Man's physical universe



A compass supported so that it can swing vertically is called a. dipping

needle. The dipping needle shows the direction of the lines of force in

the earth. Large deposits of magnetic minerals alter the direction of


111/ /^~^-\ W I I



these lines of force somewhat in certain


The Molecules of a Magnet Are Believed

to Be Magnets.

A few easily performed experiments

show that magnetism is molecular in its

nature and origin.

If we cut a magnetized

steel ware showing pronounced poles at the

ends into short pieces, each one of these

pieces will be a magnet with north and

south poles. This subdivision into separate

small magnets can be continued until

the pieces are too small to be further subdivided

by physical methods. Such experiments

form the basis for the molecular

theory of magnetism. According to this

Fig. 226. Diagrammatic

theory, each of the particles (atoms or

representation of the lines of

molecules, or groups of these, as the case

force of topical magnets.

may be) of which the iron wire is composed

is a tiny magnet having a north and a south pole. In an

unmagnetized piece of iron these small magnets point in various

directions with no resultant magnetic effect. Magnetizing of iron consists

of turning some of

these miniature magnets so that their poles

point in the same direction and therefore unite to make the whole bar

show magnetic properties. The above process of aligning the particles

is called orientation. The molecular theory seems to be quite logical

when we consider how a bar of iron is magnetized. One way of magnetizing

a body is to stroke it wdth a magnet, always drawing the

magnet the same way on the bar. Another way is to hold the bar so

that the earth's lines of force pass through it, and to tap it several


On the other hand, a magnet may be demagnetized by tapping

it while it is held in such a position that the earth's lines of force

do not pass through it lengthwise. Still another method of destroying

the magnetism of an iron bar is to heat it to redness.

All these phenomena can be explained by the molecular theory.

Stroking with a magnet tends to orient the molecules. Tapping temporarily

disturbs the forces which hold the molecules together within the

solid, thus leaving them free to rearrange themselves in accordance

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