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1633. Magnets with poles reversed neutralize each other.
If a second magnet of equal intensity with the first be laid upon
a b with its poles reversed, so that its austral pole will coincide
with b and its boreal with a, the bars AB, A'B', A"B" magnetized by
induction will instantly be reduced to their natural state, and
deprived of the magnetic influence. This is easily explained.
The attraction of the pole b, which draws towards it the austral
and repels the boreal fluids of the bar AB, is neutralized by the
attraction and repulsion of the austral pole of the second magnet
laid upon it, which repels the austral fluid of the bar AB with a
force equal to that with which the boreal fluid of the pole b
attracts it, and attracts the boreal fluid with as much force as
that with which the pole b repels it. Thus the attraction and
repulsion of the two poles of the combined magnets neutralize



MAGNETISM BY INDUCTION. 159

each other, and the fluids which were decomposed in the bar
AB spontaneously recombine ; and the same effects take place in
the other bars.

All these effects may be rendered experimentally manifest by
submitting the bars AB,A'B',A"B" to any of the tests already
explained.

1634. A magnet broken at its equator produces two magnets.
It might be supposed, from what has been stated, that if a
magnetic bar were divided at its equator, two magnets would
be produced, one having austral and the other boreal magnetism,
so that one of them would attract an austral and repel a boreal
pole, while the other would produce the contrary attraction and
repulsion. This, however, is not found to be the case. If a
magnet be broken in two at its equator, two complete magnets
will result, having each an equator at or near its centre, and
two poles, austral and boreal ; and if these be again broken, other
magnets will be formed, each having an equator and two poles
as before ; and in the same manner, whatever be the number of
parts, and however minute they be, into which a magnet is
divided, each part will still be a complete magnet, with an
equator and two poles.

1635. Decomposition of magnetic fluid not attended by its
transfer between pole and pole. It follows from this, that it
cannot be supposed that the decomposition of the magnetic fluid
which is produced when a body is magnetized, is attended with
an actual transfer of the constituent fluids towards those regions
of the magnet which are separated by its equator. It cannot,
in a word, be assumed that the boreal fluid passes to one, and
the austral fluid to the other side of the equator ; for if this were
the case, the fracture of the magnet at the equator would leave
the two parts, one surcharged with austral and the other with
boreal fluid, whereas by what has been just stated it is apparent
that after such division both parts will possess both fluids.

1636. The decomposition is molecular. The decomposition
which takes place is therefore inferred to be accomplished spon-
taneously in each molecule which composes the magnet ; each
molecule is invested by an atmosphere composed of the two fluids,
and the decomposition takes place in these atmospheres, the
boreal fluid passing to one side of the molecule, and the austral
fluid to the other. When a bar is magnetized, therefore, the
material molecules which form it are invested with the magnetic



J60 MAGNETISM.

fluids, but the austral fluids are all presented towards the austral
pole, and the boreal fluids towards the boreal pole. When the
bar is not magnetic, but in its natural state, the two fluids sur-
rounding each molecule are diffused through each other and com-
bined, neither prevailing more at one side than the other.

1637. Coercive force of iron varies with its molecular struc-
ture. Iron in different states of aggregation possesses different
degrees of coercive force to resist the decomposition and re-
composition of the magnetic fluid. Soft iron, when pure, is
considered to be divested altogether of coercive force, or at least
it possesses it in an insensible degree. In a more impure state,
or when modified in its molecular structure by pressure, per-
cussion, torsion, or other mechanical effects, it acquires more or
less coercive power, and accordingly resists the reception of
magnetism, and when magnetism has been imparted to it, retains
it with a proportional force. Steel has still more coercive force
than iron, and steel of different tempers manifests the coercive
force in different degrees, that which possesses it in the highest
degree being the steel which is of the highest temper, and
which possesses in the greatest degree the qualities of hardness
and brittleness.

