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medium, as on the temperature of that medium itself.

If the two hands be plunged, one in water at the temperature
of 200 and the other in snow, and being held there for a
certain time are transferred to water of the intermediate tem-
perature of 100, this water will appear warm to one hand and
cold to the other ; warm to the hand which had been plunged
in the snow, and cold to the hand which had been plunged in
the water at 200.

If on a hot day in summer we descend into a deep cave, it
will feel cold ; if we descend into the same deep cave on a frosty
day in winter, it will feel warm; yet a thermometer in this
case will prove that in the winter and in the summer it has
exactly the same temperature.

1609. These contradictions explained. These apparent ano-
malies are easily explained. The sensation of heat is relative.
When the body has been exposed to a high temperature, a
medium which has a lower temperature will feel cold, and
when it has been exposed to a low temperature it will feel
warm.

If in a room raised to a high temperature, as in a vapour or
hot-air bath, we touch with the hand different objects, they will
appear to have very different temperatures ; a woollen carpet
will feel cold, marble slabs warm, and metal objects very hot.
If, on the other hand, we are in a room at a very low temperature,
all these properties will be reversed ; the carpet will feel warm,
the marble slabs cold, and the metallic objects colder still.

These effects are easily explained. A woollen carpet is a
non-conductor of heat. When surrounding objects are at a
more elevated temperature than that of the body, the woollen
carpet partaking in this temperature will when touched feel
cool, because, being a non-conductor of heat, the heat which
pervades it does not pass freely to the part of the body which
touches it. A marble slab being a better conductor, and a
metallic object a still better, the heat will pass from them more
freely to the part of the body which touches them, and they
accordingly appear hotter.

But if the room be at a temperature much lower than the
body, then when we touch the woollen carpet the heat does not
pass from our body to the carpet because it is a non-conductor,
and as we do not lose heat the carpet feels warm ; but when



THE SENSATION OF HEAT. 147

we touch the marble, and still more a metallic object, the heat
passes more and more freely from our body to these objects,
and being sensible of a loss of heat more or less rapid, we feel
cold.

1610. Examples of the fallacious impressions produced by
objects on the touch. When we plunge in a cold bath, we are
accustomed to imagine that the water is colder than the air and
surrounding objects ; but if a thermometer be immersed in the
water, and another suspended in the air, they will indicate the
same temperature. The apparent cold of the water arises from
the fact that it abstracts from our bodies heat more rapidly
than air does, being a denser fluid and a greater number of
particles of it coming into contact at once with the surface of
the body. A linen feels colder than a cotton, and a cotton
colder than a flannel shirt, yet all the three are at exactly the
same temperature. Linen is a better conductor of heat than
cotton, and cotton than flannel, and, consequently, the heat
passes more freely through the first than the second, and through
the second than the third.

The sheets of a bed feel cold, and the blankets warm, and yet
they are of the same temperature, a fact which is explained
in the same manner.

The air which is impelled against a lady's face by her fan
feels cold, while the same air at rest around her feels warm ;
yet it is certain that the temperature of the air is not lowered
by being put in motion. The apparent coolness is explained
in this case by a slight evaporation, which is effected upon the
skin by the motion given to the air by the fan.

1611. Feats of fire-eaters explained. Some of the feats
performed by quacks and fire-eaters in exposing their bodies to
fierce temperatures may be easily explained upon this principle.
When a man goes into an oven raised to a very high tem-
perature., he takes care to place under his feet a cloth or mat
made of wool or other non-conducting substance upon which
he may stand with impunity at the proposed temperature. His
body is surrounded with air raised it is true to a very high
temperature, but the extreme tenuity of this fluid causes all
that portion of it in contact with the body at any given time to
produce but a slight effect in communicating heat. The exhi-
bitor always takes care to be out of contact with any good con-
ducting substance, and when he exhibits the effect produced by

H 2



148 HEAT.

the oven in which he is enclosed upon other objects, he takes as
much care to place them in a situation very different from that
which he himself has occupied. He exposes them to the effect
of metal or other good conductors.

Meat has been exhibited dressed in the apartment with the
exhibitor. A metal surface is in this case provided, and
probably raised to a much higher temperature than the atmo-
sphere in which the exhibitor is placed.



