Spark image

The magnetic field of the Earth

The Earth's magnetic field closely resembles that of a uniformly magnetised sphere, or at least one with a magnetic dipole at its centre. The field is not constant with time; it changes over periods as short as a few hundred years. It is thought that it is due to the motion of molten material within the Earth's core - a sort of self- exciting dynamo. The field has also undergone periods of reversal, the direction changing by 180o. The reasons for this are not too well understood but a study of the magnetisation of rocks, a science known as paleomagnetism, has been of considerable help in developing our knowledge of the movement of the Earth's crust: continental drift.

At any point on the Earth the resultant magnetic field may be considered in two components:
(a) the vertical component and
(b) the horizontal component. The direction of the resultant field makes an angle f with the horizontal, and this angle is known as the angle of dip.
This is related to the two components by the formula:

tan f = vertical component/horizontal component

You can see from the diagram that the Earth's magnetic north pole is not at the same place as the Earth's geographic north pole. At the moment it is to the west of geographic north and moving east but a compass needle will not point to geographic north in London again until around the beginning of the twenty second century. The first records of the position of magnetic north were made in 1659 when it was 11o 15' east. It then moved westwards to be a maximum of 24o 30' west of north in 1820.

© Keith Gibbs 2011