The Earth’s Magnetism:
In 1600, Sir William Gilbert was the first to suggest that the Earth itself is a huge magnet.
Earth behave as if a powerful magnet is placed within it. But this assumptions fails because temperature inside the earth is very high (more than Curie temperature). If there is any bar magnet inside the earth its magnetic property will be abolish at high temperature.
# The magnetic field of the Earth is due to rotating metallic fluids(iron) mainly Iron and Nickel.
# Current produced due to rotation of Ions of metallic fluids.
This approach is quite accepted because-
(a) Moon has no metallic fluids so it has no magnetic field.
(b) Venus rotational speed is low so it produce lesser current, Thus it has weak magnetic field.
(c) Jupiter rotation speed is high so it produce more current, Thus it has strong magnetic field.