the earth being a huge magnet, its north pole attracts all the north-seeking poles of the molecules, and its south poles the south-seeking poles. (The north-seeking pole of a magnet is marked N., though it is in reality the south pole; for unlike poles are mutually attractive, and like poles repellent.)
There are two forms of magnet—permanent and temporary. If steel is magnetized, it remains so; but soft iron loses practically all its magnetism as soon as the cause of magnetization is withdrawn. This is what we should expect; for steel is more closely compacted than iron, and the molecules therefore would be able to turn about more easily.12 It is fortunate for us that this is so, since on the rapid magnetization and demagnetization of soft iron depends the action of many of our electrical mechanisms.
THE PERMANENT MAGNET.
Magnets are either (1) straight, in which case they are called bar magnets; or (2) of horseshoe form, as in Figs. 50 and 51. By bending the magnet the two poles are brought close together, and the attraction of both may be exercised simultaneously on a bar of steel or iron.
LINES OF FORCE.
In Fig. 50 are seen a number of dotted lines. These are called lines of magnetic force. If you lay a sheet of paper on a horseshoe magnet and sprinkle it with iron dust, you will at once notice how the particles arrange themselves in curves similar in shape to those shown in the illustration. It is supposed (it cannot be proved) that magnetic force streams away from the N. pole and describes a circular course through the air back to the S. pole. The same remark applies to the bar magnet.
ELECTRICAL MAGNETS.
If an insulated wire