Elements of Earth’s Magnetism
The earth’s magnetic field at a place can be completely described by three parameters which are called elements of earth’s magnetic field. They are:
(i) Angle of declination
(ii) Angle of dip
(iii) Horizontal component of earth’s magnetic field.
(i) Angle of Declination: The angle between the geographical meridian and the magnetic meridian at a place is called angle of declination at that place. Geographic meridian is the vertical plane passing through the geographical north and south poles. While magnetic meridian is the vertical plane passing through the magnetic axis of a freely suspended small magnet.
Magnetic declination arises because the magnetic axis of the earth does not coincide with its geographic axis.
By knowing declination, we can determine the vertical plane in which the earth’s magnetic field lies. In India, the value of angle of declination is small. It is 0°41′ E for Delhi and 0°58′ W for Mumbai.
(ii) Angle of dip: The angle made by the earth’s total magnetic field \(\vec B\) with the horizontal direction in the magnetic meridian is called angle of dip at any place. The angle of dip is different at different places on the surface of the earth. The angle between the horizontal and the final direction of the dip needle gives the angle of dip at that given location.
At the magnetic equator, the dip needle rests horizontally so that the angle of dip is zero at the magnetic equator. The dip needle rests vertically at the magnetic poles so that the angle of dip is 90° at the magnetic poles. At all other places, the dip angle lies between 0° and 90°.
(iii) Horizontal component of earth’s
magnetic Held: It is the component of the earth’s total magnetic field \(\vec B\) in the horizontal direction in the magnetic meridian. If θ is the angle of dip at any place, then the horizontal component of earth’s field \(\vec B\) at that place is given by
and tanθ = \(\frac{B_{V}}{B_{H}}\)
or BV = BH tanθ …………. (3)
