G. P. (George Payn) Quackenbos.

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

. (page 34 of 42)
Online LibraryG. P. (George Payn) QuackenbosA natural philosphy: embracing the most recent discoveries in the various branches of physics .. → online text (page 34 of 42)
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inertia carries it past the point, and thus a series of vibra-
tions, like those of a pendulum, take place before it finally
settles. The number of such vibrations occurring in a
given time evidently depends on the intensity of the earth's
magnetic attraction. Now this number (and consequently
the intensity of terrestrial magnetism) is found to be dif-
ferent at different places, and at different times in the same
place.

The magnetic intensity varies according to the square of the number of vi-
Irations made in a given time. By applying this law, it is ascertained that
the greatest magnetic intensity thus far found on the earth's surface is three
times as great as the least. The magnetic intensity is found to be least in
Southern Africa.

Production of Artificial Magnets.

886. Artificial magnets should be made of well hard-
ened steel, of fine grain and uniform structure, free from

by the Magnetic Poles ? "Where is the earth's south magnetic pole ? By whom waa
it reached, and what was found there ? Where is the earth's north magnetic pole f
IIow near has it been reached ? With what do the changes in variation and dip seem
to be connected ? 884. What alone is affected by the magnetic attraction of tho
earth ? Give an illustration. 885. How is the intensity of the earth's magnetic at-
traction shown to be different at different places? What is the law for ascertaining



346 MAGNETISM.

flaws, and having level and polished faces. The breadth
of a bar magnet should be one-twentieth of its length, and
its thickness about one-seventieth of its length. In a horse-
shoe magnet, the distance between the poles ought not to
be greater than the breadth of one of the sides.

887. Magnetism may be imparted to steel or iron in four
different ways: 1. By induction. 2. By the sun's rays.
3. By contact with a magnet. 4. By electric currents.

888. INDUCTION, A SOURCE or MAGNETISM. A magnetic
atmosphere surrounds every magnet. A piece of iron or
steel brought within this atmosphere, even without touch-
ing the magnet, has its neutral fluid decomposed, and ex-
hibits magnetic properties. It is then said to be magnetized
by induction.

Present half a dozen bars of iron at different angles to the positive pole
of a magnet, without letting them touch it. They will all be magnetized by
induction, the ends towards the magnet becoming negative poles and the
ppposite ends positive.

Suspend two pieces of soft iron wire by threads, parallel to each other
and on the same level. On bringing either pole of a magnet a short distance
below them, they become magnetized by induction. Like poles are formed
in their contiguous extremities, and consequently instead of hanging parallel
as before, they repel each other and diverge.

Bring one end of an unmagnetized steel bar near the north pole of a mag-
netic needle, and the latter will be attracted to it. Now place the positive
pole of a powerful magnet near the other end of the bar, and the needle will
soon be repelled. This is because the bar becomes magnetized by induction.
The end nearest the needle becomes a positive pole by which the positive
pole of the latter is repelled.

889. The earth magnetizes by induction. A bar of soft
iron placed in the direction of the dipping needle, acquires
magnetic properties by the inductive influence of the earth
acting as a magnet. A few blows with a hammer on the

the magnetic intensity? What is found by applying this law ? Where is the mag-
netic intensity found to be least ? 886. Of what should artificial magnets be made ?
What should be the comparative dimensions of a bar magnet ? What is essential in
a horse-shoe magnet? 8ST. Name the four ways in which magnetism maybe im-
parted to a piece of steel or iron. 883. When is a piece of iron said to be 7nagnctizcd
l>y induction f Illustrate magnetic induction with an experiment. Describe the
experiment with two pieces of soft iron wire. What other experiment proves that a
bar may be magnetized by induction ? 8S9. How is it proved that the earth mag-
netizes by induction ? What experiment shows the inductive influence of the earth ?



mODUCTION OF AKTTFICIAL MAGNETS. 347

upper end, by causing the particles to vibrate, help them
to receive the magnetic influence.

Hold a bar of soft iron horizontally with one end near the north pole of a
magnetic needle. The iron, being unmagnetized, attracts the needle. Now
hold the bar in the direction of the dipping needle, give it one or two blows
with a hammer, and the north pole of the needle will be repelled, showing
that the bar is magnetized, and a north pole formed in its lower end, by the
inductive influence of the earth.

