1. Study of magnetic field of earth is called Geomagnetism or terrestrial magnetism.

2. William Gilbert (in about 1600) was the first to demonstrate that the entire earth behaves as an enormous magnet.

3. The magnitude & direction of Earth’s magnetic field can be obtained approximately by assuming that the earth has a magnetic dipole of dipole moment about 8 x 1022 J/T located at its centre tilted 11.5O from the spin axis of the earth as shown in the diagram.

4. The average strength of the earth’s magnetic field is about half a Gauss. Also Bequator = 30 μT, Bpole = 60 μT. Range of magnetic field is about 5 R from the radius of earth.

5. Earth’s mag. field changes both in magnitude & direction with the time. It is fairly constant over a span of few years but noticeable changes occur in say 10 yrs.

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