G. P. (George Payn) Quackenbos.

A natural philosphy: embracing the most recent discoveries in the various branches of physics .. online

. (page 33 of 42)
Online LibraryG. P. (George Payn) QuackenbosA natural philosphy: embracing the most recent discoveries in the various branches of physics .. → online text (page 33 of 42)
Font size
QR-code for this ebook

iron. Sir Isaac Newton is said to have worn, in a ring, a
piece of loadstone weighing three grains, which would lift
750 grains of iron. Their attractive power, however, does
not increase with their size. Large pieces of loadstone
never support more than five or six times their own weight,
and rarely as much. The most powerful natural magnet
known is capable of lifting 310 pounds.

861. Armature. The power of
a natural magnet is increased By
applying vertically to its opposite
polar surfaces thin strips of soft
iron, projecting a little below, and
bent, as shown in ap, b n, Fig.
302. The attractive force then
centres in p and M, which become

the new poles. This arrangement is called an Armature,
and a magnet so prepared is said to be armed.

To keep the armature in its place, metallic
bands, A B, C D (Fig. 303), are passed round the
whole. A ring, R, is attached to the top for con-
venience of handling. The effect of the magnet
is further increased by uniting its poles with a
transverse piece of soft iron, K, called the Keeper.
To this a hook is attached for suspending a scale-
pan and weights.

of iron or steel brought in contact with
a natural magnet or very near it, ac-
quires its peculiar properties, and will
itself attract steel-filings, needles, &c.
Soft iron loses these properties on be-
ing withdrawn from the magnet ; but a piece of steel re-
tains them permanently, nor does the natural magnet from

broken, what is said of the fragments ? 860. What is the power of natural magnets,
when small ? When large ? How much is the most powerful natural magnet known
capable of lifting ? 8G1. How is the power of a natural magnet increased? Describe
the armature and the arrangement for securing it in its place. IIow is the effect of

Fig. 30a





which it receives them suffer any diminution of power in

A piece of iron or steel to which magnetic properties
have been imparted in any way, is called an Artificial

863. Kinds of Artificial Magnets. There are several
kinds of artificial magnets, called from their shape Bar Mag-
nets, Horse-shoe Magnets, and Magnetic Needles. The first
two are most powerful when formed of several similar pieces
riveted together, in which case they are called Compound

Fig. 304 Fig. 305.

G ' ' ~~



Fig. 304 represents a Compound Bar Magnet ; Fig. 305,
a Compound Horse-shoe Magnet. N, S, represent the poles.
The Horse-shoe magnet has an armature, A, attached, which
increases and preserves its power, and should always be
kept on when the magnet is not in use.

Magnetic Needles are very light magnetic
bars (see Fig. 306), poised at their centre on a
pivot, on which they move freely either hori-

F5g. 306. zontally or up and down.

In the former case, they
are called Horizontal Nee-
dles; in the latter, Verti-
cal or Dipping Needles.

8G4 Artificial magnets are more
efficient and regular in their action
than natural ones, and are therefore

*==* preferred for purposes of experi-

MAGNETIC NEEDLE. mcnt. The horse-shoe is more

the magnet further increased? 862. How may magnetic properties be imparted to a
piece of iron or steel ? What is the difference between soft iron and steel in this
connection ? What is an Artificial Magnet ? 863. Name the different kinds of artifi-
cial magnets. What are Compound Magnets? What do Figs. 304, 305, represent ?
What are Magnetic Needles ? Into what two classes are they divided ? 864. How
do artificial magnets compare in efficiency with natural ones ? IIow does tho horse-


powerful than the bar magnet. A horse-shoe of one pound has been known
to sustain 26y 2 pounds.

865. Poles. The poles of an artificial magnet, that is,
the points in which the greatest attractive force resides,
are found to be about one-tenth of an inch from the ex-
tremities. In very long bar magnets, besides the two poles
always situated near the extremities, two other poles, nearer
the centre, are sometimes, though rarely, found.