Iron bars that have long stood in a vertical position, or in the direction
of the dipping needle, often acquire magnetic properties in an inferior de-
gree. The same may be said of iron bars raised to a red heat and allowed
to cool in the positions above mentioned, as well as of augers, gimlets, &c.,
that have been much used. Iron wire is frequently made magnetic by twist-
ing it till it breaks. All these are instances of magnetism by induction.

890. THE SUN'S RATS, A SOURCE OF MAGNETISM. Sun-
light constitutes a second source of magnetism. The violet
rays of the solar spectrum, concentrated by lenses on steel
needles, have been found to endow them with magnetic
properties.

891. CONTACT WITH A MAGNET, A SOURCE OF MAGNET-
ISM. A third and more efficient mode of exciting magnet-
ism in iron or steel is by bringing it in contact with a mag-
net. Till recently this was the way in which artificial
magnets were almost exclusively produced.

There are several different ways of magnetizing by con-
tact. The principal are as follows :

892. Magnetizing Needles. An ordinary sewing needle
may be magnetized by simply touching one of its ends to
either pole of a powerful magnet. The end in question be-
comes negative if touched to the positive pole, and positive
if touched to the negative.

893. Magnetizing Bars. Steel bars maybe magnetized
either by single touch or double touch. Single Touch con-
sists in applying but one pole of a magnet to the bar, or
one pole to one-half, and the opposite pole to the other.



Give some further instances of magnetism by the Inductiva influence of the earth.
S90. What is a second soiirce of magnetism ? How may sun-light be made to mag-
netize steel needles ? 891. What is a third source of magnetism ? 892. What is the
mode of magnetizing needles? 893. What two modes are there of magnetizing steel




348 MAGNETISM.

Double Touch consists in applying both poles at the same
time throughout the whole length of the bar.

894. To magnetize a lar ~by single toucJi, apply midway of its length one
of the poles of a magnet, and draw it to either end. Return it through the
air to the middle of the bar, and draw it again to the same end as before.
Repeat this process several times, always using the same pole and drawing
it in the same direction. Then place the other pole on the middle of the bar,
and draw it to the opposite extremity, repeating the strokes as in the former
case. This must be done on both sides of the bar.

Fig. 311. ^ Another mode is repre-

sented in Fig. 311. The op-
posite poles of two magnets,
kept about one-fourth of an
inch apart by apiece of wood,
are placed on the centre of
the bar A B, so as to form angles of about 30 degrees with its surface. They
are then slowly drawn in contrary directions from the middle to the extrem-
ities. This process is repeated several times, the magnets being raised when
they reach the ends and replaced in the middle. The bar is then turned over,
and the same thing done on the other side. The process is facilitated by
resting the ends of the bar on the opposite poles of two other magnets, as
shown in the figure.

895. To magnetize a bar T)ij double touch, apply the opposite poles of two
magnets as just described, only let them be perpendicular to the surface.
Then, instead of drawing them to opposite extremities as before, move them
together from the middle to one end, then through the air to the opposite ex-
tremity, and over the bar to the same end again, and so on drawing them
in the same direction over the bar, letting neither of the applied poles pass
beyond its extremity, and finally stopping in the middle.

896. Magnetizing Horse-shoe Bars.
Horse-shoe magnets are produced by
placing a piece of soft iron, as a keeper,
across the ends of a steel bar bent injthe
proper form ; and then, as shown in Fig.
312, applying perpendicularly to the ex-
tremities a horse-shoe magnet, whose
arms are the same distance apart. Move
it slowly to the bend, then carry it back through the air to
the extremities, and draw it to the bend again. This must



bars? In what does Single Touch consist ? In what, Double Touch 1 894. Describe
the process of magnetizing a bar by single touch. What other mode is described ?
69& How is a bar magnetized by double touch ? 896. How are horse -shoe magneta




I




PRODUCTION OP ARTIFICIAL MAGNETS. 349

be done about a dozen times ; then, without removing the
keeper, turn the bar over and do the same on the other side.
The poles of the magnet produced will in this case be of
the same character as those respectively brought in con-
tact with them.

897. The best mode of magnetizing a
horse-shoe bar is represented in Fig. 313.
Lay the horse-shoe, A B, flat on a table,
with its ends in contact with the poles of
a horse-shoe magnet, N, S. Then place
a piece of soft iron on these poles, and
draw it slowly six or eight times towards
the bend of the bar, in the direction of the arrow, raising it as often as it
reaches the bend, and replacing it as at first. This process performed on
both sides endows the horse-shoe with strong magnetic properties. The end
which touches the positive pole of the horse-shoe magnet becomes negative,
and the other positive.

