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Table of Contents.
¶ U.S. GOVERNMENT PRINTING OFFICE; 1945 - 618779
An ancient legend tells us that nearly 5,000 years ago an Emperor
of China had a small statue of a man mounted on his chariot. This statue
was pivoted at the base and one outstretched arm always pointed to the
south. In those ancient times, this action must have seemed truly miraculous
- probably the Emperor used his statue more to impress his subjects
than he did to find his way. This legend is the first report of man's
use of a black or lead-colored stone called MAGNETITE.
the time of Christ, magnetite was rediscovered by a Grecian shepherd.
He noticed that the iron of his staff was attracted to certain stones.
But for nearly another 1,000 years, no particular use was made of this
In about the twelfth century the European sailors
used a crude form of compass. They carried a piece of magnetite and
a thin piece of iron aboard their ships. By stroking the iron with the
magnetite and then floating the iron on a chip of wood in a bowl of
water, these sailors made a rough but serviceable compass. The iron
had become magnetic and floated around until it stopped in a north-south
line. Because the stone, magnetite, furnished the power of direction
to the iron, it was called LODESTONE - meaning "leading-stone."
The sailors didn't know anything about magnetism. However, they
did know how to use their com-pass, and they also knew what it would
do for them.
Surprising as it is, modern science doesn't know
much more about the lodestone than did the sailors of the twelfth century.
Modern science knows what magnetism DOES, how it ACTS, and how to PRODUCE
it. But the "why" of magnetism is still in the realm of theory.
Figure 64. - Natural and artificial magnets.
Those old sailors on their wooden and canvas ships made an ARTIFICIAL
MAGNET every time they stroked the sliver of iron with the lodestone.
It was necessary to make an artificial magnet, because a piece of magnetite
has too many POLES to be used as a compass. Poles are points on a magnet
where the magnetism CONCENTRATES. Compare the natural and artificial
magnets of figure 64. Notice that they both attract and hold iron tacks
ONLY AT CERTAIN POINTS. These points are their POLES. You can see how
impossible it would be to use the lodestone as a compass. It has so
many poles, a sailor would never know which one to follow. But usually
a sliver of iron has only two poles - and, as you know - lines up in
a north and south direction. Here are two fundamental facts about magnetism
1. MAGNETISM IS CONCENTRATED AT POINTS CALLED
2. ARTIFICIAL MAGNETISM CAN BE PRODUCED BY
CONTACT WITH ANOTHER MAGNET. This
magnetism is called INDUCED MAGNETISM.
You can, and probably
have, made magnets by INDUCTION. Starting with any unmagnetized piece
of iron and steel, stroke it against a magnet. It is necessary to always
keep the motion in ONE direction. This means that on the back-stroke
the IRON must be lifted free of the magnet. Figure 65 explains just
how this is done. Study the diagram and then try producing a magnet.
Your knife blade and an old horseshoe magnet are good materials.
Figure 65. - Making a magnet by induction.
Many times, mere contact between an unmagnetized object and a magnet
will produce induced magnetism. For example, if you lay the blade of
a screwdriver across the poles of a magnet - the screwdriver becomes
magnetic. This is a handy thing to know when you have to place a screw
in some out of the way spot. Magnetize a screwdriver and let it carry
the screw where your fingers can't.
Figure 66. - Making a magnet by the coil
There is still a third method of producing induced magnetism. If
you coil wire around a bar of iron and pass a current through the coil,
the iron bar will become magnetic. This is the method used to produce
the strongest artificial magnets. Figure 66 shows the production of
an artificial magnet by a current-coil.
Some materials make strong
magnets - but many materials will not make magnets at all. The materials
which make good magnets are MAGNETIC SUBSTANCES. The materials which
will not make magnets are NON-MAGNETIC SUBSTANCES. Iron, of course,
is the most common magnetic material. It makes a good magnet, but when
it's pure - soft IRON - it quickly loses its magnetism. Soft iron, therefore,
forms only a TEMPORARY magnet. Magnets made of hard steel containing
iron and carbon hold their magnetism almost indefinitely. They are PERMANENT
magnets. In recent years many alloys of iron have been developed for
making permanent magnets. The best is ALNICO - a combination of iron,
aluminum, and nickel. In fact, nickel is a fair magnetic material even
when it is not combined with iron.
Strong permanent magnets are
used in compasses, electrical measuring instruments, telephones, gasoline
ignition systems, and radios. As a matter of fact, magnetism is so closely
connected to electricity that if you are to understand the one you must
know about the other.
