PART V

HOW WE USE NATURE'S FORCES

CHAPTER XXIII

THE MARINER'S COMPASS

215. What is a Magnet? - Did you ever have a knife with a magnetized blade? If you did, you will remember how the blade attracted and picked up

needles, tacks, and other small objects

made of iron or steel. Perhaps you

have owned a horseshoe magnet

(Fig. 124); this could pick up a great

deal more than the knife blade. Beside these you may have had a pocket compass, and enjoyed finding where true north was, even when the sun

Both poles

FIG. 124. of the magnet can pick

up particles of iron or
steel.

125).
compass is a magnet, like the
magnetized knife blade and
the horseshoe magnet; but it
is suspended or supported so
in the plane of our horizon.

that it can swing around
What does this mean? See § 84.

The compass used on ships (mariner's compass; Fig. 126) is supported so that it will remain level, no matter how much the ship tosses about in the sea.

NW

NNW

N by W

NW by N

NW by W

WNW

W by N

W

W by S

WSW

SW

SW by W

SW by S

N by E
NNE

NE by N

NE
>NĚ by E
>ENE

E by N

E

E by S

ESE

SE by E

SE

SE by S

FIG. 126. -The thirtytwo points of a compass, according to which ships are steered at sea.

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Experiment. Examine a bar magnet and a horseshoe magnet, and pick up nails and tacks with them. Note that a nail which is being held by a magnet is itself a magnet and will pick up other nails.

Try picking up a brass key with a magnet. Do you succeed? Try also the following: a gold ring or pin; a silver coin; a copper coin; a nickel; an ordinary pin; a needle. Which are attracted by the magnet?

Magnetize a needle by bringing it near, or touching, a large magnet. The magnet may be either a bar magnet or a horseshoe magnet. The best way is to stroke the needle, from the middle to t'e point, with one end of the magnet. Repeat this several times. Test the end of the needle which you stroked with the larger magnet; is it itself a magnet? Test the other end of the needle; is it a magnet, or not? Find out how large magnets are magnetized (see § 226).

The bar of soft iron which is laid against the two ends of a horseshoe magnet when the magnet is not in use is called an armature (pronounced är'mă-tiūr). Find out why it is used.

How did people ever learn about magnets? The ancients were acquainted with them, and magnets received their name from certain pieces of iron ore which were found in Magnesia, in Asia Minor. These pieces had the peculiar property of drawing small pieces of iron and steel to themselves. Later, in England, such natural magnets