Fundamentals of Geomagnetism
The Magnetic field of the earth is also called Geomagnetism. The strength of the Earth’s magnetic field changes from place to place and its value is in order of 10-5 T.
The scientific reason for the earth magnetic field is still not satisfactory. Earlier is was the large bar magnet is present along the axis of rotation of earth deep inside the earth. But it is scientifically correct.
Also by theory of Dynamo effect, it was though that magnetic field is arised by the electrical current generated by convective motion of metallic fluids consisting of iron and nickel in molten form.
The magnetic field of lines shows that the magnetic dipole located at the centre of the earth. But the axis of the dipole does not coincide with the axis of rotation of earth as it is tilted to 11.3°.
The place of the north magnetic pole is at a latitude of 79.74° N and a longitude of 71.8° W, a location in northern Canada. The magnetic south pole is at 79.74° S, 108.22° E in Antarctica.
The pole near the geographic north of the earth is the North Magnetic Pole and similarly the pole near the geographic south pole is called South Magnetic Pole.
There is some confusion in the nomenclature of the poles this is because if one looks at the magnetic field lines of the earth, the field lines go into the earth at the north magnetic pole (Nm) and come out from the south magnetic pole (Sm).
This convention comes as the magnetic north was the direction to which the north pole of a magnetic needle-pointed and the north pole of a magnet was so named as it was the north-seeking pole and practically, the north magnetic pole behaves like the south pole of a bar magnet inside the earth and vice versa.
Magnetic declination in surveying
When a point is taken into account on the earth’s surface. At such a point, the direction of the line of the longitude circle judges the geographical north-south direction.
The line of longitude towards the pole being the direction of true north. The vertical plane containing the line of the longitude circle and also the axis of rotation of the earth is termed the Geographic Meridian.
Similarly, the Magnetic meridian of an area is the vertical place which passes through the imaginary line connecting the magnetic north and the south poles and this plane would cross the surface of the earth in a longitude sort of a circle.
A magnetic needle, which is free to swing horizontally, would then lie in the magnetic meridian and also the north pole of the needle would point towards the magnetic north pole.
Since the line joining the magnetic poles is titled with relevance to the geographic axis of the earth, the magnetic meridian at a point makes angle with the geographic meridian.
This, then, is the angle between the true geographic north and the north shown by a compass needle. This angle is called the magnetic declination or simply declination.
The declination is larger at higher latitudes and smaller close to the equator. The declination in India is little, it is at Delhi with 0°41′ E and 0°58′ W at Mumbai. Thus, in each of these places, a magnetic needle shows the true north quite accurately.
There is an added amount of interest. If a magnetic needle is perfectly balanced about a horizontal axis so that it can swing in a plane of the magnetic meridian, the needle would make an angle with the horizontal and is known as the angle of dip or inclination.