We know that a freely suspended bar magnet aligns itself along the geographical north-south direction. This is due to the fact that earth is a huge magnet. This fact was first discovered by Sir William Gilbert in 1600 A. D. although the effect was used in navigation for several centuries ago. The earth behaves as if there were a huge magnetic dipole embedded in it, with magnetic N-pole near the geographical south pole and magnetic S-pole near the geographic north pole.
The pattern of magnetic field lines of the earth. A magnetized needle, capable to rotate about its C.G. in the vertical plane, dips downward towards the north to the horizontal. The angle increases with latitude. The point where the angle of dip is 90 is called magnetic poles of the earth. They are fairly near to the geographical poles. The magnetic axis passes through these two poles, which makes an angle with the geographic axis.
The magnetic poles of the earth do not coincide with the geographic poles; in fact, these are widely separated. The magnetic north pole, for example, is now located nearly 1800 km from the geographic pole at a point in Hudson Bay in northern Canada. The south magnetic pole is located south of Australia. The difference between the geographic north and the geomagnetic north varies from point to point on the earth's surface, and the difference is known as magnetic declination. The magnetic field of the earth is considered due to a dipole of magnetic moment. The strength of the field at the surface varies from 3 near the equator to 6 at the poles. However, this magnitude as well as direction changes as time passes. The studies of magnetic rocks have revealed that the magnetic field can reverse its direction. This reversal occurs about every 100,000-1,000,000 years. It appears that in the past 76 years, about 171 such reversal have taken place. The latest reversal is believed to be having occurred around 10,000 years ago.
Various theories have been proposed to explain the source of the earth's magnetic field, but none has given the satisfactory explanation. The earth has large deposits of iron or deep beneath the surface, but high temperature in the core prevents it from retaining permanent magnetization as a bar magnet. Scientists consider that the true source of the earth's magnetic field is the convection current in its core. The charged ions or electrons circulating in the liquid interior could produce a magnetic field just as a current loop.
Elements of terrestrial magnetism
The quantities needed to specify the earth's magnetic field at a place completely are called the elements of terrestrial magnetism. These are declination, inclination and horizontal component of the earth's magnetic field.
Declination
It is the angle between the magnetic meridian and the geographic meridian of the earth. This arises because of the earth's magnetic axis not coinciding with its geographic axis. The magnetic meridian is a vertical plane at a place which passes through the axis of a freely suspended bar magnet. The geographic meridian is the vertical plane at a place to place in the earth and even at a place, it shows the periodic variation.
Inclination or Angle of Dip
The angle of dip is defined as the angle between the earth's magnetic field and the horizontal direction in the magnetic meridian.
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