The Earth as a Magnet

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About AD1600, William Gilbert, who was a court physician of Queen Elizabeth of England, performed certain experiments using spherical lodestones. When he placed small, pivoted magnetic needles at different positions on this sphere, he found that their behavior was quite similar to that of magnetized needles at corresponding places on the earth's surface. Later he concluded that the earth behaves like a huge magnet.

The cause of the earth's magnetism has not yet been understood clearly. It is certainly not due to the presence of a bar magnet deep within the earth. In the earth's outer core there is high concentration of ferromagnetic material in molten state. It is believed that as a result of the rotation of the earth, the molten material flows in the form of currents around the solid inner core. These currents are supposed to be responsible for the terrestrial magnetic field. This magnetic field appears to enter the earth at the north pole and leaves at the south pole. It should be noted that the magnetic poles do not coincide with the geographic poles. The magnetic poles move slightly from year to year. The branch which studies the terrestrial magnetism is known as Geomagnetism.

As the geographic and magnetic poles are not at the same place, the imaginary planes drawn on the surface of the earth pointing at these poles also do not coincide. These planes are respectively known as the Geographic meridian and the Magnetic meridian.

Geographic meridian is the vertical plane containing the geographic north and south poles of the earth and passing through the given place on the surface of the earth.

Magnetic meridian is a vertical plane containing the magnetic axis of a freely suspended magnet at rest under the earth's field.

The angle between these two meridians is known as the 'angle of declination or deviation'.

In other words, this is the angle between the geographic north and the magnetic north.

The angle of declination varies all over the world. Seamen need to know the declination to navigate by compass. Maps are therefore prepared periodically showing the declination at different parts of the world.

Angle of Dip

If a non-magnetic piece of metal like copper is suspended at its center of gravity, the copper plate remains horizontally in equilibrium. If on the other hand, a magnetic needle is suspended at its center of gravity so that it is free to take up any position in the magnetic north-south plane, it makes an angle.

The earth's magnetic field is not parallel to the earth's surface excepting at the magnetic equator. As a result, excepting at the magnetic equator, a freely suspended needle makes an angle with the horizontal.

The angle between the horizontal and the earth's magnetic field is known as angle of dip.

The angle of dip is different at different places on the surface of the earth. At the magnetic equator it is 0^o and increases and at the magnetic pole it becomes 90^o.

You would be surprised to know that, though scientists have not been able to discover the mechanism, some of the migratory birds are capable of detecting changes in the angle of dip as well as angle of declination. This helps them in navigating long distances while they migrate.