Magnetism

Chapter IX. Magnetism.


(1) General Properties of Magnets

199. Magnets.?Since the times of the early Greek philosophers men have known of certain stones that have the property of attracting to themselves objects of iron and steel. Such stones are called natural magnets. It is thought by many that the name magnet is derived from Magnesia in Asia Minor, where these stones are abundant, though this is but tradition.
It was also learned long ago that iron and steel objects when rubbed with natural magnets become magnetized, that is, acquire the properties of magnets. These are said to be artificial magnets.
Fig. 169.?A bar magnet.
Fig. 170.?A horseshoe magnet.
Some 800 years ago it was discovered that magnets, natural or artificial, when suspended so as to turn freely, always come to rest in a definite position pointing approximately north or south. This is especially noticeable when the magnet is long and narrow. Because of this property of indicating direction, natural magnets were given the name of lodestone (lode-leading).
Artificial magnets are made by rubbing steel bars with a[Pg 229] magnet or by placing the steel bar in a coil of wire through which a current of electricity is flowing. The magnetized steel bars may have any form, usually they are either straight or bent into a "U" shape. These forms are known as bar and horseshoe magnets. (See Figs. 169 and 170.) Magnets retain their strength best when provided with soft-iron "keepers," as in Fig. 171.
Fig. 170.?A horseshoe magnet.Fig. 171.?Bar magnets with keepers.
200. Magnetic Poles.?If a magnet is placed in iron filings and removed, the filings will be found to cling strongly at places near the ends of the magnet, but for a portion of its length near the middle no attraction is found. (See Fig. 172.) These places of greatest attraction on a magnet are called poles. If a bar magnet is suspended so as to swing freely about a vertical axis the magnetic pole at the end pointing north is called the north-seeking pole; at the other end, is the south-seeking pole. In most places the needle does not point to the true north, but somewhat to the east or west of north. The direction taken by a magnetic needle is parallel to the magnetic meridian.
Fig. 172.?Iron filings attracted to the poles of a magnet.
201. Law of Magnetic Action.?The north pole of a magnet is usually marked. If a marked bar magnet be held in the hand and its north-seeking pole be brought near the north-seeking pole of a freely suspended bar magnet, the two poles will be found to repel each other, as will also two south-seeking poles, while a north-seeking and a south-seeking pole attract each other. (See Fig. 173.) This action leads to the statement of the Law of Magnetic Action:[Pg 230] Like poles repel, while unlike poles attract each other. The force of attraction or repulsion lessens as the distance increases. The force of the action between magnetic poles is inversely proportional to the square of the distance between them. Compare this with the law of gravitation (Art. 88).
Fig. 173.?Like poles of two magnets repel.
Fig. 174.?A magnetoscope.
202. Magnetic Substances and Properties.?It is found that if an iron or steel magnet is heated red hot that its magnetic properties disappear. Accordingly one method of demagnetizing a magnet is to raise it to a red heat. If a magnet that has been heated red hot and then cooled is brought near a suspended bar magnet, it is found to attract either end, showing that it has regained magnetic properties even though it has lost its magnetic polarity. A suspended bar magnet used to test the magnetic properties of a body is called a magnetoscope. (See Fig. 174.) The needle of a magnetic compass serves very well as a magnetoscope. Magnetic properties are most strongly exhibited by iron and steel, though nickel and cobalt show some magnetic effects. There is a peculiar alloy of copper, aluminum, and manganese, known as Heusler's Alloy, that is also magnetic. However, of all substances, iron and steel show the strongest magnetic effects.
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203. Magnetic Induction.?Let the north-seeking pole of a bar magnet support an iron nail by its head. (See Fig. 175.) Test the point of the nail for polarity. See whether a second nail can be attached by its head to the point of the first. Test the polarity of the point of this nail. Find by trial how many nails can be suspended in succession from the magnet. Test in each case for polarity. Withdraw carefully the magnet from the first nail?the string of nails will fall apart. Repeat the test with a thickness of paper between the magnet and the first nail. Results similar to those secured at first will be found, though probably fewer nails will be supported. The presence of paper between the magnet and nails simply weakens the action. Test the action of the magnet upon the nail when there is between them a piece of glass, one's thumb, thin pieces of wood, copper, zinc, etc. The magnetizing of a piece of iron or steel by a magnet near or touching it is called magnetic induction. This action takes place through all substances except large bodies of iron or steel hence these substances are often used as magnetic screens. The pole of the new induced magnet adjacent to the bar magnet is just opposite to the pole used. Thus the N.-pole of the magnet used will produce a S.-pole at the near end of the nail and a N.-pole at the end farther away. (See Fig. 175.) On removing the magnet, the nails are found to retain a part of their induced magnetism.
Fig. 175.?Nails magnetized by induction.
204. Retentivity.?In several of the foregoing paragraphs it has been seen that a piece of iron or steel when once magnetized does not entirely lose its magnetism when the magnetizing force is removed. Different pieces of[Pg 232] iron and steel vary greatly in this respect, some remaining strongly magnetized, others losing much of their magnetism. This property of retaining magnetism is called retentivity. Hardened steel has a high degree of retentivity, while soft iron retains but little magnetism.