When a magnet is used as a compass, the
pole (or end) which points north is named the NORTH-SEEKING POLE. Or
more simply, it's usually shortened to just NORTH or + pole. (THIS +
HAS NOTHING TO DO WITH CURRENT - DO NOT CONFUSE THE TWO IDEAS.) The
other pole pointing south is called the SOUTH-SEEKING POLE - shortened
to SOUTH or - pole.
All magnetic poles are either N or S. Usually,
there is only one N pole at one end of the magnet, and only one S pole
at the opposite end of the magnet.
Magnetism is a force - and like mechanical force, the force of gravity,
and electromotive force - it is invisible. You cannot see the push that
sends electrons along a wire; nor can you see the force that pulls objects
toward the earth. And you cannot SEE the FORCE that a magnet exerts.
Yet magnetic force is just as real as the force of gravity. You have
no doubt that there is a force of gravity when you land on the deck
after losing your footing! You have experienced the EFFECTS of the force
of gravity. You can also experience the EFFECTS of magnetic force. Magnetic
force acts like the other forces you are familiar with. Study the different
forces in figure 67.
Figure 67. - Forces as vectors.
Everyone of these forces is represented by a straight line arrow.
This arrow tells you two things-the head tells you the DIRECTION of
the force-and the line of the arrow, by its length, tells you the STRENGTH
of the force. Arrows used in this sense are called VECTORS. If you wanted
to represent the collision of two ships by vectors, your diagram would
look like figure 68.
This diagram shows the direction of the
ships' headings and tells you that ship A has the most force. Probably
ship B got the worst damage.
Figure 68. - Ships' forces as vectors.
These ships' forces are easily recognized. You can see and measure
the exact heading of a ship. But to "see" the heading and strength of
invisible magnetic force you must make the force VISIBLE. And doing
this is quite easy. Place the magnet under a glass plate as in A of
figure 69. Now sprinkle iron filings over the plate. The attraction
of the magnetic force will cause the filings to line up on the LINES
OF FORCE. Figure 69 B shows clearly the STRENGTH and SHAPE of the magnetic
Figure 69. - Magnetic field of force.
Iron filings do not indicate force direction - there are no arrow
heads on iron filings. Even today, scientists are not positive about
the direction of the lines of magnetic force, so arbitrarily they are
said to go FROM THE N POLE TO THE S POLE. Now, this gives you as much
knowledge about magnetic force as you have about any force.
forces can be represented by lines and arrows the same as other forces.
Figure 70 shows the vector-picture of the magnetic force of figure
69. This pattern of force is called a MAGNETIC FIELD OF FLUX, a MAGNETIC
FIELD, a FIELD or a FIELD OF FLUX. There are three important facts you
should note -
Figure 70. - Flux pattern of bar magnet.
1. NO LINES CROSS.
2. ALL LINES
3. ALL LINES LEAVE THE MAGNET AT RIGHT
ANGLES TO THE MAGNET.
These three facts apply to ALL fields and
ALL COMBINATIONS of fields.
Magnetic lines are like rubber bands
- they can be stretched, distorted, or bent. But they always tend to
spring back into form. Also like rubber bands-too much stretching will
break lines of force. Using fields of force as the basis of magnetism,
you can understand the many characteristics and actions of magnets.
Figure 71. - Unlike poles-flux pattern.
ATTRACTION AND REPULSION
Place two magnets under a glass plate with the North pole of one
next to the South pole of the other. Now sprinkle iron filings over
the plate. The pattern of the iron filings is like figure 71 A. The
field pattern is shown in figure 71 B.
This flux pattern shows
that the forces of both poles are in the same direction - they should
pull together. That two opposite poles are attracted is proved by the
diagram in figure 72. Notice that one magnet is free to turn on its
suspension string. The poles of this free magnet are ATTRACTED to the
OPPOSITE poles of the stationary' magnet.
UNLIKE POLES ATTRACT!
Figure 72. - Unlike poles attract.
Now take the same two magnets and turn one around so that the two
N poles are adjacent. The flux pattern would look like figure 73. This
pattern shows that the forces are in opposite' directions and oppose
each other. The two magnets should push a part. They do exactly that,
as shown by figure 74.
LIKE POLES REPEL!
Figure 73. - Like poles-flux pattern.
Let a bar of iron be placed in a magnetic field, as in figure 75.
