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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.
About 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 POLES.
2. ARTIFICIAL MAGNETISM CAN BE PRODUCED BY CONTACT WITH ANOTHER MAGNET. This
magnetism is called
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
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 method.
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 -
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 force.
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
Magnetic forces can be represented by lines and arrows the same as other forces.
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 ARE COMPLETE.
3. ALL LINES LEAVE THE MAGNET AT RIGHT
ANGLES TO THE MAGNET.
These three facts apply to ALL fields and ALL COMBINATIONS of fields.
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
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
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 SOUTH pole.
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 sun.
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!
Deviation 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
THEORY OF MAGNETISM
Theory helped you understand current and theory may help you understand magnetism.
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.
In 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 INDUCTION.
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 magnet.
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.
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 will make.
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.
IMPORTANT MAGNETIC TERMS
||The concentration of the lines of force - the strongest magnetic point.
||ALL UNLIKE magnetic poles attract each other.
||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
||Re-alinement of molecules in magnetic substances to PRODUCE A MAGNET.
||The pole at which the magnetic force leaves the magnet.
||The pole at which the magnetic force re-enters the magnet.
||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 ,permeability substances.
||Materials which cannot be magnetized-high reluctance substances.
||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 strength.
||The property of retaining magnetism after magnetization has stopped.
||The magnetism left in a magnet after magnetization has stopped.
Chapter 11 Quiz