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THE EARTH'S MAGNETISM (OR) TERRESTRIAL MAGNETISM:In 1600 William Gilbert who was a physician to queen Elizabeth1 of England, showed that the force acting on a suspended magnet is not due to the influence of the pole star but to a magnetic field which is present all over the surface of the earth, as if the earth was magnetized. Gilbert made a large spherical ball from magnetite, the poles of which were situated on two opposite ends. By placing a small magnetic compass on this magnetite sphere, Gilbert showed that the compass took up a definite direction at different points on the sphere as it does in the case of the earth. According to his view the magnetic field on the surface of the earth can be approximately represented as if caused by a huge imaginary bar magnet in the interior of the earth but inclined to the geographic north and south axis of the earth. This concept of terrestrial magnetism has been illustrated as shown in figure. The north end of the imaginary magnet has a south polarity while the south end possesses the north polarity
Declination:In order to understand the term declination, let us first define two terms. Geographic meridian and magnetic meridian.Geographic meridian at any place is a vertical plane passing through the geographic north and south poles of the earth. Magnetic meridian at any place is a vertical plane passing through the magnetic north and south poles of the earth.The angle between the magnetic meridian and the geographic meridian at a place is known as declination or variation,In figure, ABCD is the magnetic meridian and ABCdashDdash represents the geographic meridian at a place. The angle BABdash=θ is the angle of declination. The angle of declination varies from place to place on the earth's surface. Gilbert believed that the declination at any one place was constant in time. But this belief was soon proved wrong. There were several periodic variations in the value of declination for a given location.