1638. Effect of induction on hard iron or steel. If a bar of
hard iron or steel be placed with its end in contact with a
magnet in the same manner as has been already described with
respect to soft iron, it will exhibit no magnetism ; but if it be kept
in contact with the magnet for a considerable length of time, it
will gradually acquire the same magnetic properties as have been
described in respect to bars of soft iron, with this difference,
however, that having thus acquired them, it does not lose them
when detached from the magnet, as is the case with soft iron.
Thus it would appear, that it is not literally true that a bar of
steel when brought into contact with the pole of a magnet
receives no magnetism, but rather that it receives magnetism
in an insensible degree ; for if continued contact impart sensible
magnetism, it must be admitted that contact for shorter intervals
must impart more or less magnetism, since it is by the accumu-
lation of the effects produced from moment to moment that the
sensible magnetism manifested by continued contact is pro-
duced.

It appears, therefore, that the coercive energy of the bar of
steel resists the action of the magnet, so that while the pole of



MAGNETISM BY INDUCTION. 161

the magnet accomplishes the decomposition of the magnetic
fluid in a bar of soft iron instantaneously, or at least in an in-
definitely small interval of time, it accomplishes in a bar of steel
the same decomposition, but only after a long protracted interval,
the decomposition proceeding by little and little, from moment
to moment during such interral.

Various expedients, as will appear hereafter, have been con-
trived, by which the decomposition in the case of steel bars
having a great coercive force is expedited. These consist gene-
rally in moving the pole of the magnet successively over the
various points of the steel bar, upon which it is desired to pro-
duce the decomposition, the motion being always made with the
contact of the same pole, and in the same direction. The pole
is thus made to act successively upon every part of the surface
of the bar to be magnetized, and being brought into closer con-
tact with it acts more energetically ; whereas when applied to
only one point, the energy of its action upon other points is en-
feebled by distance, the intensity of the magnetic attraction
diminishing, like that of gravity, in the same proportion as the
square of the distance increases.

Since steel bars having once received the magnetic virtue in
this manner retain it for an indefinite time, artificial magnets
can be produced by these means of any required form and
magnitude.

1639. Forms of magnetic needles and bars. Thus a mag-
netic needle generally receives the form of a lozenge, as repre-
sented in fig. 464., having a co-
nical cap of agate at its centre,
which is supported upon a pivot
in such a manner as that the
needle is free to turn in a hori-
zontal plane, round the pivot as
a centre. In this case the weight
of the needle must be so regu-
lated as to be in equilibrium on

Figl 464> the pivot.

Bar magnets are pieces of steel in the form of a cylinder or
prism whose length is considerable compared with their depth
or thickness. In producing such magnets certain processes are
necessary, which will be explained hereafter.

1640. Compound magnet. Several bar magnets, equal and



J f

I



162 MAGNETISM.

similar in magnitude, being placed one upon the other with
their corresponding poles together, form a compound magnet.

1641. Effects of heat on magnetism. It is evident from
what has been stated respecting the various degrees of coercive
force manifested by the same metal in different states of aggre-
gation, that the magnetic qualities depend upon molecular
arrangement, and that the same body in different molecular
states will exhibit different magnetic properties.

Since the elevation or depression of temperature by pro-
ducing dilatation and contraction affects the molecular state of
a body, it might be expected to modify also its magnetic pro-
perties, and this is accordingly found to be the case.

1642. A red heat destroys the magnetism of iron. If a
magnet, no matter how powerful, natural or artificial, be raised
to a red heat, it will lose altogether its magnetic virtue. The
elevation of temperature and the molecular dilatation conse-
quent upon it destroys the coercive force and allows the
recombination of the magnetic fluid. When after such change
the magnet is allowed to cool, it will continue divested of its
magnetic qualities. These effects may, however, be again
imparted to it by the process already mentioned.

1643. Different magnetic bodies lose their magnetism at dif-
ferent temperatures. M. Pouillet found that this phenomenon

is produced at different temperatures for the different bodies
which are susceptible of magnetism. Thus the magnetism of
nickel is effaced when it is raised to the temperature of 660,
iron at a cherry red, and cobalt at a temperature much more
elevated.

1644. Heat opposed to induction. But not only does in-
creased temperature depi'ive permanent magnets of their mag-
netism, but it renders even soft iron insusceptible of magnetism
by induction, for it is found that soft iron rendered incandescent
does not become magnetic when brought into contact or con-
tiguity with the pole of a magnet.

