Geomagnetism or Terrestrial magnetism. There are three quantities required to specify the magnetic field of the Earth on its surface, which are often called as the elements of the Earth’s magnetic field. They are (a) magnetic declination ( D ) (b) magnetic dip or inclination ( I ) (c) the horizontal component of the Earth’s magnetic field ( B H ) Figure . Declination angle Geographic meridian Magnetic meridian D = Angle of declination D Magnetic meridian Magnetic meridian Geographic meridian Geographic meridian Geographic Equator Magnetic Equator Angle of declination Earth magnetic south pole Magnetic field lines “True” North pole - the Earth rotates around this axis Earth Axis around which the Earth rotates once a day Earth magnetic north pole “True” South pole Bar magnet Day and night occur because Earth spins about an axis called geographic axis.
A vertical plane passing through the geographic axis is called geographic meridian and a great circle perpendicular to Earth’s geographic axis is called geographic equator. The straight line which connects magnetic poles of Earth is known as magnetic axis. William Gilbert in proposed that Earth itself behaves like a gigantic powerful bar magnet. But this theory is not successful because the temperature inside the Earth is very high and so it will not be possible for a magnet to retain its magnetism.
Gover suggested that the Earth’s magnetic field is due to hot rays coming out from the Sun. These rays will heat up the air near equatorial region. Once air becomes hotter, it rises above and will move towards northern and southern hemispheres and get electrified. This may be responsible to magnetize the ferromagnetic materials near the Earth’s surface.
Till date, so many theories have been proposed. But none of the theorium completely explains the cause for the Earth’s magnetism. 12th - 12th - - - - - Unit Magnetism and magnetic effects of electric current A vertical plane passing through magnetic axis is called magnetic meridian and a great circle perpendicular to Earth’s magnetic axis is called magnetic equator. When