Domain:

Domain: Atoms of ferromagnetic substance have a permanent dipole moment i.e. they behave like a very small magnet. The atoms form a large no. of effective regions called domain in which 10¹⁸ to 10²¹ atoms have their dipole moment aligned in the same direction. The magnetism in ferromagnetic substance, when placed in a magnetic field, is developed due to these domain by (i) the displacements of boundaries of the domains (ii) the rotation of the domains.

Curie temperature: As temperature increases, the magnetic property of ferromagnetic substance decreases and above a certain temperature the substance changes into paramagnetic substance. This temperature is called Curie temperature.

·         Permanent magnets are made of steel, cobalt steel, ticonal, alcomax and alnico.

·         Electromagnets, cores of transformers, telephone diaphragms, armatures of dynamos and motors are made of soft iron, mu-metal and stalloy.

Terrestrial Magnetism: Our earth behaves as a powerful magnet whose south pole is near the geographical North Pole and whose north pole is near the geographical South Pole. The magnetic field of earth of a place is described in the terms of following three elements.

(i)                  Declination: The acute angle between magnetic meridian and geographical meridian at a place is called the angle of declination at that place.

(ii)                Dip or inclination: Dip is the angle which the resultant earth’s magnetic field at a place makes with the horizontal. At poles and equator, dip is 90˚ and 0˚ respectively.

(iii)                Horizontal component of earth’s magnetic field: At a place it is defined as the component o earth’s magnetic field along the horizontal in the magnetic meridian.

Its valve is different at different places. (Approximately 0.4 gauss or 0.4 x 10⁻⁴ tesla).