Notice how the flux field concentrates in order to pass through the
iron. Flux always prefers iron to air for a path. This is because iron
has a high PERMEABILITY. Which means it is easier for flux to go through
iron than it is for flux to go through air. All magnetic substance -
iron, cobalt, nickel, and alnico -are highly permeable.
Figure 74. - Like poles repel.
You may look at permeability this way - a field of flux has a certain
amount of force. Borneo! this force is used up in going from the N pole
to the S pole. If the flux must travel in AIR, a good deal of the force
is used up. But if it can travel in IRON, only a small amount of force
is used up in traveling through the more permeable substance. All magnetic
machinery is made of iron or steel in order to save as much of the flux
strength as possible.
Figure 75. - Permeability of iron.
Now let a piece of glass be placed in a magnetic field as in figure
76. No change in the form of the field takes place. Glass is a HIGH-RELUCTANCE
(or low permeability) material. That is, flux lines pass through glass
with difficulty. Air is also a high-reluctance material. You might say
that since both glass and air are high-reluctance materials the flux
lines don't care which one they go through - a good proportion of the
force is going to be expended in travel anyway. Paper, copper, and tin
are other high reluctance materials.
Figure 76. - Reluctance of glass.
NOTICE - All high-reluctance materials reduce the strength of the
flux field. If you want to waste flux, use a high-reluctance material.
For example, compare the two. magnets in figure 77. In A, the flux travels
through the high-reluctance air, and the magnet will soon become weak
because of the losses. But in B, an iron KEEPER provides a low reluctance
path for the flux. This reduces the loss of magnetic power and this
magnet will remain stronger much longer than the magnet in A.
Figure 77. - Keeper-reducing reluctance.
THE EARTH'S MAGNETISM
The earth's core is a huge magnet, and surrounding the earth is
the field of flux produced by this core. An artist's conception of what
this core and field look like is shown in figure 78. Notice that the
core is irregular in shape and is located at an angle to the axis of
the earth's rotation. This accounts for certain irregularities in the
field's pattern and also for the "off-center" position of the magnetic
poles. The North and South GEOGRAPHIC poles are at either ends of the
axis of rotation of the earth. But the north MAGNETIC pole is 100° south
and 40° east of the geographic pole. And the south MAGNETIC pole is
180° north and 30° west of the geographic pole. This places the magnetic
poles about 1,400 miles from the corresponding geographic poles. You
will see later that this offset of the magnetic poles introduces an
error, which must be corrected for purposes of navigation.
Figure 78. - Magnetic and geographic poles
of the earth.
The earth's magnetic field is just like the field of any magnet
- only LARGER and STRONGER. A compass is simply another magnet. And
the principles of attraction and repulsion govern the earth magnet and
the compass magnet exactly as though they were the two magnets of figures
71 and 73. The earth magnet is considered stationary. Therefore, the
compass magnet's north pole is attracted to the earth's south pole and
the compass' south is attracted to the earth's north. Which means that
the compass' magnet, which is free to turn, always points north. The
confusing part of this is that the NORTH POLE of the compass points
to the NORTH POLE of the earth. This apparently says "North attracts
North." Of course, this is NOT true. The magnetic pole near the north
geographic pole is ACTUALLY A SOUTH MAGNETIC POLE. Common usage has
named this "the North Pole" - just remember that MAGNETICALLY it's a
Figure 79. - The pocket compass.
The compass itself is a strong magnet (or magnets) pivoted at the
center. In the small hand type or pocket type compass, the magnet is
pivoted on a hard metal point with a jeweled bearing. This allows the
magnet to swing freely and always line up on the North-South line. Notice
in figure 79 that the COMPASS CARD is a part of the case - it does not
swing with the magnetic NEEDLE. In using this compass, the N pole of
the compass needle (black or blue) always points to the South magnetic
pole. (Remember that the SOUTH MAGNETIC pole is near the NORTH GEOGRAPHIC
pole.) You can see that the accuracy of such a compass depends upon
the extremely small amount of friction at the pivot bearing. The needle
must be free to swing to the attraction of magnetic poles. Most of these
compasses have a LOCK which lifts the needle free of its bearing and
holds it stationary when not in use. This lock prevents damage to the
bearing in case of shock.
Figure 80. - The spirit compass.
The metal-jewel bearing type of compass has the marked disadvantage
of jamming when the compass is tilted. Jamming simply means that the
needle scrapes against the card and sticks. This makes it practically
useless for shipboard use because of the pitch and roll of a vessel.