Important Topics

1. Magnet; natural, artificial, bar, horseshoe.
2. Magnetic poles; north seeking, south seeking.
3. Law of action, magnetoscope, retentivity, induced magnet.

Exercises

1. Make a summary of the facts of magnetism presented in this lesson.
2. Is magnetism matter, force, or energy? How do you decide? To what other phenomenon that we have studied is it similar? How?
3. Make a simple magnetoscope for yourself by suspending a thin steel needle or rod 5 to 10 cm. long, with a light thread or silk fiber at its center, so that it will hang level. Then magnetize the needle, and keep the magnetoscope in your book.
4. Name three uses for magnets or magnetism.
5. Mention three uses for a magnetoscope.
6. Are all magnets produced by induction? Explain.
7. In what magnetic devices is a high retentivity desirable?

(2) The Theory of Magnetism and Magnetic Fields

205. The Theory of Magnetism.?If a magnetized watch spring is broken in two, each part is found to be a magnet. If one of these parts be broken and this process of breaking be continued as far as possible, the smallest part obtained has two poles and is in fact a complete magnet. (See Fig. 176.) It is supposed that if the division could be continued far enough that each of the molecules of the steel spring would be found to have two poles[Pg 233] and to be a magnet. In other words, magnetism is believed to be molecular. Other evidence supporting this idea is found in the fact that when a magnet is heated red hot, to a temperature of violent molecular motion, its magnetism disappears. Also if a long, fine soft iron wire be strongly magnetized, a light jar causes its magnetism to disappear. This would lead us to believe that magnetism is not a property of the surface of the body, but that it depends upon molecular structure or the arrangement of the molecules.
Fig. 176.?Effect of breaking a magnet.
Fig. 177.?Possible arrangement of molecules in an unmagnetized iron bar.
It is believed also that the molecules of a magnetic substance are magnets at all times; that before the body is magnetized the molecules are arranged haphazard (see Fig. 177) but that when a magnet is brought near, the molecules tend to arrange themselves in line, with their north-seeking poles pointing in the same direction. (See Fig. 178.) If the magnet is jarred some of the molecules tend to get out of line, perhaps to form little closed chains of molecules. (See Fig. 177.)
Fig. 178.?Arrangement of molecules in a saturated magnet.
206. Magnetic Fields and Lines of Force.?The behavior of magnets is better understood after observing and[Pg 234] studying the lines of force of a magnet. The earliest descriptions of these are by William Gilbert, the first Englishman to appreciate fully the value of making experimental observations. He wrote a book in 1600 called De Magnete in which he published his experiments and discoveries in magnetism. (See p. 217.)
Magnetic lines of force may be observed by placing a magnet upon the table, then laying upon it a sheet of paper and sprinkling over the latter fine iron filings. On gently tapping the paper, the filings arrange themselves along curved lines extending from one end of the magnet to the other. These are called the magnetic lines of force. (See Fig. 179.) The space about a magnet in which the magnetic lines are found is called the magnetic field. (See Fig. 180.)
Fig. 179.?Iron filings on paper over a bar magnet.
Many interesting things have been discovered concerning the lines of force. Some of the facts of magnetic action are given a simple explanation if we think of them as due to the magnetic lines of force. A summary of several discoveries concerning magnetic fields follows:
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(A) Magnetic lines of force run side by side and do not cross one another. (See magnetic fields.)
(B) Magnetic lines of force are believed to form "closed curves" or to be continuous. The part outside of the magnet is a continuation of the part within the magnet. (See Fig. 180.)
Fig. 180.?Diagram of the field of a bar magnet.
(C) The attraction of a magnet is strongest where the magnetic lines are thickest, hence they are believed to be the means by which a magnet attracts.