866. The power of a magnet, whether natural or artifi-
cial, may be increased by daily adding a little to the weight
which it will support. If, for instance, a given magnet just
sustains two pounds of iron, by putting on a small addi-
tional weight every day, we may perhaps make it sustain
three or even four pounds. If, on the other hand, we over-
load it, so that the armature falls off, the power of the mag-
net will be impaired. Any rough treatment, such as ham-
mering the magnet, rubbing it violently, or letting it fall,
has the same effect. Heat, also, diminishes the power of a
magnet. Red heat destroys it altogether, even after the
magnet has cooled.

867. Air is not essential to the action of a magnet; all
its phenomena may be exhibited in a vacuum.

o2 tlae Magaiet.

888. ATTRACTION. As stated above, all magnets attract
unmagnetic iron. They are also attracted by it.

Suspend a magnetic needle by a thread. Bring a piece of iron near either
extremity, and the needle will be drawn towards it.

869. Magnetic attraction acts with undiminished power
through any thin substance.

In the last experiment interpose a piece of glass or paste-board between
the iron and the needle ; the latter will be attracted none the less.

thoe compare with the bar magnet ? 865. Where do the poles of an artificial magnet
lie ? What are sometimes found in very long bar magnets ? 866. What is the effect
of adding a little daily to the weight which a magnet supports ? Give an example.
What is the effect of overloading a magnet ? Of treating it roughly ? Of heating it ?
86T. Is air essential to the action of a magnet ? Prove it. 863. "What is the first prop-
erty of magnets ? "What experiment shows the attraction of iron for a magnet ?
BOD. What is tho effect of interposing any thin substance ? How may this fact bo





Hold a piece of paper over q
bar magnet, and dust on it some
steel filings. Under the influence
of the magnetic attraction trans-
mitted through the paper, they
will arrange themselves in regu-
lar lines, as shown in Fig. 307.
These lines are called Magnetic
Curves. The superior attractive
power of the poles is also shown
by this experiment ; for the filings are thickest directly over those points, the
curves appearing to converge there from all directions.

Magnetic figures of any description may be formed on a steel plate by
marking on it with one of the poles of a bar magnet, and then sprinkling
iron filings on the surface. They will at once adhere to the lines which the
magnet has traced. The result is the same if paper is laid on the steel sur-
face before the bar is drawn over it, the magnetic influence being transmitted
through the paper^

870. Law. Magnetic attraction decreases in intensity
as the square of the distance from the magnet increases.

If two similar substances are situated respectively 1 inch and 2 inches
from a given magnet, the former will be attracted 4 times as strongly as the
latter. This law corresponds with that of gravitation, light, and heat.

871. POLARITY. A magnetic needle, left free to move,
always points north and south, or nearly so. Often as it
may be disturbed from its natural position, it invariably re-
sumes it after a few vibrations. This property is called
Magnetic or Directive Polarity.

It is to be observed in connection with magnetic polarity
that the same extremity of the needle always points to the
north, and the same extremity to the south. That which
points north is called the North Pole ; and that which points
south, the South Pole. Turn the needle round till its north
pole points south, and it will not rest till it has traversed
a semicircle and got round again to the north.

872. If the poles of a bar or horse-shoe magnet be pre-
'sented successively to the north pole of a magnetic needle,

illustrated ? How are Magnetic Curves formed ? "W hat does this experiment show ?
How may magnetic figures be formed ? What is tho effect of Interposing paper be-
tween the magnet and the steel surface ? 870. What is the law of magnetic attrac-
tion? Give an example. 871. What is meant by Magnetic or Directive Polarity?
What is to bo observed in connection with magnetic polarity ? What name is given


one of them will be found to attract it and the other to
repel it. If the experiment be tried with a number of dif-
ferent needles, the same pole will always be found to at-
tract, and the same to repel. This shows that the two poles
of the magnet have different properties, which we indicate
by giving them different names. The one that attracts the
north pole of the needle we call the South Pole of the
magnet, and the one that repels it, the North Pole.

873. General Law. Like poles of magnets repel each
other, and unlike poles attract each other. This law corre-
sponds with that of electrical attraction and repulsion.

Balance a bar magnet with weights on a pair of scales. Beneath its pos-
itive pole bring the positive pole of another magnet, and the scale containing
the bar will rise owing to the repulsion of the like poles. Substitute the neg-
ative pole, and the scale will descend owing to the attraction of the unlike poles.