Two straight bars may be readily magnetized at once in the same way,
by placing one extremity of each against the poles of the horse-shoe magnet,
and connecting the opposite ends with a keeper.

898. ELECTRIC CURRENTS, A SOURCE OF MAGNETISM.
A bar of iron or steel is endowed with magnetic properties
in the highest degree, by passing a current of voltaic elec'
tricity over a conductor placed in a certain position rela*
tively to the bar. The details of this process belong to
that branch of the science which is known as Electro-
magnetism.

Electro-magnetism.

899. Electro-magnetism treats of the phenomena and
principles of magnetism excited by the passage of electric
currents.

900. EFFECTS OF ELECTRIC CURRENTS ON THE MAGNETIC
NEEDLE. As a science, Electro-magnetism owes its origin
to a discovery made in 1819 by Prof. Oersted, of Copen-
hagen. He found that a wire along which a voltaic current

produced? What will be the character of the poles in the magnet produced?
897. With Fig. 313, describe the best mode of magnetizing a horse-shoo bar. How
may two straight bars be magnetized at once ? 898. How is a bar of steel endowed
with magnetic properties in the highest degree ? 899. Of what does Electro-magnet-
ism treat ? 900. To what does electro-magnetism owe its origin ? Give an account



350 ELECTEO-MAGNETISSr.

Was passing tended to turn the magnetic needle from its
natural position to one perpendicular to the direction of
the current. The conducting wire, of whatever metal it
might be, was thus rendered magnetic by the electric cur-
rent which it transmitted. It was subsequently found to
attract iron filings ; which, when the battery was in full
action, clustered around it to the thickness of a quill, but
gradually thinned off as the energy of the battery dimin-
ished, and left it entirely bare the moment the circuit was
broken.

The direction in which the needle is turned depends on its position rela-
tively to the wire, and the direction in which the current is passing. When
the needle is on a different level from the wire, that is, directly above or be-
low it, it retains its horizontal position ; but its north pole is turned east or
west, according to whether it is above or below the wire, and according to
the direction in which the current moves. When the needle is on the same
level with the wire, but on one side of it, it does not then swerve east or
Vvest ; but its north pole is made either to dip or to rise, according to the
Bide of the wire it is on and the direction in which the current moves. The
following rule enables us always to determine the direction in which the
needle will be turned :

Imagine yourself, with arms extended, perpendicularly, lying along tlie
conducting wire, with your head towards the point from which the current is
coming, and your face turned towards the north pole of the needle ; then this
north pole will be deflected in the direction of your right hand, ^vhether it be
up or down, east or west.

The magnetic influence of the electric current is not therefore exerted in
the plane of the conducting wire, but rather perpendicularly to that plane,
so as to produce circular motion round the wire.

901. The deflection of the needle by an electric cur-
rent may be shown w^ith the apparatus represented in
Fig. 314.

A brass wire is bent into rectangular form, and provided with a screw-
cup at each extremity, P, N, for the reception of the wires from a galvanic
battery, so that a current may be passed above and below a magnetic needle,
N, S, suspended within the rectangle. The arms proceeding from P and N



of Oersted's discovery. How was it proved that the conducting wire was rendered
magnetic by the electric current ? On what does the direction in which the needle
turns depend ? How does it turn, when on a different level from the wire ? How,
when on the same level with the wire, but on one side of it ? State the rule for de-
termining the direction in which the needle will be turned? How is the magnetic
Influence of the electric current exerted ? 901. Illustrate the deflection of the needle





THE GALVAKOMETEE. 351

are insulated from each other Fig. 314

where they cross. No sooner is a

positive current passed over the

upper wire from north to south,

than the needle is turned, its

north pole deviating towards the

cast and its south pole to the

west.

Here the under current, pass-
ing in the opposite direction to
the upper one, tends to turn the
needle in the same direction ; and the deflecting force, as it is called, is there-
fore twice as great as if the current passed in one direction only. If the wire
be bent so as to make two rectangles about the needle, the deflecting force
will be twice as great as when but one is
formed ; if five rectangles are made, as in
Fig. 315, it will be five times as great, &c.
In these cases, the wire must be covered
with silk thread, or some other non-con-
ductor, so as to insulate its arms from
each other, and oblige the current to traverse its whole length. It is on this
principle that the Galvanometer is constructed.