Figure 80 shows a SPIRIT compass used aboard ship. In addition to the
metal-jeweled bearing suspension, the compass floats in a liquid-usually
water and alcohol. The liquid suspension dampens oscillation and absorbs
pitch and roll. The compass card, in this case, is attached to the magnets
and turns with the magnets.
Figure 81. - Compass on 170° heading.
The case of the spirit compass is marked with a reference line which
is parallel to the keel of the ship. This is called the LUBBER'S LINE.
The compass card turns with the magnets and the N-S line of the card
is always on the earth's N-S line. The number of degrees between the
N pole reading of the card and the lubber's line is the ship's heading.
Figure 81 shows the compass of a ship on a course of 170°.
Figure 82. - Agonic line.
There is only one line across the face of the earth where a compass
points to the true, or geographical, north pole. Figure 82 shows this
AGONIC line. If you are on the agonic line, your compass points to both
the geographic and magnetic poles. Figure 82 shows that if you are on
the agonic line, you are lined up with both poles. Now, if you move
to right or left (east or west), you get out of this magnetic - geographic
line - up. Your compass would continue to point to the MAGNETIC pole,
but it would be at an angle to the GEOGRAPHIC pole. The amount of this
angle is called the VARIATION. Through studies of all locations on the
earth's surface, the variations are known and marked on charts. Lines
drawn through points of EQUAL VARIATION are ISOGONIC lines. Figure 83
shows the isogonic lines of the United States.
Figure 83. - lsogonic lines of U.S.
Say you were sailing in the northern part of Lake Michigan. You
would be on or near the agonic line. Your compass would read true north
- ZERO VARIATION. Now move your ship to just off New York Harbor. You
would be on or near the 10°-west isogonic line. Your compass would read
10° west of true north - 10° W variation.
CHANGES IN VARIATION
The exact amount of variation for each spot on the earth is NOT
a constant value. First, there is a slow, regular change throughout
the years. And charts showing the isogonics are revised every few years
to keep them correct. Then there are small, sharp, temporary changes
which may occur throughout the day. When these daily variations are
large, they are probably caused by MAGNETIC STORMS. Magnetic storms
are somehow connected with sunspots or some other excitement on the
VARIATION is caused by influences OUTSIDE the ship or airplane.
DEVIATION is caused by influences INSIDE the ship or airplane. Large
masses of iron or pieces of electrical equipment-the hull, engines,
guns, motors, radios, and lights - all have magnetic influence. They
throw a compass off because they compete with the earth's field. By
experimenting, the amount of deviation is determined for every ship
and airplane for all headings. A chart is made up of the deviations
and called a DEVIATION CHART. Then the deviation is corrected by adding
the error to, or subtracting it from, the compass reading.
Figure 84. - Compass deviation.
A better method for correcting for deviation is COMPENSATION. In
compensation, a weak magnet outside the compass is placed just the right
distance from the compass to cancel the deviation effect. Say that the
iron and steel in the engine of a landing craft has a strong north attraction
as in figure 84. This pulls against the compass and causes a large deviation.
To compensate for the engine's magnetism, a small magnet will be mounted
near the compass with its south pole closest to the compass. Now the
south pole of the compensating magnet cancels the north-pole attraction
of the engine. Usually compensating magnets are mounted so that their
position can be shifted to compensate for various deviations. Imagine
how the compass "acts-up" on a tank landing craft - 20 to 50 tons of
iron coming aboard after the compass is all compensated!
causes so much error that on large ships the GYRO-COMPASS is used. The
gyro does not IINO magnetism in its operation, therefore, deviation
can be ignored. However, regardless of the advantages of the gyro, all
ships are equipped with a magnetic compass for stand-by service.
THEORY OF MAGNETISM
Theory helped you understand current and theory may help you understand
According to the accepted theory of magnetism, every
atom and molecule has a weak north pole and a weak south pole. Actually
that is saying that atoms and molecules are tiny magnets.
an ordinary piece of unmagnetized iron, the molecules are jumbled together
with no particular arrangement. This condition would look like figure
85. Notice that the north poles (black) and the south poles (white)
cancel each other's force. Now, suppose you magnetize this piece of
iron with the north pole of another magnet. When you stroke the magnet
along the piece of iron, the strong north pole of the magnet attracts
all the molecular south poles in the iron. The molecules shift around
so that their south poles point toward the magnet's north. The molecules
do not move from place to place but they do shift or turn. After each
stroke, more and more molecules are found to have shifted around so
that all their south ends are pointing one way and all their north ends
the other way. The iron bar's molecules would now look like figure 86.
