The action of the earth on a compass needle is exactly the same as that of a permanent magnet. The fact that a magnetized needle places itself in the magnetic meridian is because the earth is a great magnet with lines of force passing in a north and south direction.
The compass needle does not generally point exactly toward the true North. This is because the magnetic pole of the earth toward which the needle points is not situated at the same place as the geographical pole.
Magnetic Dip. If a sewing-needle is balanced so as to be perfectly horizontal when suspended from a silk thread and is then magnetized, it will be found that it has lost its balance and that the north end points slightly downward.
Fig. 16.—A Simple Dipping Needle.
This is due to the fact that the earth is round and that the magnetic pole which is situated in the far North is therefore not on a horizontal line with the compass, but below such a line.
A magnetic needle mounted so as to move freely in a vertical plane, and provided with a scale for measuring the inclination, is called a
Dipping Needle. A dipping needle may be easily made by thrusting a knitting-needle through a cork before it has been magnetized.
A second needle is thrust through at right angles to the first and the arrangement carefully balanced, so that it will remain horizontal when resting on the edge of two glasses.
Then magnetize the first needle by stroking it with a bar magnet. When it is again rested on the glasses it will be found that the needle no longer balances, but dips downward.