874. Like poles neutralize each other's attraction for
unmagnetic iron.

Immerse the positive poles of two magnets separately in iron filings. On
withdrawing them, both will be covered with large tufts. Now bring them
together, and the filings will immediately drop off from both. The result
will be the same if the experiment be tried with the negative poles of two
magnets. If the positive pole of one magnet and the negative of the other
be used, the filings, instead of falling off, will join in a festoon between the
two unlike poles.

875. The Astatic Needle. The
polarity of two needles of equal
power may be neutralized by sup-
porting them on the same pivot, one
above the other, parallel and with
unlike poles pointing in the same
direction. An instrument so formed
is called the Astatic Needle.

Fig. 308 represents an astatic needle. The
north pole of the upper one points the same
way as the south pole of the under one, and

to the two poles of the needle ? 872. How is it shown that the poles of a bar magnet
have different properties ? How are these poles distinguished ? 873. What is the
law of magnetic attraction and repulsion? Illustrate this law with an experiment.
874. What is the effect of like poles on each other's attraction ? Show this experi-
mentally. 875. How may the polarity of two needles of equal power be destroyed ?


vice versa. The consequence is that the polarity of both is destroyed ; the
heedles will remain in whatever direction they are placed.

876. When a magnet is divided, each portion becomes
a perfect magnet in itself, and has its own poles, even though
the parts in which the new poles lie exhibited no magnetic
attraction at all before the division. Those extremities of
the divided portions which lie towards the north pole of
the original magnet will all be north poles, and the extrem-
ities towards its south pole will all be south poles.

877. Magnetic Variation. In a given place, all mag-
netic needles point in the same direction. This direction
is called the Magnetic Meridian.

In some parts of the earth the magnetic meridian runs
due north and south ; that is, a plane extended in the di-
rection in which the needle stands would pass through the
north and the south pole of the earth. The magnetic me-
ridian would then correspond with the geographical me-
ridian. In most places, however, the magnetic meridian
deviates more or less from the geographical meridian. This
deviation is called the Variation of the Needle, or Magnetic

The variation of the needle is different at different places on the earth's
surface, and is constantly changing at the same place. Recorded observa-
tions in the old world show that for a series of years the needle kept varying
more and more towards the west; till, having attained its western limit, it
turned back towards the east, in which direction it is now moving. The
cause of this periodical change and the law which regulates it are as yet un-
known. At Washington City the variation is now between 2 and 3 degrees
west; that is, the needle points between 2 and 3 degrees west of north.
Every year it becomes somewhat greater, the annual rate of increase being
about 3'.

Two irregular lines (which are constantly changing) may be traced on
the earth's surface, one in each hemisphere, along which the needle points
due north and south. They are called Lines of no Variation.

878. Magnetic Dip. An ordinary steel needle, poised

Describe the Astatic Needle. 876. When a magnet is divided, what is said of each
portion ? Which extremities of the divided portions will be north poles, and which
south ? 877. What is the Magnetic Meridian ? In some parts of the earth how does
the magnetic meridian run ? How, in others ? "What is meant by Magnetic Varia-
tion? What do recorded observations show? What is the present variation at
"Washington City, and how is it changing from year to year ? "What is meant by



Fig. 309.

on its centre of gravity so as to move freely up and down,
remains in any position in which it may be placed ; if mag-
netized, in most parts of the earth it inclines more or less
towards the horizon. This inclination is called the Dip of
the Needle, or Magnetic Dip. It was discovered in 1576,
by an optician of London.

With the Dipping Needle and graduat-
ed scale attached, represented in Fig. 309,
the magnetic dip at any given place can be
measured. Experiments with this instru-
ment show that there are two points, one
in the northern hemisphere (latitude 70),
the other in the southern (lat. 75), in
which the needle stands vertical, and the
dip is therefore 90 degrees. That, on the
contrary, there is a circle of points near
the equator, at which the needle is par-
allel to the horizon, and the dip is ;
this line is called the Magnetic Equator.
At different intermediate points the dip is
different, increasing, though not regular-


ly, as the distance from the magnetic equator increases. The dip, like the
variation, keeps changing at a given place. In the latitude of New York it
is now about 70 degrees, and is constantly decreasing.

879. The Compass. The polarity ol the magnetic nee-
dle, applied in the Compass, enables us to determine, at
any place, a given direction or the bearing of a given object.

