Earth's Magnetic Fields

Magnetic Axis and Geographic Axis

Compass 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 South Pole near the geographical North, and a magnetic North 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 not coincide. The axis of the Earth's magnetic field is inclined at an angle of about 15 degrees with the geographical axis. Due to this a freely suspended magnet makes an angle of about 15 degrees 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.

Cause of Earth's Magnetism

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.

Elements of Earth's Magnetic Field

To understand the Earth's magnetic field at any place, we should know the following two quantities or elements:

  • Declination
  • Angle of dip (or Inclination)

Declination The vertical plane passing through the axis of a freely suspended magnet is called magnetic meridian. The direction of Earth's magnetic field lies in the magnetic meridian and may not be horizontal. The vertical plane passing through the true geographical North and South (or geographical axis of Earth) is called geographical meridian. The angle between the magnetic meridian and the geographic meridian at a place is called declination at that place.

The value of the angle of declination is different at different places on Earth. To find the exact geographic directions (North and South) at a place by using a magnetic compass, we should know the angle of declination at that place. The Declination is expressed in degrees East ( o E) or degrees West (o W). For example a declination of 2 o E means the compass will point 2 degrees East of true geographical North. Thus, the knowledge of declination at a place helps in finding the true geographical directions at that place. In every map used by surveyors, mariners and air pilots, declination for different places is indicated. It should be noted that at the places of zero declination, the compass North will coincide with the true geographical North.

Angle of Dip (or Inclination)

Inclination 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.

Angle of Dip at the Poles

The magnetic lines of force at the poles of Earth are vertical due to which the magnetic needle becomes vertical. The angle of dip at the magnetic poles of Earth is 90 degrees.

Angle of Dip at the Equator:

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 degrees. The angle of dip varies from place to place.