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CHAPTER 5
MAGNETISM
NATURAL MAGNETS
It may seem strange to you that, for centuries, magnets
were of little practical value. Magnetism's first real
use was in a compass to guide the ancient mariners.
The first of these devices was little more than a magnetized needle on a block of wood floating in a dish of
water. Crude as they were, these early compasses were
the first step toward modern navigation.
Little by little other uses were found, but it was not
until the 19th century that new discoveries and inventions
showed magnetism to be one of the foundations of the
science of electricity.
Not only are magnets important in many electrical
devices that you will use directly, but also in hundreds
of hidden and indirect ways, such as the generation of an
emf to light the incandescent bulbs about the ship.
The discovery of magnetism dates back to the ancient
shepherds of Asia Minor. They noticed that the iron
tips of their staffs were attracted to certain types of
stones. These stones were NATURAL MAGNETS known as
LODESTONES, meaning "leading stone."
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The shepherds also observed that the iron tips of the
staffs, if left in contact with lodestones, soon acquired the
ability to attract other pieces of iron. While these
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Figure 33 -Lodestones.
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ancients did not understand WHY these things happened,
they were observing two types of magnets, NATURAL and
ARTIFICIAL.
A lodestone, the NATURAL magnet, is a piece of rich
IRON ORE, magnetite, and its source of magnetism is the
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Figure 34.-Earth's magnetic and geographic poles.
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earth itself. As illustrated in figure 34, the core of the
earth is assumed to consist of iron, or a high grade
iron ore.
During the past ages, the core became magnetized.
The EFFECT of this magnetism seems to be concentrated
in two areas, which are located near the north and the
south GEOGRAPHIC poles.
The area near the north geographic pole is called the
NORTH MAGNETIC POLE and the other the SOUTH MAGNETIC
POLE.
ARTIFICIAL MAGNETS
Most NATURAL magnets have many north and south
poles. The nails, sticking to the lodestone in figure 33,
indicate the presence of three poles. Actually it may
have many more. In addition to having many poles, the
magnetic strength of a lodestone is too weak to be useful.
A few metals-iron, cobalt, and nickel-have the ability
to become magnetized. They are ARTIFICIAL magnets,
having but two poles and a greatly increased magnetic
strength.
THEORY OF MAGNETISM
Probably no one knows exactly what happens inside
an iron bar when it becomes magnetized, but a good
explanation has been given. A piece of iron is supposed
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Figure 35.-Unmagnetized iron bar.
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to be made of millions of small magnets. When the bar
is unmagnetized, these small magnets have a "helter-skelter" arrangement as illustrated in figure 35. The
magnetic forces of one molecule cancel the field of its
neighbor.
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Figure 36.-Magnetized iron bar.
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When the bar is magnetized, the small magnets are
arranged so that ALL the north poles point in one direction, and all the south poles in the opposite direction.
This systematic "line-up" of the individual magnets
causes the whole bar to act as a SINGLE MAGNET. All the
magnetism seems to be concentrated at the two ends of
the bar, with one end designated as NORTH and the other
SOUTH.
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Figure 37.-Magnetic Poles.
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Breaking the bar in half or into many pieces does not
separate one pole from the other. As illustrated in figure
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37, a magnet may be cut into many pieces, and each will
have a north and a south pole.
MAKING A MAGNET
A bar of iron may be magnetized by stroking it with a
lodestone or with another magnet. You must be careful
always to stroke in the same direction, as illustrated in
figure 38. It doesn't make any difference which way you
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Figure 38.-Making a magnet by induction.
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stroke the bar-just be sure you LIFT the stroking bar
several inches away at the end of each stroke.
The stroking arranges the molecular magnets within
the bar so that the N poles point in one direction and the
S poles in the other.
If a bar of iron lies in contact with another magnet,
the bar will in time become magnetized.
You may also produce a magnet by heating the bar to
red heat and then placing it parallel to the magnetic field
of the earth-that is, in a north and south direction.
Heating the bar frees the molecular magnets so that they
may arrange themselves in order with greater ease.
MAGNETIC FIELD
A magnet extends its influence a considerable distance
away from the bar. This area of influence is known as
the MAGNET'S FIELD.
If you place a pane of glass over a bar magnet, figure
39A, and sprinkle iron filings on the glass, you will get a
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Figure 39.-Magnetic field.
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pattern like figure 39B. The filings arrange themselves
in DEFINITE LINES, with the GREATEST CONCENTRATION at
the ENDS of the bar. The lines DO NOT cross, but run from
one end to the other.
Notice only a few scattered filings are directly over
the bar itself-indicating the presence there of only a
few magnetic lines of force.
NORTH AND SOUTH MAGNETISM
One pole of a magnet is designated as being NORTH,
and the other SOUTH. Just how it was decided which
should be S and which N is not definitely known, but all
the laws of magnetism have been built around this
convention.
