Physics Part 37

(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_.

[Ill.u.s.tration: FIG. 169.--A bar magnet.]

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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_.

[Ill.u.s.tration: 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).

[Ill.u.s.tration: FIG. 173.--Like poles of two magnets repel.]

[Ill.u.s.tration: 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 compa.s.s_ 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 gla.s.s, 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: FIG. 176.--Effect of breaking a magnet.]

[Ill.u.s.tration: 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.)

[Ill.u.s.tration: 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 s.p.a.ce about a magnet in which the magnetic lines are found is called the _magnetic field_. (See Fig. 180.)

[Ill.u.s.tration: 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.)

[Ill.u.s.tration: 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.)

[Ill.u.s.tration: 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 compa.s.s in different portions of a magnetic field). Note the position of the needle and the lines of force. This fact indicates that the compa.s.s needle points north on account of its tendency to turn so as to be parallel to the earth's magnetic held.