1645. Induced magnetism rendered permanent by hammering
and other mechanical effects. If a bar of soft iron when
rendered magnetic by induction be hammered, rolled, or
twisted, it will retain its magnetism. It would follow, there-
fore, that the change of molecular arrangement produced by
these processes confers upon it a coercive force which it had not
previously.



MAGNETISM BY INDUCTION. 163

1646. Compounds of iron differently susceptible of magnet-
ism. Compounds of iron are in general more or less sus-
ceptible of magnetism, according to the proportion of iron they
contain.

Exceptions, however, to this are presented in the peroxide,
the persulphate, and some other compounds containing iron in
small proportion, in which the magnetic virtue is not at all
present.

1647. Compounds of other magnetic bodies not susceptible.
Nickel, cobalt, chromium, and manganese are the only simple
bodies which, in common with iron, enjoy the magnetic pro-
perty, and this property completely disappears in most of the
chemical compounds of which they form a part.

Magnetism, however, has been rendered manifest under a
great variety of circumstances connected with the development
of electricity which will be fully explained in a subsequent
Book.

1648. Magnets with consequent points. In the production
of artificial magnets it frequently happens that a magnetic bar
has more than one equator, and consequently more than two
poles. This fact may be experimentally ascertained by ex-
posing successively the length of a bar to any of the tests already
explained. Thus, if presented to the test pendulum, it will
be attracted with a continually decreasing force as it approaches
each equator, and with an increasing force as it recedes from it.
If the bar be rolled in iron filings, they will be attached to it in
a succession of tufts separated by spaces where none are attached,
indicating the equators.

If it be placed under a glass plate or sheet of paper on which
fine iron filings are sprinkled, they will arrange themselves
according to a series of concentric curves, as represented in
fig. 465.



164



MAGNETISM.



It is evident that the magnetic bar in this case is equivalent
to a succession of independent magnets placed pole to pole.
The equators in these cases are called consequent points.



CHAP. III.

TERRESTRIAL MAGNETISM.

1649. Analogy of the earth to a magnet. If a small and
sensitive magnetic needle, suspended by a fibre of silk so as
to be free to assume any position which the attractions that act
upon it may have a tendency to give to it, be carried over a
magnetic bar from end to end, it will assume in different positions
different directions, depending on the effect produced by the
attractions and repulsions exercised by the bar upon it.

Let ab,Jig. 466., be such a needle, the thread of suspension oe
being first placed vertically over the equator E of the magnetic







1



E
Fig. 466.

bar AB. The austral magnetism of AE will attract the boreal
magnetism of be and will repel the austral magnetism of ae;
and in like manner the boreal magnetism of BE will attract the
austral magnetism of ae and will repel the boreal magnetism of
be. These attractions and repulsions will moreover be re-
spectively equal, since the distances of ae and be from BA and
BE are equal. The needle a b will therefore settle itself
parallel to the bar AB, the pole a being directed to B, and the
pole b being directed to A.

If the suspending thread oe be removed towards A to PC,
the attraction of A upon b will become greater than the at-



TERRESTRIAL MAGNETISM. 165

traction of B upon a, because the distance of A from b will be
less than the distance of B from a ; and, for a like reason, the
repulsion of A upon a will be greater than the repulsion of B
upon b. The needle a b will therefore be affected as if the end
b were heavier than a, and it will throw itself into the inclined
position represented in the figure, the pole a inclining down-
wards.

If it be carried still further towards A, the inequality of the
attractions and repulsions increasing in consequence of the
greater inequality of the distances of a and b from A and B,
the inclination of b downwards will be proportionally aug-
mented, as represented at p'.

In fine, when the thread of suspension is moved to a point
p" over the pole A, the needle will become vertical, the pole b
attracted by A pointing downwards.

If the needle be carried in like manner from E to B, like
effects will be manifested, as represented in the figure, the pole
a inclining downwards arising from the same causes.

A magnetic needle similarly suspended, carried over the surface
of the earth in the directions north and south, undergoes changes
of direction such as would be produced, on the principles ex-
plained above, if the globe were a magnet having its poles at
certain points, not far distant from its poles of rotation.

