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Page 28
The properties of the magnet are best observed in magnetised steel; and
when we proceed to test its magnetic power, it will be found that it is
most active at the extremities of the bar, which are hence called its
poles, and hardly, if at all, at the centre; that while both poles
attract certain substances and repel others, the one always points
nearly north and the other nearly south when the bar is horizontally
suspended; and that, when we break the bar into two or any number of
pieces, however small, each part forms into a complete magnet with its
virtue active at the poles, which, when suspended, preserves its
original direction; so that of two particles one is, in that case,
always north of the other; nay, it is probable that each of these has
its north pole and its south, as constant as those of the earth itself,
which, too, is a large magnet.
The magnet acts through media and at a distance, as well as in contact;
and it has an especial attraction for iron, the more so when the
conducting medium is solid, such as a table; and so when the magnet is
horizontally suspended, or poised, in the vicinity of iron, its tendency
to point north and south is seriously disturbed. The disturbance of the
bar, or needle, in such a case, is called its _deflection_; and it is
corrected by so placing a piece of soft iron or another magnet in its
neighbourhood as to neutralise the effect, and leave said bar, or
needle, free to obey the magnetism of the earth. The needle, it is to be
remarked, does not point due north and south, neither, when poised
freely on its centre, does it lie perfectly horizontal; in our latitude
it points at present 20� west of north, which is called its
_declination_, and its north pole slopes downwards at an angle of 68�,
which is called its _dip_.
By holding a rod of iron, or a poker, for a length of time parallel to
the direction of the needle, so as to have the same declination and the
same dip, it will gradually assume and display magnetic virtue, and this
will ere long become fixed and powerful under a succession of vibratory
shocks. There is a beautiful experiment in which a needle, when
magnetised, can be made to float on water, when it adjusts itself to the
magnetic meridian, and will incline north and south the same as the
needle of the compass.
_The Chemical Action of Electricity and Magnetism_.--These agents
possess powers which develop wonderfully in connection with chemical
combination. Thus, if we suspend a piece of iron in a vessel which
contains oxygen gas, and apply to the metal an electric current, it will
immediately begin to unite rapidly, and form an oxide with oxygen,
emitting, during the process, intense heat and a bright flame. Zinc,
too, when similarly acted on, will ignite in the common atmosphere and
burn away, though with less intensity, till it also is, under the
electric force, reduced to an oxide. It is presumed that many other
chemical combinations take place because of the simultaneous joint
development of electric agencies, as in copper, water, and aquafortis,
nitrate of copper, &c. So also it happens that, when a plate of iron is
for some time immersed in a copper solution, it comes out at length
covered over with a coating of copper. And it is because there is
electricity at work that a silver basin will be coated with copper when
we pour into it a copper solution, and at the same time place in it a
rod of zinc, so that it rests on the side and bottom, though no coating
will form at all when there is no rod present to excite the electric
current. The same phenomena will appear if we deposit a silver coin in
the solution in question: the coin will come out unaffected, unless we
excite affinity by means of a rod of iron. It is under the action of an
electric current that one metal is coated with another. The metal,
copper say, is steeped in a solution of the coating substance, and
connected by means of wires with a galvanic battery, under the action of
which the metal in solution unites with the surface of the plate
immersed in it. Heat also is developed under magnetic influence, and
that often of great intensity. Thus, if we connect the poles of a
voltaic battery by means of a platinum wire, heat will develop to such a
degree that the platinum will almost instantaneously become red hot and
emit a bright light, and that along a wire of some considerable length.
A similar effect is noticeable when we substitute other metals, such as
silver or iron, for platinum. And the _electric light_, which flashes
out rays of sunlike brilliance, is the result of placing a piece of
compact charcoal between the separated but confronting poles of a
powerful galvanic battery, light, developing more at the one pole and
heat more at the other of the incandescent substance.
Kindred, though much milder, results will show themselves under simpler,
though similar, contrivances. A flounder will jump and jerk about
uneasily if we lay it upon a piece of tinfoil and place over it a thin
plate of zinc, and then connect the two with a bent metal rod; which
will happen to an eel also, if we expose it to a gentle current from a
battery.
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