Theory of magnetism is divided into two of them
- Magnetism Theory: Based on magnetism theory, if a magnet is divided into small pieces, the pieces remain magnetic, and still have the north and the south poles, so a magnet consists of tiny magnets which are called elementary magnets. Bodies such as iron, steel, and several other metals are ferromagnetic bodies, that as we know are materials which are easly attracted by magnets. In bodies which have not become magnets, the composition of the elementary magnets is not in order. If the ferromagnetic bodies lie in a magnetic field, the bodies will be induced, so the elementary magnets will rotate to arrange themselves (they arrange themselves in parallel to magnetic poles exerted from the outside). When the elementary magnets have been arranged, the bodies will become magnets.
- Earth Magnetism Theory: A physicist named William Gilbert (1544-1603) proposed that the earth is a giant magnet. The earth is a natural magnet. The north pole of earth magnet lies on the south pole of earth and the south pole of earth magnet lies on the north pole of the earth. Hence, a compass’s needle always points to the north and south directions, and the north pole of compass needle points to the north pole of earth because it undergoes an attractive force against the south pole of earth magnet which lies on the north pole of earth, while the south pole of compass needle points to the south pole of earth because is undergoes an attractive force against the north pole of earth magnet on the north pole of earth. Study the figure
From the figure, obviously the magnet axes or magnetic poles of the earth do not coincide with the rotational axic or earth’s poles. The magnitude of declination angle between them is 15o. Hence, the location (direction) of compass needle does not precisely show the direction of the north and south of poles of the earth. The declination magnitude of compass needle direction in every place of earth is not equal. The closer to the equator, the smaller the declination. This declination angle is called the angle of declination.
Referensi: Irawan, Etsa Indra dan Sunardi. 2008. Pelajaran IPA-Fisika Bilingual untuk SMP/MTs. Kelas IX. Bandung: CV.Yrama Widya.
