« ΠροηγούμενηΣυνέχεια »
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
S true north was, even when the sun
FIG.124.-Both poles woods after of the magnet can pick dark (Fig.
up particles of iron or 125). The
compass is a magnet, like the - A pocket compass. When the needle of the compass
magnetized knife blade and is free to move, it points toward the horseshoe magnet; but it the north.
is suspended or supported so that it can swing around in the plane of our horizon. 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
NW by N
N by W
NE by N
NW by W
NĚ by E
W by N
E by N
W by S
E by S
SE by E
SW by W
SW by s
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 mag
net. Do you succeed? Try also the folFig. 126. — The thirty- lowing: a gold ring or pin; a silver coin; two points of a compass, according to which ships are
a copper coin; a nickel; an ordinary pin ; a steered at sea.
needle. Which 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