For a Ferromagnetic material, Magnetization curve is the graphical representation of flux density (B) and magnetizing force (H) or magnetic field strength. This curve is also known as B-H curve or saturation curve.
For representing a magnetization curve, we should consider a toroidal solenoid of ferromagnetic material. That means materials should be shaped in the form of a ring.
The magnetization curves can be determined by (i) By means of a ballistic galvanometer and (ii) By means of a fluxmeter.
The graph in the figure represents the relationship between the flux density(B) and the magnetic field strength (H) obtained for different qualities of iron like mild steel, wrought iron, sheet steel stalloy (iron and silicon alloy).

AMPERE’S WORK LAW OR AMPERE’S CIRCUITAL LAW: This law states that the m.m.f produced in a closed loop due to current passes through that loop is equal to the algebraic sum of all currents of that closed loop.
H⃗ .ds⃗ = I
Where H ⃗ representing magnetic field strength in dot product with vector d s ⃗ of the close loop s.
By this circuital law we can calculate m.m.f around (i) long straight current carrying conductor (ii) a long solenoid.
(i)In case of long straight current carrying conductor:
If there is N number of conductor carrying I ampere current upwards, it will creates magnetic lines of force around it.
Now at any point, suppose r meter from the centre of the conductor where field strength is H newton,
Now, m.m.f is force X distance= I.
=H X 2πr= I,
H=I/2πr A/m,
For N number of conductor,
H= NI/2πr A/m.
B=µ0NI/2πr wb/m2 or tesla in air.
= µ0µrNI/2πr wb/m2 or tesla in a medium.

(ii) In case of long solenoid:
In case of long solenoid if L is the length of the solenoid and magnetic field strength all over the solenoid be H,
Then m.m.f = H X L = ampere.
Again the ampere turn linked with the path amp- turn= NI.
So, H X L = N X I,
Or, H =NI/L Amp/ meter or Oersted.
So, B= µ0NI/L wb/m2 or tesla in a medium.
And, B= µ0µrNI/L wb/m2 or tesla in a medium.

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