Sir William Gilbert was the first to suggest that earth itself is a huge magnet. His statement was based on the following evidence-
A magnet suspended from a thread comes to rest along the North-South direction. This is as if a huge fictitious magnet is buried along the diameter of the earth. The North-pole of this fictitious magnet must be towards the geographic South, so as to attract the South Pole of suspended magnet and vice versa.
When a soft iron core is buried under the earth in the North-South direction, it is found to acquire the properties of the magnet after some time.
The branch of physics which deals with the study of earth’s magnetism is called geomagnetism. The strength of the earth’s magnetic field is ≅ 10-4 Tesla or 1 gauss.
Cause of Earth’s Magnetism:
The exact cause of earth’s magnetic field is not yet known. However, some important postulates in this respect are-
There is metallic molten charged fluid in the core of the earth. The radius of this fluid is about 3500 km. Due to the motion of this fluid inside the earth, large circulating currents are produced. These currents magnetise the earth.
In the outer layers of the earth’s atmosphere, gases are in an ionised state. Due to the rotation of the earth, these ions also move and hence they constitute the current. These currents may be the reason for the magnetism of the earth.
Every substance is made up of protons and electrons. When the earth rotates, every substance on/inside the earth will rotate. Hence there is a motion of protons and electrons and due to this, currents are produced. This may be the reason for the magnetism of the earth.