149



BOOK THE SECOND.

MAGNETISM.



CHAP. I.

DEFINITIONS AND PRIMARY PHENOMENA.

1612. Natural magnets Loadstone. Certain ferruginous
mineral ores are found in various countries, which being brought
into proximity with iron manifest an attraction for it. These
are called NATURAL MAGNETS or LOADSTONES ; the former term
being derived from MAGNESIA, a city of Lydia, in Asia Minor,
where the Greeks first discovered and observed the properties
of these minerals.

1613. Artificial magnets. The same property may be im-
parted to any mass of iron having any desired magnitude or
form, by processes which will be explained hereafter. Such
pieces of iron having thus acquired these properties are called
ARTIFICIAL MAGNETS ; and it is with these chiefly that scientific
experiments are made, since they can be produced in unlimited
quantity of any desired form and magnitude, and having the
magnetic virtue within practical limits in any desired degree.

1614. Neutral line or equator Poles. This attractive
power, which constitutes the peculiar character of the magnet,
whether natural or artificial, is not diffused uniformly over
every part of its surface. It is found to exist in some parts
with much greater force than in others, and on a magnet a
certain line is found where it disappears. This line divides the
magnet into two parts or regions, in which the attractive power
prevails in varying degrees, its energy augmenting with the
distance from the neutral line just mentioned.

This neutral line thus dividing the magnet into two different
regions of attraction may be called the EQUATOR of the magnet.

H 3



150 MAGNETISM.

The two regions of attraction separated by the equator are
called the poles of the magnet.

Sometimes this term pole is applied, not generally to the two
parts into which the magnet is divided by the equator, but to
two points upon or within them, which are the centres of all
the magnetic attractions exercised by the surface, in the same
manner as the centre of gravity is the centre of all the gravi-
tating forces which act upon the particles of a body.

1615. Experimental illustration of them. The neutral line
and the varying attraction of the parts of the surface of the
magnet which it separates may be manifested experimentally
as follows. Let a magnet, whether natural or artificial, be
rolled in a mass of fine iron filings. They will adhere to it,
and will collect in two tufts on its surface, separated by a space
upon which no filings will appear. The thickness with which
the filings are collected will increase as the distance from the
space which is free from them is augmented.

This effect, as exhibited by a na-
tural magnet of rough and irregular
form, is represented in^. 456. ; and
as exhibited by an artificial magnet
in the form of a regular rod or
cylinder whose length is considerable
as compared with its thickness, is
Fig. 456. represented in fig. 457. ; the equator

being represented by EQ, and the poles by A and B.





Fig. 457.

1616. Experimental illustration of the distribution of the mag-
netic force. The variation of the attraction of different parts
of the magnet may also be illustrated as follows. Let a magnet,
whether natural or artificial, be placed under a plate of glass or
a sheet of paper, and let iron filings be scattered on the paper or
glass over the magnet by means of a sieve, the paper or glass
being gently agitated so as to give free motion to the particles.



DEFINITIONS AND PRIMARY PHENOMENA. 151

They will be observed to affect a peculiar arrangement corre-
sponding with and indicating the neutral line or equator and the
poles of the magnet as represented in fig. 458., where E Q is the
equator, and A and B the poles of the magnet.




Fig. 458.

1617. Varying intensity of magnetic force indicated by a
pendulum. The varying intensity of the attraction of different
parts of the surface of the magnet may be ascertained by pre.
senting such surface to a small ball of iron suspended by a
fibre of silk so as to form a pendulum. The attraction of the
surface will draw this ball out of the perpendicular to an extent
greater or less, according to the energy of the attraction. If
the equator of the magnet be presented to it, no attraction will
be manifested, and the force of the attraction indicated will be
augmented according as the point presented to the pendulum
is more distant from the equator and nearer to the pole.

1618. Curve representing the varying intensity. This vary-
ing distribution of the attractive force over the surface of a
magnet may be represented by a curve whose distance from the
magnet varies proportionally to the intensity of this force.
Thus if, in fig. 459., EQ be the equator and A and B the poles
of the magnet, the curve ECDF may be imagined to be drawn
in such a manner that the distance of its several parts from the
bar EB shall be everywhere proportional to the intensity of the



152 MAGNETISM.

attractive force of the one pole, and a similar curve EC'D'F'
will in like manner be proportional to the varying attractions




Fig. 459.

of the several parts of the other pole. These curves necessarily
touch the magnet at the equator E Q, where the attraction is
nothing, and they recede from it more and more as their distance
from the equator increases.

