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Contents. • U.S. Government Printing Office; 1945 - 618779
In the light of recent urgent news about an unexplained, sudden uptick in the migration rate of Earth's magnetic field, this chapter on magnetism from the U.S. Navy's training course is especially interesting. Figure 83 is a snapshot of the magnetic variation (aka declination) isogonic lines as they were around 1945, when this manual was published. I say "snapshot" because those lines are constantly changing. Magnetic declination (variation) is the difference between magnetic north (or south) pole as indicated by a magnetic compass, and the true geographic north (or south) pole around which the earth rotates. Magnetism records locked up in rocks and plants, combined with records kept by ancient mariners who compared compass readings with those obtained from sextants provide the data. As you can see in the animation posted on Wikipedia, the magnetic declination changes significantly over time. The advent of satellite-based navigation has largely diminished the need for a magnetic compass, but aircraft and ships are required by law to carry a magnetic compass as a last-ditch backup navigation instrument, so magnetic variation information is still required.
Magnetism - Its HistoryAn 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 discovery.
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.
Figure 65. - Making a magnet by induction.
Figure 66. - Making a magnet by the coil method.
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 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.
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.
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.
Figure 67. - Forces as vectors.
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.
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.
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 68. - Ships' forces as vectors.
Figure 69. - Magnetic field of force.
Figure 70. - Flux pattern of bar magnet.
Figure 71. - Unlike poles-flux pattern.
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.
Magnetic 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 -
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.
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.
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.
Figure 72. - Unlike poles attract.
Figure 74. - Like poles repel.
Unlike Pole 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.
Figure 75. - Permeability of iron.
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.
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.
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.
Figure 77. - Keeper-reducing reluctance.
Figure 78. - Magnetic and geographic poles of the earth.
Figure 79. - The pocket compass.
Figure 80. - The spirit compass.
Figure 81. - Compass on 170° heading.
Figure 82. - Agonic line.
Figure 83. - Isogonic lines of U.S.
Figure 84. - Compass deviation.
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.
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.
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.
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.
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.
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°.
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.
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.
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 stand-by service.
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.
Figure 85. - Iron - unmagnetized.
Figure 86. - Iron - magnetized.
Figure 87. - Magnetic poles in a broken magnet.
Figure 88. - Polarity of induced magnetism.
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.
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 reinforce 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.
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.
When inducing magnetism, more strokes will produce more magnetism. It seems that each stroke forces more molecules into alignment. 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 magnetos 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
|Poles||The concentration of the lines of force - the strongest magnetic point.|
|Attraction||ALL UNLIKE magnetic poles attract each other.|
|Repulsion||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.|
|Induction||Re-alinement of molecules in magnetic substances to PRODUCE A MAGNET.|
|North Pole||The pole at which the magnetic force leaves the magnet.|
|South Pole||The pole at which the magnetic force re-enters the magnet.|
|Permanent magnets||Magnets which retain their magnetism a long time - years.|
|Temporary magnets||Magnets which lose their magnetism after a short time - minutes or days.|
|Complete path||All magnetic lines leave a north pole and enter a south pole.|
|Magnetic lines||Magnetic lines never cross magnetic lines; two lines may blend together, add together, or cancel but they CANNOT CROSS.|
|Magnetic substances||Materials which can be magnetized - high ,permeability substances.|
|Non-magnetic substances||Materials which cannot be magnetized-high reluctance substances.|
|Reluctance||The amount of resistance offered to the passage of lines of flux.|
|Permeability||The ease of passage of flux.|
|Saturation||The holding of flux-the limit of magnetic strength.|
|Retentivity||The property of retaining magnetism after magnetization has stopped.|
|Residual magnetism||The magnetism left in a magnet after magnetization has stopped.|
Chapter 11 Quiz
Posted January 24, 2019