902. Tlie Galvanometer. The Galvanometer is an in-
strument for measuring the force of galvanic currents by the
deflection of the magnetic needle. It consists of a long
wire bent into an oval or rectangular coil, the parts of
which are prevented from touching by being wound with
silk. The wire terminates in screw-cups, for convenience
of connection with a galvanic battery. Within the coil a
magnetic needle is delicately poised ; and the instrument
is placed so that the wire may have the same direction as
the needle. They retain this direction till a galvanic cur-
rent passes over the wire, when the needle is turned to-
wards the east more or less, according to the force of the
current. A graduated scale fixed below the needle, with
its circumference divided into degrees, measures the de-
flection, and consequently the quantity of electricity passing
over the wire.



with Fig. 314. What is the effect of having two currents, one above and one below ?
What is the effect of having two rectangles ? Five rectangles ? In these cases, what
precaution must be taken? What instrument is constructed on this principle?
U02. What is the Galvanometer ? Describe the galvanometer. 903. How is the gal-



352



ELECTRO-MAGNETISM.




GALVANOMETER WITH ASTATIC
NEEDLE.



903. Galvanometer with Astatit
Needle. Instead of the ordinary nee-
dle, an astatic needle (see 875) is
sometimes used in the galvanometer.
In this case, the needle, having its
polarity neutralized, is more readily
turned. The instrument is consequent-
ly more sensitive, indicating the pres-
ence of electric currents which would
otherwise entirely escape detection.

Fig. 316 represents the Galvanom-
eter with the Astatic Needle. The nee-
dles are suspended by two parallel silk
threads from r, so that one of them
may hang directly over the top of the
coil z c, and the other below it. p q arc
the screw-cups terminating the wire
which forms the coil, and ss is the
graduated scale. The upper needle
hangs above the coil; but as its poles
point in opposite directions to those of the under one, it will tend to move in
the same direction as the latter when galvanic action takes place.

904. CONNECTION BETWEEN ELECTRICITY AND MAGNET-
ISM. That there is an intimate connection between elec-
tricity and magnetism, was established by Oersted's experi-
ment. It is further shown by the fact that compass-needles
often have their poles reversed or their polarity weakened
by lightning ; that a spark has been drawn from a magnet ;
that a charge of electricity passed through a needle renders
it magnetic ; and that a bar may be permanently magnet-
ized with an electric current more efficiently than in any
other way.

These facts have led to the theory that magnetism is
not an independent agent, but simply one of the forms as-
sumed under certain circumstances by that subtile all-
pervading agent which we call THE ELECTRIC FLUID. Ac-
cording to this theory, frictional electricity, voltaic elec-
tricity, thermo-electricity, magneto-electricity, and electro-
magnetism, are all one and the same thing, identical in

vanometer made more sensitive, and why ? Describe the Galvanometer with the
Astatic Needle. 904. What was established by Oersted's experiment ? How is the
eonnection between electricity and magnetism further shown ? What theory has



ELECTTEO-MAGNETIC ROTATION. 353

kind, but differing in intensity, quantity, and properties,
in consequence of the different modes in which they are
developed.

905. ELECTRO-MAGNETIC ROTATION. When a magnetic
pole and a wire over which an electric current is passing
are brought near each other, the pole tends to revolve
round the wire, and the wire has a similar tendency to
revolve round the magnet in a plane perpendicular to
the direction of the current. With suitable apparatus, the
following phenomena of electro-magnetic rotation may be
exhibited :

1. The conducting wire being fixed, the magnet will
revolve about it.

2. The magnet being fixed, the conducting wire will
revolve about it.

3. Both magnet and wire being left free to move,
tney will revolve in the same direction round a com-
mon centre, each appearing to pursue and be pursued by
the other.

4. The conducting wire being dispensed with, a magnet
may be made to turn on its own axis by the passage of an
electric current along half its length.