The Land or Surveyor's Compass is simply a magnetic
needle set in a shallow case covered with glass, on the bot-
tom of which is a circular card, having its circumference
divided into 360 degrees. At a distance of one-fourth of
the circumference apart stand the letters N", E, S, "W, de-
noting the four cardinal points North, East, South, West.
As the needle is stationary, while the card moves, the order
of the points is reversed ; that is, when we hold the instru-
ment so as to have the point S next to us, E is on the left,
and W on the right.

Lines of no Variation ? How many are there ? 8T8. "What is the Dip of the Needle ?
When and by whom was it discovered? How may the dip at any given place bo
measured ? What is shown by experiments with the- dipping needle ? How great
is the dip in f he latitude of New York ? 879. In what instrument is the polarity of



880. It is to the navigator, who relies entirely on it for
guidance over the trackless ocean to his desired port, that
the compass is most important. Arranged for his use, it is
called the Mariner's Compass.

Pig. 810. In the mariner's

compass, repre-
sented in Fig. 310,
the circular card
is attached to the
needle and turns
with it. The cir-
cumference of the
card is divided into
32 equal parts, de-
noted by marks and
sometimes subdi-
vided into halves
and quarters. These
marks have names
given to them, in-
dicating the dif-
ferent directions,
which are called
Points of the Com-
pass. Mentioning
THE MARINER'S COMPASS. the pomtg of tne

compass in their order is called boxing tlie compass. The compass box is
suspended within a larger box by means of two brass hoops, or gimbals as
they are called, supported at opposite points on pivots, so that however the
vessel may roll or pitch the needle may retain its horizontal position.

It is believed that the Chinese were the first to avail themselves of the
magnet in navigation, many hundred years before the Christian era ; and
that from them various other eastern nations learned to use it for the same
purpose. The compass of these early times was probably nothing more than
a piece of loadstone mounted on a cork and allowed to float on water. The
magnetic needle and the card attached to it were no doubt the inventions of
Europeans, among whom a knowledge of the rude compass used in the East
appears to have been introduced in th twelfth century after Christ. Flavio
Gioia [flaJi -ve-o jo -ya7i\, a Neapolitan who flourished about the year 1300,

the magnetic needle applied ? Describe the Land Compass. SSO. To whom is the
compass most important? Describe the Mariner's Compass. What is meant by
boxing the compass f How is the compass box suspended ? Who are thought to
have been the first to use the magnet in navigation ? What did this ancient compass
probably consist of? When did it first become known in Europe ? What improve-
ments were soon made ? How did the name of Flavio Gioia become connected with


by some regarded as the inventor of the compass, probably merely improved
its construction, or extended its use among the maritime nations of Europe.

No one can estimate how much the invention of the mariner's compass
has contributed to the progress of the world. Relying on his little needle,
which never betrays its trust, the mariner is no longer obliged to keep his
bark within sight of laud, and to direct his course by sun and star which
clouds may obscure for days and nights together. He fearlessly ventures
into unknown seas, explores the remotest regions, pursues his way under
lowering skies and in utter darkness, well knowing whither he is sailing and
how to steer when he wishes to retrace his course. This simple instrument
has thus made the ocean a safe and frequented highway, extended the com-
merce and knowledge of the world, linked its most distant families in friendly
intercourse, and brought whole continents virtually into being.

881. The compass needle, like all other magnetic nee-
dles, is subject to variation and dip.

Its variation seems to have been known two hundred years before the
time of Columbus ; but that this variation differs in different places was dis-
covered by that navigator on his memorable voyage across the Atlantic in
1492. As he went westward, he observed that the variation increased from
day to day. The fact was soon discovered by his crew, and filled them with
consternation. It seemed * as if the very laws of nature were changing, and
they were entering a new world subject to mysterious influences'. It re-
quired all the ingenuity of Columbus to induce them to proceed ; which he
did by allaying their fears with an explanation of the phenomenon satisfac-
tory to them, though it was far from satisfying himself.

As the compass needle must be perfectly horizontal, the dip is counter-
balanced by loading the end that tends to rise with a small weight, which
may be shifted to suit any latitude.

Tiaeory of Magraetism.