(D) Since like poles repel and unlike poles attract, it is known that the action along a line of force is not the same in both directions. It has therefore been agreed by physicists to indicate by an arrow head (Fig. 180), the direction that a north-seeking pole tends to move along a line of force. The lines of force are considered as leaving the north-seeking pole of a magnet and entering the south-seeking pole. (See Figs. 181 and 182.)
Fig. 181.?Magnetic field between like poles showing repulsion.
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(E) A freely suspended small magnet in a magnetic field places itself parallel to the lines of force. (Test this by holding a magnetic compass in different portions of a magnetic field). Note the position of the needle and the lines of force. This fact indicates that the compass needle points north on account of its tendency to turn so as to be parallel to the earth's magnetic held.
Fig. 182.?Magnetic field between unlike poles showing attraction.
(F) Each magnet is accompanied by its own magnetic field. When a piece of iron is brought within the field of a magnet the lines of force passing through the iron tend to arrange the iron molecules in line or to magnetize the iron.
207. Magnetic Induction.?The action of magnetic lines of force in magnetizing iron when they pass through it, is called Magnetic Induction. This may now be defined as the production of magnetism in a body by placing it within a magnetic field. Freely suspended magnets place themselves parallel to the lines of force in a magnetic field, therefore when an iron rod is placed in a weak field, or one with few lines of force, the iron is but slightly magnetized; that is, but few molecules are brought into[Pg 237] line. Increasing the strength of the magnetizing field, gives stronger magnetization to the iron up to a certain point. After this, stronger fields give no increase in magnetizing effect. When iron exhibits its greatest magnetization it is said to be saturated.
Fig. 183.?Effect of a piece of iron in a magnetic field.
208. Permeability.?If a piece of iron is placed between the poles of a horseshoe magnet, the "field" obtained by sprinkling iron filings upon a sheet of paper over the magnet resembles that shown in Fig. 183. The lines in the space between the poles of the magnet seem to crowd in to the piece of iron. The property of the iron by which it tends to concentrate and increase the number of lines of force of a magnetic field is called permeability. Soft iron shows high permeability. Marked differences in behavior are shown by different kinds of iron and steel when placed in a magnetic field. Very pure iron, or soft iron, is strongly magnetized by a magnetic field of medium strength. Its magnetism, however, is quickly lost when the magnetizing field is removed. This indicates that soft-iron molecules are easily swung into line, but also disarrange themselves as easily when removed from a magnetizing force. Soft-iron magnets having high permeability quickly lose their magnetism. They are therefore called temporary magnets. On the other hand a hardened steel bar is difficult to magnetize, but when once magnetized retains its magnetism permanently, unless some action weakens the magnet. Such magnets are called permanent magnets.
Note.?The term "line of force" as used in this text means the same as "line of induction" as used in more advanced texts.
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Important Topics

1. Molecular theory of magnetism, saturation, permeability.
2. Magnetic fields and lines of force.
3. Six facts concerning magnetic fields.

Exercises

1. Name an object whose usefulness depends upon its retentivity. Explain.
2. How do you explain the retentivity of hard steel?
3. Are the molecules of a piece of iron magnetized at all times? Explain.
4. When a piece of iron is magnetized by induction does any magnetism enter the iron from the magnet? Does the magnet lose as the iron gains magnetism? Explain.
5. Have all magnets been produced by induction? Explain.
6. Why will tapping a piece of iron when in a magnetic field increase the amount it will be magnetized?
7. Express in your own words the theory of magnetism.
8. Place two bar magnets in a line 5 cm. apart, unlike poles adjacent; obtain the magnetic field with iron filings. Sketch it.
9. Repeat Exercise No. 8 using like poles. Describe the appearance of a field that gives attraction; of a field that gives repulsion.

