MAGNETIZATION AND MAGNETIC INTENSITY

The material whose magnetic properties are t be studied is generally taken in the form of a toroidal ring, known as Rowland ring. Magnetic Field is approximately uniform in the region inside a toroidal winding. The following figure shows such a ring with its winding in which
current I(f) is flowing.  Winding on the ring is called magnetizing winding and the current, I(f), is called magnetizing current.The magnetic field  inside the toroidal region due to current in the  absence of any material in the ring is Bf = u0 n I(f) = m0 i(f) ( for a toroid with a large radius and small cross-sectional area ),where, n = number of turns per unit length on the circumference of the toroid
i(f) = current per unit lengthA small separate winding around the main winding with a sensitive galvanometer shown in the figure is used to magnetic induction ( magnetic field or magnetic flux density ) and its changes. When the toroidal winding is on the ring of some material, the magnetic field  generated by the winding current If induces current loops inside the material. Such current induced in the
material due to the external magnetic field is called bound current ( Ib ). Let ib denote such bound current per unit length. Then the magnetic field inside the toroidal region is due to combined effect of i(f) and i (b)
B = Bf + Bb = m0 ( i(f) + i(b)) = u0 n( If + I b ) ... ... ... ( 1 )
where, Bb is the magnetic field produced in the material due to the bound current. The corresponding induced magnetic dipole moment, M1, due to bound current i b per unit length of the circumference of the material is given by
M1 = i b A, where A is the area of cross-section of the ring. The volume corresponding to the unit length of the circumference is V = 1 * A units. Therefore the induced dipole moment per unit volume