As to the magnetism, which seems produced by electricity, my real
opinion is, that these two powers of nature have no affinity with
each other, and that the apparent production of magnetism is purely
accidental. The matter may be explained thus:
1st, The earth is a great magnet.
2dly, There is a subtile fluid, called the magnetic fluid, which
exists in all ferruginous bodies, equally attracted by all their
parts, and equally diffused through their whole substance; at least
where the equilibrium is not disturbed by a power superior to the
attraction of the iron.
3dly, This natural quantity of the magnetic fluid, which is contained
in a given piece of iron, may be put in motion so as to be more
rarefied in one part and more condensed in another; but it cannot
be withdrawn by any force that we are yet made acquainted with, so
as to leave the whole in a negative state, at least relatively to
its natural quantity; neither can it be introduced so as to put the
iron into a positive state, or render it _plus_. In this respect,
therefore magnetism differs from electricity.
4thly, A piece of soft iron allows the magnetic fluid which it
contains to be put in motion by a moderate force, so that being
placed in a line with the magnetic pole of the earth, it immediately
acquires the properties of a magnet; its magnetic fluid being drawn
or forced from one extremity to the other; and this effect continues
as long as it remains in the same position, one of its extremities
becoming positively magnetised, and the other negatively. This
temporary magnetism ceases as soon as the iron is turned east and
west, the fluid immediately diffusing itself equally through the
whole iron, as in its natural state.
5thly, The magnetic fluid in hard iron, or steel, is put in motion
with more difficulty, requiring a force greater than the earth to
excite it; and when once it has been forced from one extremity of the
steel to the other, it is not easy for it to return; and thus a bar
of steel is converted into a permanent magnet.
6thly, A great heat, by expanding the substance of this steel, and
increasing the distance between its particles, affords a passage
to the electric fluid, which is thus again restored to its proper
equilibrium; the bar appearing no longer to possess magnetic virtue.
7thly, A bar of steel which is not magnetic, being placed in the same
position, relatively to the pole of the earth, which the magnetic
needle assumes, and in this position being heated and suddenly
cooled, becomes a permanent magnet. The reason is, that while the bar
was hot, the magnetic fluid which it naturally contained was easily
forced from one extremity to the other by the magnetic virtue of the
earth; and that the hardness and condensation, produced by the sudden
cooling of the bar, retained it in this state without permitting it
to resume its original situation.
8thly, The violent vibrations of the particles of a steel bar, when
forcibly struck in the same position, separate the particles in such
a manner during their vibration, that they permit a portion of the
magnetic fluid to pass, influenced by the natural magnetism of the
earth; and it is afterwards so forcibly retained by the re-approach
of the particles when the vibration ceases, that the bar becomes a
permanent magnet.
9thly, An electric shock passing through a needle in a like position,
and dilating it for an instant, renders it, for the same reason, a
permanent magnet; that is, not by imparting magnetism to it, but by
allowing its proper magnetic fluid to put itself in motion.
10thly, Thus, there is not in reality more magnetism in a given
piece of steel after it is become magnetic, than existed in it
before. The natural quantity is only displaced or repelled. Hence it
follows, that a strong apparatus of magnets may charge millions of
bars of steel, without communicating to them any part of its proper
magnetism; only putting in motion the magnetism which already existed
in these bars.
I am chiefly indebted to that excellent philosopher of Petersburgh,
Mr. Æpinus, for this hypothesis, which appears to me equally
ingenious and solid. I say, _chiefly_, because, as it is many years
since I read his book, which I have left in America, it may happen,
that I may have added to or altered it in some respect; and if I have
misrepresented any thing, the error ought to be charged to my account.
If this hypothesis appears admissible, it will serve as an answer to
the greater part of your questions. I have only one remark to add,
which is, that however great the force is of magnetism employed, you
can only convert a given portion of steel into a magnet of a force
proportioned to its capacity of retaining its magnetic fluid in the
new position in which it is placed, without letting it return. Now
this power is different in different kinds of steel, but limited in
all kinds whatever.