(3) The Earth's Magnetism

209. The Earth's Magnetic Field.?Dr. William Gilbert's famous book, De Magnete, contains many helpful and suggestive ideas, none perhaps more important than his explanation of the behavior of the compass needle. He assumed that the earth is a magnet, with a south-seeking pole near the geographical north pole, and with a north-seeking pole near the geographical south pole. This idea has since been shown to be correct. The north magnetic (or south-seeking) pole was found in 1831, by Sir James Ross in Boothia Felix, Canada. Its approximate present location as determined by Captain Amundsen in 1905 is[Pg 239]
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latitude 70? 5? N. and longitude 96? 46? W. The south magnetic pole is in latitude 72? S., longitude 155? 16? E. The north magnetic pole is continually changing its position. At present it is moving slowly westward.
Fig. 184.?Magnetic map of the earth for 1910. Isogonic lines ??? Isoclinic lines - - - -
210. Direction of the Earth's Magnetic Field.?Reference has been made to the fact that the compass does not always point exactly north. This indicates that the earth's magnetic field varies in its direction. Columbus discovered this fact upon his first voyage. The discovery alarmed the sailors since they feared they might come to a place where the compass would be unreliable. This variation is called declination. It is defined as the angle between the direction of the needle and the geographical meridian. Declination is due to the fact that the geographical and magnetic poles do not coincide. What is meant by a declination of 90?? Lines drawn upon a map so as to pass through places of the same declination are called isogonic lines. The line passing through points where the needle points north, without declination, is the agonic line. The agonic line is slowly moving westward. It now passes near Lansing, Michigan; Cincinnati, Ohio; and Charleston, S. Carolina. (See Fig. 184.) At all points in the United States and Canada east of the agonic line the declination is west, at points west of the agonic line the declination is east.
211. The Dipping Needle.?Mount an unmagnetized steel needle on a horizontal axis so as to be in neutral equilibrium, that is, so as to remain balanced in any position in which it is left. Upon being magnetized and placed so that it can swing in a north and south plane, the north-seeking pole will now be found to be depressed, the needle forming an angle of nearly 70? with the horizontal. (See Fig. 185.) The position assumed by the needle indicates that the earth's magnetic field instead of[Pg 241] being horizontal in the United States dips down at an angle of about 70?. Over the magnetic pole, the dipping needle as it is called, is vertical. At the earth's equator it is nearly horizontal. The angle between a horizontal plane and the earth's magnetic lines of force is called the inclination or dip.
Fig. 185.?A dipping needle.
212. Inductive Effect of the Earth's Magnetic Field.?The earth's magnetic lines of force are to be considered as filling the space above the earth, passing through all objects on the surface and into and through the earth's interior. The direction of the earth's field is shown by the compass and the dipping needle. Magnetic lines of force tend to crowd into and follow iron and steel objects on account of their permeability. Therefore, iron or steel objects, such as posts, columns, etc., are permeated by the earth's lines of force, which in the United States enter at the top of these objects and leave at the bottom. The lines of force passing through these bodies arrange their molecules in line or magnetize the bodies. The inductive effect of the earth's magnetism indicates how lodestones or natural magnets acquire their magnetized condition. So far as is known, magnetism produces no effect upon the human body. It can therefore be studied only by observing its effects upon magnets or bodies affected by it.

Important Topics

The earth's magnetic field, dip, declination, agonic line, induction by the earth's field.
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Exercises

1. How would a dipping needle be of assistance in locating the magnetic poles of the earth?
2. Will a dipping needle weigh more before or after it is magnetized? Explain.
3. It is said that induction precedes attraction. Using this idea, explain how a magnet attracts a piece of soft iron.
4. Devise an experiment to show that a piece of iron attracts a magnet just as a magnet attracts a piece of iron.
5. Give two methods for determining the poles of a magnet.
6. State three of the most important points in the theory of magnetism. What evidence supports each?
7. Why is a permanent magnet injured when it is dropped?
8. Name two important uses of the earth's magnetic field.
9. What magnetic pole would you find at the top of an iron post that has stood for some time in the ground? What pole at the bottom? How would you test this?

 

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