1619. Magnetic attraction and repulsion. If two magnets,
being so placed as to have free motion, be presented to each
other, they will exhibit either mutual attraction or mutual
repulsion, according to the parts of their surfaces which are
brought into proximity. Let E and ^',fig. 460., be two magnets,



A E B

Fig. 460.

their poles being respectively A B and A' B'. Let the two poles
of each of these be successively presented to the same pole of a
third magnet. It will be found that one will be attracted and
the other repelled. Thus, the poles A and A' will be both
attracted, and the poles B and B' will be both repelled by the
pole of the third magnet, to which they are successively pre-
sented.

1620. Like poles repel, unlike attract. The poles A and A',
which are both attracted, and the poles B B', which are both
repelled by the same pole of a third magnet, are said to be like
poles ; and the poles A and B', and B and A', one of which is
attracted and the other repelled by the same pole of a third
magnet, are said to be unlike poles.

Thus the two poles of the same magnet are always unlike
poles, since one is always attracted, and the other repelled by
the same pole of any magnet to which they are successively
presented.



DEFINITIONS AND PRIMARY PHENOMENA. 153

If two like poles of two magnets, such as A and A' or B and B',
be presented to each other, they will be mutually repelled ; and
if two unlike poles, as A and B' or B and A', be presented to each
other, they will be mutually attracted.

Thus it is a general law of magnetic force, that like poles
mutually repel and unlike poles mutually attract.

1621. Magnets arrange themselves mutually parallel with
poles reversed. If a magnet A B be placed in a fixed position
on a horizontal plane, and another magnet be suspended freely
at its equator E' by a fibre of untwisted silk, the point of
suspension being brought so as to be vertical over the equator
E of the fixed magnet, the magnet suspended being thus
free to revolve round its equator E' in a horizontal plane, it
will so revolve, and will oscillate until at length it comes to
rest in a position parallel to the fixed magnet AB; the like
poles, however, being in contrary directions, that is to say, the
pole A' which is similar to A being over B, and the pole B' which
is similar to B being over A. This phenomenon follows
obviously from what has been just explained ; for if the magnet
A' B' be turned to any other direction, the arm E B attracting the
unlike arm E' A', and at the same time the arm E A attracting
the unlike arm E' B', the suspended magnet A' B' will be under the
operation of forces which have been already described (160),
and which are called a couple, consisting of two equal and con-
trary forces whose combined effect is to turn the magnet round
E' as a centre. When, however, the magnet A' B' ranges itself
parallel to A B, the like poles being in contrary directions, the
forces exerted balance each other, since the pole A attracts B' as
much as the pole B attracts A'.

1622. Magnetic axis. It has been already stated that certain
points within the two parts into which a magnet is divided by
the equator, which are the centres of magnetic force, are the
magnetic poles. A straight line joining these two points is
called the magnetic axis.

1623. How ascertained experimentally. If a magnet have
a symmetrical form, and the magnetic force be uniformly diffused
through it, its magnetic axis will coincide with the geometrical
axis of its figure. Thus, for example, if a cylindrical rod be
uniformly magnetized, its magnetic axis will be the axis of the
cylinder ; but this regular position of the magnetic axis does
not always prevail, and as its direction is of considerable iin-

H 5



154



MAGNETISM.



portance, it is necessary that its position may in all cases be
determined. This may be done by the following expedient :
Let the magnet, the direction of whose axis it is required to
ascertain, be suspended as already described, with its equator
exactly over that of a fixed magnet resting upon a horizontal
plane. The suspended magnet will then settle itself into such
a position that its magnetic axis will be parallel to the magnetic
axis of the fixed magnet which is under it. Its position when
thus in equilibrium being observed, let it be reversed in the
stirrup, so that without changing the position of its poles, its
under side shall be turned upwards, and vice versa. If after
this change the direction of the bar remains unaltered, its
magnetic axis will coincide with its geometrical axis ; but if, as
will generally happen, it take a different direction after being
reversed, then the true direction of the magnetic axis will be
intermediate between its directions before and after reversion.

