For any ferromagnetic material there is a definite value of flux density (B), corresponding to a specified value of magnetising force (H). These values can be ascertained from graphs of B against H for each material. A BH curve can only be obtained using a piece of material that has never been magnetised before. Once the material has been magnetised and the curve obtained, the production of another BH curve, from the same piece of material, is not possible.
The BH curve is the line O to Q on the hysteresis curve shown below.
The gradient of the BH curve gives the permeability of the material. In practice it is found that the magnetic property of different specimens of the same material vary considerably. The fact that permeability varies for a given material may also be seen from the shape of the curve, if the permeability was a constant, the graph of B against H would be a straight line.