Man's physical universe

xanabras

.

MAGNETISM 481

with the forces acting on them. Heating agitates the molecules so

much that all previous orderly arrangements are broken up and thus

destroys the magnetism. The molecular theory can be demonstrated

very nicely by stroking a test tube full of iron filings with the pole of a

magnet. The iron filings are found to line up end to end, but this

orderly arrangement may be disturbed

by shaking the tube.

The molecular theory may also be dem

onstrated by suspending a large number

of small magnets on pivots on a board

and submitting them to magnetic fields.

The small magnets will spin around as a

large magnet is moved over them, or they

will become oriented when one pole of the

'/^^ \ ^.-^ /X^/_\

Unmagnetized

Magnetized

magnet IS placed at one end of the board. y- 10-7 t-u 1 1

°

111 -lit r

riG. 227. 1 he molecules are

The methods described above for mag- thought to be oriented when

netizing a piece of iron are examples of the bar of iron is magnetized.

what is known as magnetic induction. The molecules in the unmag-

The molecules of soft iron are easily

netized bar are not oriented,

oriented according to the above theory,

inasmuch as soft

,

iron acts

-^

as a magnet

"f'^^ *^°^." ^" ?" magnetized

bar are oriented,

only when it is in a magnetic field; for example, a piece of soft iron

held near a magnet acquires the property of picking up iron objects

but loses it at once when the magnet is removed. A whole chain of

tacks can be picked up by a magnet because each tack temporarily

becomes a small magnet by induction. Steel is more difficult to magnetize

but retains its magnetism for a long time once it is magnetized.

According to the molecular theory the molecules in steel are held

together more rigidly than are the molecules in soft iron.

A magnet is said to be saturated when it can acquire no more magnetism,

or, in other words, when the maximum fraction of the molecules

has been oriented.

A magnetic field can freely penetrate nonmagnetic substances such

Some substances are more readily penetrated

as paper, wood, or glass.

by magnetic fields than others. Thus iron has a permeability (attraction

for lines of force) several hundred times that of air. This is shown

by the bending of the earth's lines of force as they pass over a deposit

of iron ore.

In some parts of the world the compass is rendered useless

for telling directions because of the attraction of large deposits of

magnetite for the compass needle.

The magnetic field in the gap of a broken ring is very intense.

fact is applied advantageously in the use of horseshoe magnets.

This

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