To render this more clear, let A ~R,fig. 461., be the geometrical
axis of a regularly shaped prismatic magnet, and let it be
M required to discover the direction of its

^ * A magnetic axis. Let a b be the poles, and
JL- -J*^^/ the line JIN passing through them there-

\ j~ 7 fore its magnetic axis.
*\l I If this magnet be reversed in the man-
y / ner already described over a fixed magnet,
A / its magnetic axis in the new position will
/ \ / coincide with its direction in the first posi-
/ \ / tion, and the magnet when reversed will
/ \ / take the position represented by the dotted
\ / \l \l line, the geometrical axis being in the
A direction A' B', intersecting its former
i / \ direction A B at o. The poles a b will
/ \ coincide with their former position, as
\ I \ will also the magnetic axis M N. It is
,\ / J evident that the geometric axis o A will
-~-~r~ f orm with the magnetic axis oa the same
angle as it forms with that axis in the
second position, that is to say, the angle
Fig. 461. AOM will be equal to the angle A'OM;

and, consequently, the magnetic axis M N will bisect the angle
A o A', formed by the geometric axis of the magnet in its second
position.



DEFINITIONS AND PRIMARY PHENOMENA. 155

1624. Hypothesis of two fluids, boreal and austral. These
various phenomena of attraction and repulsion, with others
which will presently be stated, have been explained by different
suppositions, one of which assumes that all bodies susceptible of
magnetism are pervaded by a subtle imponderable fluid, which is
compound, consisting of two constituents called, for reasons
which will hereafter appear, the austral fluid and the boreal
fluid. Each of these is self-repulsive ; but they are reciprocally
attractive, that is to say, the austral fluid repels the austral, and
the boreal the boreal ; but the austral and boreal fluids recipro-
cally attract.

1625. Condition of the natural or unmagnetized state.
When a body pervaded by the compound fluid is in its natural
state and not magnetic, the two fluids are in a state of com-
bination, each molecule of the one being combined by attract-
tion with a molecule of the other ; consequently, in such state,
neither attraction nor repulsion is exercised, inasmuch as what-
ever is attracted by a molecule of the one is repelled by a
molecule of the other which is combined with it.

1626. Condition of the magnetized state. When a body
is magnetic, and manifests the powers of attraction and repul-
sion such as have been described, the magnetic fluid which
pervades it is decomposed, the austral fluid being directed on
one side of the equator, and the boreal fluid on the other. That
side of the equator towards which the austral fluid is directed is
the austral, and that towards which the boreal fluid is directed
is the boreal pole of the magnet.

If the austral poles of two magnets be presented to each other,
they will mutually repel, in consequence of the mutual repulsion
of the fluids which predominate in them ; and the same effect
will take place if the boreal poles be presented to each other.
If the austral pole of the one magnet be presented to the boreal
pole of another, mutual attraction will take place, because the
austral and boreal fluids, though separately self-repulsive, are
reciprocally attractive.

It is in this manner that the hypothesis of two self-repulsive
and mutually attractive fluids supplies an explanation of the
general magnetic law, that like poles repel and unlike poles
attract.

It must be observed that the attraction and repulsion in this
hypothesis are imputed not to the matter composing the mag-

H 6



156 MAGNETISM.

netic body, but to the hypothetical fluids by which this matter
is supposed to be pervaded.

1627. Coercive force The force with which the particles

of the austral and boreal fluids are combined, varies in different
bodies, in some being so slight that their decomposition is readily
effected, in others being so energetic that it is only accomplished
with considerable difficulty. It is found that in bodies where
the decomposition of the magnetic fluids is resisted, its recom-
position is also resisted, and that where the fluids are separated
with difficulty, when once separated they are recombined with
difficulty.

This force, by which the decomposition and recomposition of
the constituents of the magetic fluid are resisted, is called the
coercive force.

A different and more probable hypothesis for the explanation
of the phenomena will be explained hereafter.

1628. Coercive force insensible in soft iron most active in
highly tempered steel. Of the magnetic bodies, that in which
the coercive force is most feeble is soft iron, and that in which
it is manifested with greatest energy is highly tempered steel.