906. To show the revolution of a magnet about a
conducting wire, Faraday used the apparatus repre-
sented in Fig. 317. A magnet, n S, is immersed in a I |p~
vessel of mercury, with its north pole, n, a short dis-
tance above the liquid, and its south pole, S, connect-
ed by a silk thread with the conducting wire C, which
passes through the bottom of the vessel, a b is an-
other conducting wire, which enters the mercury from
above. When a b is connected with the positive pole
of a galvanic battery, and C d with the negative, a de-
scending current of positive electricity passes along
the conductor (the mercury completing the circuit),
and the north pole, n, will revolve round the fixed
wire, a b, in the direction of the hands of a watch. If,
on the contrary, a & be connected with the negative

been based on these facts ? 905. What follows when a magnetic pole and a wire over
which an electric current is passing are brought near each other ? With suitable ap-
paratus, what phenomena connected with electro-magnetic rotation may be exhibit-
ed ? 90G. Describe Faraday's experiment for showing the revolution of a magnet




354



ELECTRO-MAGNETISM.




Fig. 319.



pole, and C d with the positive, an ascending current will be formed, and the

magnet will revolve in the opposite direction.

Mercury is used in this experiment, because, being a liquid, it allows the

magnet to move through it, while at the same time, being a conductor, it
completes the circuit, and carries off the magnetic in-
fluence from the south pole immersed in it. Were it
not for this, the south pole, by its tendency to move
in the opposite direction to the north, would keep the
magnet stationary.

907. Fig. 318 illustrates the revolution of a con^
ducting wire around a fixed magnet. Again we have
a vessel of mercury, with a conducting wire, d, passing
through its bottom, and another wire, a b, suspended
from a hook directly over the magnet, entering the
mercury from above, n is the north pole of the fixed
magnet. On connecting the hook and the wire d with
the poles of a galvanic battery, the wire will revolve

round the magnet, the direction depending, as before, on whether the electric

current is ascending or descending.

908. By ingeniously combining
the two pieces of apparatus just de-
scribed, we may exhibit the simulta-
neous revolution of both magnet and
wire round a common centre. The
magnet, M, is immersed in a vessel
of mercury about half its length, that
the current may affect only one pole.
It is connected at the bottom with a
conducting wire and screw-cup, C, in
such a way as to allow it freedom of
revolution. The wire, W, is sus-
pended from a hook, so as to move
freely. On transmitting a current,
which is done by connecting A and
C with the poles of a battery, both
the magnet and the wire commence
revolving in the same direction as if
chasing one another

909. EFFECT OF ELECTRIC
CURRENTS ON STEEL AND SOFT
IRON. The deflection of a
magnetic needle by a wire

fcbout a conducting wire. "Why is mercury used in this experiment ? 907. Describe
the experiment which shows the revolution of a conducting wire around a fixed
bagnct. 908. What does Fig. 319 represent ? Describe the experiment with this




oy



THE HELIX. 353




over which an electric current is passing, has been de-
scribed in 900. If a bar of soft iron is placed across such
a wire, it becomes a temporary magnet, as is shown by its
attracting iron filings. A bar of steel so placed is made a
permanent magnet.

910. The Helix. The magnetizing power of the wire is
greatly increased, if, instead of touching the bar in but a
single point where they Fig. 320.

cross, it is wound a number
of times spirally round the
latter, as shown in Fig. 320.
Such a coil of wire is called a Helix (plural, hel'-i-ces).

A helix may be familiarly made by winding some copper wire tightly
round a small bottle, and then drawing the bottle out. As the magnetizing
power of the helix increases with the number of times that the electric cur-
rent passes round the bar, each turn of the wire is pushed close up to the
one before it ; and, to increase the effect still further, several coils or layers
of wire may be formed, one on top of another. Direct communication be-
tween contiguous parts of the wire must be prevented by winding silk or
some other insu- Fig. 821.

lating material
round it. When
the ends of the
wire are connect-
ed with the poles
of a galvanic bat-
tery, the current
is thus obliged to
pass through its
whole length. Fig.
321 represents a

helix mounted on a stand. An iron bar extending through the centre is seen
projecting at each end.

911. Magnetizing Power of the Helix,. A steel bar
introduced within a helix becomes permanently magnetized
the moment an electric current is passed over the wire. A
needle laid inside of it is sometimes so powerfully acted on

apparatus. 909. What is the effect of a wire over which a ciirrent is passing on a bar
of soft iron placed across it ? On a bar of steel so placed ? 910. How is the effect
greatly increased ? What is such a coil of wire called? How may a helix be made ?
How is the effect of the helix increased ? With what is the wire covered, and why?



Online LibraryG. P. (George Payn) QuackenbosA natural philosphy: embracing the most recent discoveries in the various branches of physics .. → online text (page 34 of 42)