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It is common experience that a bar magnet or a magnetic needle suspended freely in air always aligns itself along geographic N-S direction.
It is common experience that a bar magnet or a magnetic needle suspended freely in air always aligns itself along geographic N-S direction.
If it has a freedom to rotate about horizontal axis, it inclines with some angle with the horizontal in the vertical N-S plane.
If it has a freedom to rotate about horizontal axis, it inclines with some angle with the horizontal in the vertical N-S plane.
This fact clearly indicates that there is some magnetic field present everywhere on the Earth .
This fact clearly indicates that there is some magnetic field present everywhere on the Earth .
This is called Terrestrial Magnetism.
This is called Terrestrial Magnetism.
It is extremely useful during navigation.
It is extremely useful during navigation.
Magnetic parameters of the Earth are described below. Unless and otherwise stated, the directions mentioned (South, North, etc.) are always, Geographic.
Magnetic parameters of the Earth are described below. Unless and otherwise stated, the directions mentioned (South, North, etc.) are always, Geographic.
Magnetic Axis :-
Magnetic Axis :-
The Earth is considered to be a huge magnetic dipole. Fictitious magnetic north pole (N) of the Earth is located below Antarctica while the south pole (S) is below Norway.
The Earth is considered to be a huge magnetic dipole. Fictitious magnetic north pole (N) of the Earth is located below Antarctica while the south pole (S) is below Norway.
The straight line NS joining these two poles is called the magnetic axis, MM'.
The straight line NS joining these two poles is called the magnetic axis, MM'.
Magnetic equator :-
Magnetic equator :-
A great circle in the plane perpendicular to magnetic axis is magnetic equatorial circle, AA'.
A great circle in the plane perpendicular to magnetic axis is magnetic equatorial circle, AA'.
It happens to pass through India near Thiruvananthapuram.
It happens to pass through India near Thiruvananthapuram.
Geographic Meridian:-
Geographic Meridian:-
A plane perpendicular to the surface of the Earth (vertical plane) perpendicular to geographic axis is geographic meridian.
A plane perpendicular to the surface of the Earth (vertical plane) perpendicular to geographic axis is geographic meridian.
Magnetic Meridian:-
Magnetic Meridian:-
A plane perpendicular to surface of the Earth (Vertical plane) and passing through the magnetic axis is magnetic meridian.
A plane perpendicular to surface of the Earth (Vertical plane) and passing through the magnetic axis is magnetic meridian.
Direction of resultant magnetic field of the Earth is always along or parallel to magnetic meridian.
Direction of resultant magnetic field of the Earth is always along or parallel to magnetic meridian.
Magnetic declination:-
Magnetic declination:-
Angle between the geographic and the magnetic meridian at a place is called ‘magnetic declination’.
Angle between the geographic and the magnetic meridian at a place is called ‘magnetic declination’.
The declination is small in India.
The declination is small in India.
It is 0 58 west at Mumbai and 0041 east at Delhi.
It is 0 58 west at Mumbai and 0041 east at Delhi.
Thus, at both these places, magnetic needle shows true North
Thus, at both these places, magnetic needle shows true North
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