882. The theory of magnetism is analogous to that of
electricity. An agent, which for convenience' sake we call
the magnetic fluid, may be supposed to pervade all things.
In its quiescent state it is a combination of two fluids, which
may be distinguished as North or Positive, and South or
Negative. When combined, these fluids neutralize each
other, and no magnetic phenomena are exhibited; but

the compass ? What is said of the effects which this simple instrument has wrought?
881. To what is the compass needle subject ? How long ago was the variation of the
needle known ? What discovery did Columbus make respecting it ? What was the
effect of this discovery on his crew ? How is the dip counterbalanced in the com-
pass needle ? 8S2. State the theory of magnetism. How are the phenomena exhib-


when through any agency they are separated, the substance
containing them displays magnetic properties, and is said
to be magnetized.

In loadstone, as found in nature, the two fluids do not
combine at all. In soft iron and steel they are easily sep-
arated, but in the former re-combine as soon as the separ-
ating agency is withdrawn, while in the latter they remain
permanently apart. In most substances they "are united so
strongly as to be almost incapable of separation, and such
substances are magnetized with the greatest difficulty.

When the magnetic equilibrium is disturbed, and the two fluids are sep-
arated as described above, the^ seem to take up their abode in opposite sides
of the individual particles of the magnetized body, the positive fluid taking
the same side throughout, so that the positive pole of one particle is contigu-
ous to the negative pole of the next Both fluids remain in the body, but
without combining ; one is not wholly expelled, as in the case of the electric
fluid. The opposite fluids nullify each other at the centre of the magnetized
body, but not at the extremities, which become their chief seats of action.
If a new extremity is formed by breaking a magnet, a new pole is formed,
opposite in kind to the one at the other end. When a piece of iron or steel is
brought near the positive pole of a magnet, its neutral magnetic fluid is de-
composed. The negative portion is attracted by the positive pole towards
the end nearest it, which consequently becomes a negative pole ; while the
positive element is repelled towards the other end, and forms there a posi-
tive pole.

883. TEKKESTEIAL MAGNETISM. The polarity of the
needle is best explained by supposing the earth itself to be
a vast magnet. At the magnetic equator, as at the centre
of a bar magnet, the two fluids neutralize each other, and
there are no magnetic phenomena. Hence at this line there
is no dip. The chief seats of magnetic energy are two
points which lie towards the geographical poles of the
earth, and which are called its Magnetic Poles.

That point of the earth which attracts the north or positive pole of the
needle, must be its south or negative magnetic pole. It lies near Hudson's

ited by loadstone, soft iron, and steel, explained? How is it with most substances?
When the two magnetic fluids are separated, where do they seem to take up their
abode ? Where do the two fluids nullify each other, and where not ? "What follows
if a new extremity is formed by breaking a magnet ? What follows when a piece of
steel is brought near the positive pole of a magnet ? 883. How is the polarity of the
needle explained? Why is there no dip at the magnetic equator ? What is meant


Bay, in 70 degrees of north latitude, and was reached by Captain Ross dur-
ing his Arctic expedition of 1829. At this point he found the dipping needle
to stand vertical, with its north pole towards the earth. The north or posi-
tive magnetic pole of the earth has never been exactly reached, but is sup-
posed to lie south of New Holland, in about 75 south latitude. The dipping
needle would there also stand vertical, but with its south pole towards the
earth. A point has been found near the region alluded to, in which the
needle is very nearly vertical, the dip being 88 2 /a degrees.

The changes in the variation and dip appear to be in some way connected
with the solar heat received by the earth.

884. Magnets draw small pieces of iron to themselves ; but it must be
remembered that the magnetic attraction of the earth only affects the direc-
tion, and does not tend to change the actual position. A magnetic needle
mounted on a cork and placed on the surface of a pond, is made to point north
and south by the earth's magnetic attraction, but is not drawn to the north
side of the pond.

885. Magnetic Intensity. A magnetic needle suspend-
ed by a delicate fibre, when turned from the direction in
which it naturally rests, resumes it, but not immediately.
The magnetic attraction of the earth brings it back, but its

Online LibraryG. P. (George Payn) QuackenbosA natural philosphy: embracing the most recent discoveries in the various branches of physics .. → online text (page 33 of 42)