Sir William Gilbert was the first to suggest in the year 1600 that earth itself is a huge magnet. His statement was based on the following evidence:
(i) A magnet suspended from a thread and free to rotate in a horizontal plane comes to rest along ht north-south direction. On disturbing the magnet returns quickly to its north-south direction aging. This is as if a huge bar magnet is placed approximately along the axis of rotation of earth and deep in its interior. The north pole of this fictitious magnet must be towards geographic south so as to attract worth plot of the suspended magnet and vice-versa. We must add here that fictitious explain the earth’s magnetic field and its peculiarities. In reality there is no such her magnet in the interior of the earth.
(ii) When a soft iron piece is buried under the surface of earth in the north south direction, it is found to acquire the properties of a magnet after some time.
(iii) When we dare field lines of a magnet, we come across neutral points. At these points magnetic field due to the magnet is neutralized or canceled exactly by the magnetic field of earth. If earth had no magnetism of its own we would never observe neutral points.
The branch of physics which deals with the study of magnetism of earth is called terrestrial magnetism or geomagnetism.
It has been established that earth’s magnetic field is fairly uniform. The strength of this field at the surface of earth is approximately 10-5 tesla. The field is not confiding only to earth’s surface. It extends upto a height nearly 5 times the radius of earth = (x 6400 km = 32000 km.)
Cause of earth’s magnetism
The exact cause of earth’s magnetism is not yet known. However some important postulates in this respect are as follows:
(a) According to proof bracket earth’s magnetism may be due to rotation of earth about its axis this because every substance is made up of charged particles (protons and electrons) therefore a substance rotating about an axis is equivalent to circulating currents which are responsible for the magnetization.
(b) In the outer layers of earth’s atmosphere gases are in the ionized state primarily on account of consume rays. As earth rotates strong electric currents are set up due to movement of (charged ions. These currents might be magnetizing the earth.
(c) The earth’s core is extremely hot and molten. Circulating ions of iron and nickel in the highly conducting liquid region of the earth’s core might be forming current loops and producing earth’s magnetic fied. This hypothesis is supported by the following facts.
(i) Moon has no molten core and hence no magnetic field.
(ii) Venus has a slower rate of rotation and hence a weaker magnetic field.
(iii) Amongst the planets Jupiter has the fastest rotation rate. Therefore it has a fairly strong magnetic field.
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