A freely suspended magnet always points in the North-South direction even in the absence of any other magnet. This suggests that the Earth itself behaves as a magnet which causes a freely suspended magnet (or magnetic needle) to point always in a particular direction: North and South. The shape of the Earth's magnetic field resembles that of a bar magnet of length one-fifth of the Earth's diameter buried at its center.
The South Pole of the Earth's magnet is in the geographical North because it attracts the North Pole of the suspended magnet and vice versa. Thus, there is a magnetic S-pole near the geographical North, and a magnetic N-pole near the geographical South. The positions of the Earth's magnetic poles are not well defined on the globe; they are spread over an area. The axis of Earth's magnet and the geographical axis do no coincide. The axis of the Earth's magnetic field is inclined at an angle of about 15o with the geographical axis. Due to this a freely suspended magnet makes an angle of about 15o with the geographical axis and points only approximately in the North-South directions at a place. In other words, a freely suspended magnet does not show exact geographical South and North because the magnetic axis and geographical axis of the Earth do not coincide.
It is now believed that the Earth's magnetism is due to the magnetic effect of current which is flowing in the liquid core at the center of the Earth. Thus, the Earth is a huge electromagnet.
So far we have only considered one type of magnetic needle which can move only in the horizontal place and points approximately in the North-South direction. Now, if we take a magnetic needle which is free to rotate in the vertical plane, then it will not remain perfectly horizontal. The compass needle makes a certain angle with the horizontal direction. In fact, in the Northern Hemisphere of Earth, the North Pole of the magnetic needle dips below the horizontal line. At any place, the magnetic needle points in the direction of the resultant intensity of Earth's magnetic field at the place.
The lines of force around the magnetic equator of the Earth are perfectly horizontal. So the magnetic needle will become horizontal there. Thus, the angle of dip at the magnetic equator of the Earth will be 0 o. The angle of dip varies from place to place.
