 
its north pole. If it be faid that the magnetic needle 
in this cafe is affieCted directly by the directive power of 
the magnet, as it would take the above pofition though 
the key were not prefent; to fhow that the effeft is pro¬ 
duced through the medium of the key, remove the needle 
into another fituation, as b, and it will Hill arrange itfelf 
with the fame pole C ; and, if it be carried to the proximate 
extremity of the key, as at a, it will turn round, and pre¬ 
fent its north pole to B ; thus (Lowing that it is, at lealt in 
fome meafure, influenced by the key. 
In general, when a piece of iron is prefented to the.pole 
of a magnet, the extremity next that pole is poflefled of 
the contrary polarity, and the remote extremity has ac¬ 
quired a fimilar polarity. The fituation of the poles., how¬ 
ever, depends much on the form of the piece of iron, and 
on the part of its furface which is prefented to the pole of 
the magnet. If the form be that of an oblong bar, one 
extremity of which is prefented to the pole, which is the 
mod ufual cafe, the circumftances will be as we have juft 
mentioned. If the oblong bar be prefented to the pole 
in a perpendicular direction, with its middle very near 
the pole of the magnet, this middle point will be poflefled 
of a polarity contrary to that of the adjacent pole, while 
the two extremities have acquired the fame polarity. If 
the prefented iron be in the form of a circular plate, and 
its centre be held near the pole of the magnet, this centre 
will have the contrary polarity, and every point of the 
circumference the lame polarity. If the plate have its 
circumference fafhioned into points, each of thefe points 
will acquires very ftrong polarity, contrary to that of the 
pole near which the centre of the plate is held. 
The communication of magnetic power from the mag¬ 
net to the key in the foregoing experiments, will be fill! 
more ftrongly illultrated by holding another piece of wire 
to the wire that is already fufpended by the key. The 
new piece of wire will alfo be fufpended, and fo feveral 
more may be fufpended by one another, like the links of 
a chain, according to the ftrength of the magnet. This 
fact was known to the ancients, who fpeak of a loadftone 
caufing an iron ring to carry another ring; and that a. 
third, till the whole puts on the appearance of a chain. 
It will be found that the magnet has loft: none of its 
powerby producing magnetifm in the iron ; and, of courfe, 
that nothing has been transferred, from the magnet to the 
iron. The magnetifm of the iron thus caufed by its jux¬ 
tapofition to a magnet is called induced magnetifm, or magne- 
tj'm by induElion. 
There is an apparent exception to the univerfality of the 
above propofition. If the key be held in fuch a pofition 
as that it (hall be perpendicular to the magnet, with one 
extremity either oppofite one of the poles, or a little above 
the centre of the magnet, the bit of wire will not be at¬ 
tracted by that extremity, and we may hence fuppofe that 
the key has acquired no magnetic power by its proximity 
to the magnet. But, if we bring a needle or a piece of 
iron wire near its remote end, it will be ftrongly attracted, 
and fhow that end to have the fame polarity with the 
neareft pole of the magnet. Now, the ends both of the 
key and the wire that are next the magnet, having the 
fame polarity with the pole of the magnet neareft them, 
cannot attract each other, but on the contrary will repel 
each other ; and therefore the wire cannot adhere to the 
key, though, by the change produced by the other extre¬ 
mity, it is evident that the key has acquired magnetic 
power. 
There is, however, one real exception. If the key in 
the firlt experiment, with the wire hanging to it, be car¬ 
ried from any ot the fituations there deferibed, towards 
the middle of the magnet, the wire will fall off as foon as 
it arrives very near the middle, if we fuppofe a plane to 