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Figure 40.-Flux pattern about a bar magnet
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The lines of magnetic force are called FLUX. Like
current, the flux is said to flow. The direction of flow
is FROM the NORTH pole TO the SOUTH pole.
The arrows on the lines of force in figure 40 indicate
the flux to be LEAVING the NORTH and ENTERING the SOUTH
pole.
The STRENGTH of a magnet is expressed by the NUMBER
OF LINES OF FORCE in the CROSS SECTION AREA of the field.
A FLUX DENSITY of 10,000 lines means there are 10,000
lines of magnetic force in a square inch cross section area
of the field.
LIKES REPEL-UNLIKES ATTRACT
If you bring two north poles or two south poles together, a force of repulsion will exist between them. In
figure 41, when the north pole of the suspended magnet
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Figure 41.-Likes repel-unlikes attract.
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is brought near the north pole of the bar magnet, the
needle will swing AWAY. The same thing is true when
you bring two south poles together. But if the north
pole of the suspended magnet is brought near the south
pole of the bar magnet, the needle will move TOWARD the
bar, indicating a force of ATTRACTION.
When iron filings are sprinkled over the ends of two
like poles, figure 42A, the filings arrange themselves in a
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Figure 42.-Likes repel, unlikes attract.
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manner that indicates a REPULSION. But with unlike
poles, figure 42B, an ATTRACTION is evident.
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Figure 43.-North magnetic pole has south pole magnetism.
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THE EARTH'S MAGNETISM
You were told in the first page of this chapter that the
earth is a huge magnet and that the NORTH MAGNETIC POLE
is near the north geographic pole. That is very true, but
the north magnetic pole has SOUTH POLE MAGNETISM.
To help clear this confusion, look at figure 43. Think
of the earth as having a huge bar magnet extending from
pole to pole with the SOUTH POLE OF THE MAGNET pointing toward the NORTH geographic pole.
Now put together two things you know-
1. Unlikes attract.
2. Lines of magnetic force leave at the North pole
and enter at the South pole.
Which end of a compass points north? The NORTH!
Applying the first of the above points, the north magnetic
pole must have SOUTH POLE MAGNETISM or it could not
attract the north magnetism of the compass needle.
Second, experiments show the earth's lines of force
moving from the SOUTH GEOGRAPHIC to the NORTH GEOGRAPHIC pole. Since magnetic flow is from north magnetism to south magnetism, the north geographic pole
must have SOUTH POLE MAGNETISM.
MAGNETIC MATERIALS
Only a few substances are capable of being magnetized.
The most common is iron. Nickel and cobalt also have
magnetic properties.
While both iron in various forms, and its derivative,
STEEL, are magnetic, they have different characteristics.
Soft iron is easily magnetized, but it also loses its magnetism very quickly. Certain forms of steel require a
great deal of energy to magnetize them, but when magnetism is once established it remains a long time.
Since steel keeps its magnetism a long time, it is said
to have a high degree of RETENTIVITY, while soft iron is
said to have little RETENTIVITY. The magnetism that remains AFTER the magnetizing force has been removed is
called RESIDUAL magnetism.
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Several ALLOYS have been developed in order to produce substances that have HIGH RETENTIVITY and others
that have LOW RETENTIVITY. A mixture of aluminum,
nickel, and cobalt, properly heat treated, produces a
metal (ALNICO ) with EXTREMELY HIGH RETENTIVITY and
field strength. An alloy of iron and nickel, properly heat
treated, produces a substance known as PERMALLOY. It is
magnetized very easily, but loses its strength the instant
the magnetizing force is removed.
ALNICO is used in the construction of loudspeakers,
while PERMALLOY is used for transformer cores. More
will be said about this later.
RELUCTANCE AND PERMEABILITY
Many metals, like copper, lead, silver, and aluminum,
are without magnetic properties. Actually they STOP
THE FLUX from passing through them.
Some substances like glass have a more or less neutral
effect on the flux. They do not seem to stop the lines of
force, neither do they aid their movement.
The DEGREE to which a substance STOPS the MOVEMENT
of flux is described as the RELUCTANCE of the material.
RELUCTANCE in magnetism can be compared with resistance in electricity. Both express the degree of opposition
to flow. Metals with extremely high reluctance are used
as MAGNETIC INSULATORS, just as substances with high
resistance are used as electrical insulators.
SOFT IRON is an extremely GOOD CONDUCTOR of FLUX, or
you may say that it is very PERMEABLE. Soft iron is so
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Figure 44.-Permeability of a piece of iron.
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permeable that when it is placed in a magnetic field, the
flux is actually concentrated into a small space. You can
observe this in figure 44.
PERMANENT MAGNETS
Magnets of the type discussed in this chapter are called
PERMANENT MAGNETS because they retain their magnetism AFTER the magnetizing force has been removed.
The magnets that lose their magnetism as soon as the
magnetizing force has been removed are temporary magnets, a common example of which is the ELECTROMAGNETS.
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