It might indeed be assumed hypothetically that the magnetic
fluid pervades all bodies whatsoever, but that its coercive force
in bodies which are said to be unsusceptible of magnetism is
such as to yield to no method of decomposition yet discovered.

1629. Magnetic substances. The only substances in which
the magnetic fluid has been decomposed, and which are there-
fore susceptible of magnetism, are iron, nickel, cobalt, chro-
mium, and manganese, the first being that in which the mag-
netic property is manifested by the most striking phenomena.



CHAP. II.

MAGNETISM BY INDUCTION.



1630. Soft iron rendered temporarily magnetic. If the ex-
tremity of a bar of soft iron be presented to one of the poles of
a magnet, this bar will itself become immediately magnetic. It
will manifest a neutral line and two poles, that pole which is in
contact with the magnet being of a contrary name to the pole



MAGNETISM BY INDUCTION. 157



which it touches. Thus, if AB,^. 462., be the bar of soft iron
which is brought in contact with the boreal pole b of the



Fig. 46'2.

magnet a b, then A will be the austral and B the boreal pole of
the bar of soft iron thus rendered magnetic by contact, and E
will be its equator, which however will not be in the middle of
the bar, but nearer to the point of contact.

These effects are thus explained by the hypothesis of two
fluids.

The attraction of the boreal pole of the magnet a b acting
upon the magnetic fluid which pervades the bar AB, decomposes
it, attracting the austral fluid towards the point of contact A,
and repelling the boreal fluid towards B. The austral fluid
accordingly predominates at the end A, and the boreal at the
end B, a neutral line or equator E separating them.

This state of the bar AB can be rendered experimentally
manifest by any of the tests already explained. If it be rolled
in iron filings, they will attach themselves in two tufts sepa-
rated by an intermediate point which is free from them ; and if
the test pendulum (1617) be successively presented to different
points of the bar, the varying intensity of the attraction will be
indicated.

If the bar AB be detached from the magnet, it will instantly
lose its magnetic virtue, the fluids which were decomposed and
separated will spontaneously recombine, and the bar will be re-
duced to its natural state, as may be proved by subjecting it
after separation to any of the tests already explained.

Thus is manifested the fact that the magnetism of soft iron
has no perceptible coercive force. The magnetic fluid is de-
composed by the contact of the pole of any magnet however
feeble, and when detached it is recomposed spontaneously and
immediately.

1631. This may be effected by proximity without contact.
It is not necessary, to produce these effects, that the bar of soft
iron should be brought into actual contact with the pole of a
magnet. It will be manifested, only in a less degree, if it be
brought into proximity with the pole without contact. If the
bar AB be presented at a small distance from the pole b, it will
manifest magnetism in the same manner ; and if it be gradually



158 MAGNETISM.

removed from the pole, the magnetism it manifests will dimi-
nish in degree, until at length it wholly disappears.

If the end B instead of A be presented to b, the poles of the
temporary magnet will be reversed, B becoming the austral,
and A the boreal.

If a series of bars of soft iron AB, A'B', A"B", be brought into
successive contiguity so as to form a series without absolute

a, e A B A. B ..A a""



Fig. 463.

contact, as represented in fig. 463., the extremity A of the first
being presented to the boreal pole b of the fixed magnet, then
each bar of the series will be rendered magnetic. The attrac-
tion of the boreal fluid at b will decompose the magnetic fluid
of the bar AB, attracting the austral fluid towards A, and re-
pelling the boreal fluid towards B. The boreal fluid thus driven
towards B will produce a like decomposition of the fluid in the
second bar A'B', the austral fluid being attracted towards A'
and the boreal repelled towards B' ; and like effects will be
produced upon the next bar A"B", and so on.

If the bars be brought gradually closer together, the intensity
of the magnetism thus developed will be increased, and will
continue to be increased until the bars are brought into contact.

1632. Induction. This process, by which magnetism is
developed by magnetic action at a distance, is called induction ;
and the bars AB, A'B', &c. are said to be magnetized by induction.



Online LibraryDionysius LardnerHand-book of natural philosophy and astronomy (Volume 2) → online text (page 17 of 45)