To render this experimentally evident, it will be necessary to
be provided with two magnetic instruments, one mounted so that
the needle shall have a motion in a horizontal plane round a
vertical axis, and the other so that it shall have a motion in a
vertical plane round a horizontal axis.

1650. The azimuth compass. The instrument called the
azimuth compass consists of a magnetic bar or needle balanced
on a vertical pivot, so as to be capable of turning freely in a
horizontal plane, the point of the needle playing in a circle, of
which its pivot is the centre.

The instrument is variously mounted and variously desig-
nated, according to the circumstances and purpose of its appli-
cation.

When used to indicate the relative bearings or horizontal
directions of distant objects, whether terrestrial or celestial, a
graduated circle is placed under the needle and concentric with
it. The divisions of this circle indicate the bearings of any
distant object in relation to the direction of the needle.



166



MAGNETISM.



The pivot in this form of compass is rendered vertical by
means of a plumb-line or spirit-level.

1651. The mariner's compass. When the azimuth compass
is used for the purpose of navigation, the pivot supporting the
needle is fixed in the bottom of a cylindrical box, which is
closed at the top by a plate of glass, so as to protect it from the
air. The magnetic bar is attached to the under side of a cir-
cular card, upon which is engraved a radiating diagram, which
divides the circle into thirty-two parts called points. The
compass box is suspended so as to preserve its horizontal posi-
tion undisturbed by the motion of the vessel, by means of two
concentric hoops called gimbals, one a little less than and in-
cluded within the other. It is supported at two points upon
the lesser hoop, which are diametrically opposite, and this lesser
hoop itself is supported by two points upon the greater hoop,
which are also diametrically opposite, but at right angles to the
former. By these means the box, being at liberty to swing in
two planes at right angles to each other, will maintain itself
horizontal, and will therefore keep the pivot supporting the
needle vertical, whatever be the changes of position of the
vessel.

This arrangement is represented in fig. 467., a vertical sec-
tion of the compass box being given in fig. 468.




Fig. 467. Fig. 468.

The sides of the cylindrical box are bb', its bottom//', and
the glass which covers it v. The magnetic bar or needle is sup-
ported on a vertical pivot by means of a conical cup, and can
be raised and lowered at pleasure by means of a screw w. The
compass card is represented in section at rr f , fig. 467., and the
divisions upon it marked by radiating lines called the rose are
represented in fig. 468.



TERRESTRIAL MAGNETISM.



167



Two narrow plates, p and p f , are attached to the sides of the
box so as to be diametrically opposed. In p there is a narrow
vertical slit. In p' there is wider vertical slit, along which is
stretched vertically a thin wire. The eye placed at o looks
through the two slits, and turns the instrument round its sup-
port until the object of observation is intersected by the ver-
tical wire, extended along the slit p'. Provisions are made
in the instrument by which the direction thus observed can
be ascertained relatively to that of the needle. The angle
included between the direction of the observed object, and
that of the needle, is the bearing of the object relatively to the
needle.

The compass box is suspended within the hoop e e, at two
points z z' diametrically opposed, and the hoop e e 1 is itself sus-
pended within the fixed hoop c c', at two points x a/, also diame-
trically opposed, but at right angles to z z 1 .

1652. The dipping needle. The apparatus represented in
fig. 469., called the dipping needle,
consists of a magnetic needle e e, sup-
ported and balanced on a horizontal
axis, and playing therefore in a ver-
tical plane. The angles through
which it turns are indicated by a
graduated circle / 1', the centre of
which coincides with the axis of the
needle, and the frame which sup-
ports it has an azimuth motion round
a vertical axis, which is indicated
and measured by the graduated
horizontal circle z z'. The instru-
ment is adjusted by means of a
spirit-level, and regulating screws
inserted in the feet.

Fig- 469. 1653. Analysis of magnetic phe-

nomena of the earth. Supplied with these instruments, it will
be easy to submit to observation the magnetic phenomena
manifested at different parts of the earth.

