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_.
[Illustration: FIG. 169.--A bar magnet.]
[Illustration: 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 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.
[Illustration: 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_.
[Illustration: 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_: _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).
[Illustration: FIG. 173.--Like poles of two magnets repel.]
[Illustration: 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.
=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.
[Illustration: 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 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_ 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.
[Illustration: FIG. 176.--Effect of breaking a magnet.]
[Illustration: 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.)
[Illustration: 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 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.)
[Illustration: 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:
(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.)
[Illustration: 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.)
[Illustration: FIG. 181.--Magnetic field between like poles showing
repulsion.]
(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.
[Illustration: 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 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_.
[Illustration: 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.
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 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.
[Illustration: 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 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._
[Illustration: 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.
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?