Magnetism - Course Of Physics
Course Of Physics
Chapter I. Introduction And Measurement. Page
Chapter II. Molecular Forces And Motions.
Chapter III. Mechanics or Liquids.
Chapter IV. Mechanics Of Gases.
Chapter V. Force And Motion.
Chapter VI. Work And Energy.
Chapter VII. Heat, Its Production And Transmission.
Chapter VIII. Heat And Work.
Chapter IX. Magnetism.
- (1) General Properties of Magnets
- (2) Theory of Magnetism, Magnetic Fields
- (3) The Earth's Magnetism
Chapter XI. Electric Currents Produced By VoltaicCells.
Chapter XII. Magnetic Effects Of Electric Currents,And Electrical Measurements.
Chapter XIII. Chemical And Heat Effects Of ElectricCurrents.
Chapter XIV. Induced Currents.
Chapter XV. Sound.
Chapter XVI. Light.
Chapter XVII. Invisible Radiations.
Chapter XVIII. Wireless Telephony And AlternatingCurrents.
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.
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.
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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.
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.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.)
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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.)
(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.)
(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.
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.
Note.?The term "line of force" as used in this text means the same as "line of induction" as used in more advanced texts.
[Pg 238]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][Pg 240] 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.
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.
Important Topics
The earth's magnetic field, dip, declination, agonic line, induction by the earth's field.[Pg 242]
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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