If the azimuth compass be placed anywhere in the northern
hemisphere, at London for example, the needle will take a certain
position, forming an angle with the terrestrial meridian, and
directing one pole to a point a certain number of degrees west




168 MAGNETISM.

of the north, and the other to a point a like number of degrees
east of the south. If it be turned aside from this direction, it
will when liberated oscillate on the one side and the other of
this direction, and soon come to rest in it.

Since an unmagnetized needle would rest indifferently in any
direction, this preference of the magnetized needle to one parti-
cular direction must be ascribed to a magnetic force exerted by
the earth attracting one of the poles of the needle in one direc-
tion, and the other pole in the opposite direction.

That this is not the casual attraction of unmagnetic ferrugi-
nous matter contained within the earth, is proved by the fact that,
if the direction of the needle be reversed, it will, when liberated,
make a pirouette upon its pivot, and after some oscillations re-
sume its former direction.

This remarkable property is reproduced in all parts of the
earth, on land and water, and equally on the summits of lofty
mountains, in the lowest valleys, and in the deepest mines.

1654. Magnetic meridian. The direction thus assumed by
the horizontal needle, in any given place, is called the MAGNETIC
MERIDIAN of that place.

The direction of a needle which would point due north and
south is the TRUE MERIDIAN, or the TERRESTRIAL MERIDIAN of
the place.

1655. Declination or variation. The angle formed by the

MAGNETIC MERIDIAN and the TERRESTRIAL MERIDIAN is Called

sometimes the VARIATION, and sometimes the DECLINATION of
the needle. We shall adopt by preference the latter term.

The declination is said to be EASTERN or WESTERN, according as
the pole of the needle, which is directed northwards, deviates to
the east or to the west of the terrestrial meridian.

1656. Magnetic polarity of the earth. To explain these
phenomena, therefore, the globe of the earth itself is considered
as a magnet, whose poles attract and repel the poles of the
horizontal needle, each pole of the earth attracting that of an
unlike name, and repelling that of a like name.

If, therefore, the northern pole of the earth be considered as
that which is pervaded by boreal magnetism, and the southern
pole by austral magnetism, the former will attract the austral
and repel the boreal pole, and the latter will attract the boreal
and repel the austral pole of the needle. Hence it will follow
that the pole of the needle which is directed northwards is the



TERRESTRIAL MAGNETISM. 169

austral, and that which is directed southwards is the boreal
pole.

16<>7. Change of direction of the dipping-needle. It was
shown in (1649) that when a needle which is free to play in
a vertical plane was carried over a magnet, it rested in the
horizontal position only when suspended vertically over the
equator of the magnet, and its austral and boreal poles were in-
clined downwards, according as the needle was suspended at the
boreal or austral side of the equator, and that this inclination
was augmented as the distance from the equator at which the
needle was suspended was increased. Now it remains to be
seen whether any phenomenon analogous to this is presented
by the earth.

For this purpose, let the dipping-needle,^. 469., be arranged
with its axis at right angles to the direction of the needle of
the azimuth compass. It will then be found, that in general
the dipping-needle will not rest in a horizontal position, but will
assume a direction inclined to the vertical line, as represented
in the figure, one pole being presented downwards, and the other
upwards.

The angle which the lower arm of the needle makes with the
horizontal line is called the dip.

If this apparatus be carried in this hemisphere northwards,
in the direction in which a horizontal needle would point, the
austral pole will be inclined downwai'ds, and the dip will con-
tinually increase ; but if it be carried southwards, the dip will
continually diminish. By continuing to transport it southwards,
the dip continually diminishing, a station will at length be found
where the needle will rest in the horizontal position. If it be
carried further southwards, the boreal pole will begin to turn
downwards ; in other words, the dip will be south instead of
north, and as it is carried further southwards, this dip will con-
tinue to increase.

If the needle be carried northwards, in this hemisphere the
dip continually augmenting, a station will at length be attained
where the needle will become vertical, the austral pole being
presented downwards, and the boreal pole upwards.

In the same manner, in the southern hemisphere, if the needle
be carried southwards, a station will at length be attained
where it will become vertical, the boreal pole being presented
downwards, and the austral pole pointing to the zenith.



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