Figure 85. - lron - unmagnetized.
Figure 86. - lron - magnetized.
According to the laws of magnetism, flux goes from the north pole
to the south pole. Considering each molecule as a magnet, the lines
of force leave the north pole of one molecule and enter the south pole
of the next molecule. This process continues through the entire length
of the bar. Finally the lines of force leave the north poles of the
molecules at the end of the bar. This flux then re-enters the bar at
the opposite end. You have a magnet. The magnet is strong because the
lines of force all reenforce each other - they are all in the same direction.
An ordinary bar of iron is made into a magnet by the simple process
of rearranging its molecules. You remember that this process is called
You can't SEE molecules, so of course, this explanation
is a theory-but, there are a number of facts to support this theory.
If you break a magnet into many pieces, as in figure 87, you will get
many small magnets. Notice that the polarity corresponds to the theory
that each molecule is a small magnet.
Figure 87. - Magnetic poles in a broken
If you hammer or heat a magnet, it loses its magnetism. You have
shaken up the tiny magnets so that they lose their alignment. Shaking
the molecules jumbles them up-you have an ordinary bar of iron again.
Figure 88 illustrates the process of inducing magnetism. Compare
"directions" in A and B. Note that the POLARITY of the magnet being
made depends upon the DIRECTION of stroking. The molecules are being
dragged into position by the magnet. Both magnetic attraction and movement
determine induced magnetic polarity.
Figure 88. - Polarity of induced magnetism.
When inducing magnetism, more strokes will produce more magnetism.
It seems that each stroke forces more molecules into alinement. BUT
- there is a limit! For any given material, there is a point beyond
which the magnetism will not get appreciably stronger. A magnet at this
point is SATURATED.
Saturation is like a sponge full of water.
No matter how many times you dip it in the pail-it will hold only so
much water. Such a sponge is saturated. A bar of iron that is magnetically
saturated is as full of magnetism as it can get. Probably all the molecules
that are ABLE to line up, are lined up. The saturation point differs
for different materials. For example, iron has a higher saturation point
than nickel, likewise Alnico has a higher saturation point than iron.
The saturation point a metal tells you exactly how strong a magnet it
Some metals hold their magnetism a long time - in fact, almost indefinitely.
Such magnets are called "permanent magnets." Others lose their magnetism
rapidly. They are called "temporary magnets." RETENTIVITY is the measure
of a magnet's permanence. All magnets lose their magnetism sooner or
later, but those which remain magnetized for a long period of time are
said to have a HIGH retentivity. And those which lose their magnetism
quickly are said to have a LOW retentivity.
The magnetism which
remains in a magnet, after magnetization has ceased, is RESIDUAL MAGNETISM.
Materials which have a high retentivity have more residual magnetism
after a given time. Permanent magnets for meters, compasses, radios,
and magnetoes must have a high retentivity. They are usually made of
hard steel or alnico.
Radios, radar, electrical meters, motors, generators, automatic
switches and many other kinds of electrical apparatus depend for their
operation on electricity AND magnetism. In fact, electricity or magnetism
alone - one without the other - is seldom found in a machine. Because
magnetism is so important in electricity, the following table reviews
the most important terms in this chapter.
||The concentration of the lines of force
- the strongest magnetic point.
||ALL UNLIKE magnetic poles attract each
||ALL LIKE magnetic poles repel each other.
|Flux, magnetic field, field
|The force pattern of a magnet - represented
by lines showing direction and strength of force.
||Re-alinement of molecules in magnetic substances
to PRODUCE A MAGNET.
||The pole at which the magnetic force leaves
||The pole at which the magnetic force re-enters
||Magnets which retain their magnetism a
long time - years.
||Magnets which lose their magnetism after
a short time - minutes or days.
||All magnetic lines leave a north pole and
enter a south pole.
||Magnetic lines never cross magnetic lines;
two lines may blend together, add together, or cancel but
they CANNOT CROSS.
||Materials which can be magnetized - high
||Materials which cannot be magnetized-high
||The amount of resistance offered to the
passage of lines of flux.
||The ease of passage of flux.
||The holding of flux-the limit of magnetic
||The property of retaining magnetism after
magnetization has stopped.
||The magnetism left in a magnet after magnetization