[Ill.u.s.tration: 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 pa.s.sing 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 pa.s.s 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 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_.

[Ill.u.s.tration: 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 s.p.a.ce 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.

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 compa.s.s needle. He a.s.sumed 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 lat.i.tude 70 5'

N. and longitude 96 46' W. The south magnetic pole is in lat.i.tude 72 S., longitude 155 16' E. The north magnetic pole is continually changing its position. At present it is moving slowly westward.

[Ill.u.s.tration: 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 compa.s.s 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 compa.s.s 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 pa.s.s through places of the same declination are called _isogonic_ lines. The line pa.s.sing through points where the needle points north, without declination, is the _agonic_ line. The agonic line is slowly moving westward. It now pa.s.ses 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 a.s.sumed by the needle indicates that the earth's magnetic field instead of 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._

[Ill.u.s.tration: 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 s.p.a.ce above the earth, pa.s.sing 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 compa.s.s 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 pa.s.sing 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.

Exercises

1. How would a dipping needle be of a.s.sistance 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?

CHAPTER X

STATIC ELECTRICITY