# Read/calculate permebility from BH curve

I need to find the permeability from a non-linear BH curve. B is given in mT. and H in A/m.

The relationship between B and H is $\mathbf{B} = \mu\mathbf{H}$. So if you have a B-H curve for a given material, you can find your permeability, $\mu$, by finding B divided by H. Keep in mind that permeability is a function of H, it is not constant for all values of H.

If you use standard SI units (Tesla for B and A/m for H), then your permeability units should be H/m.

Permeability is the slope of a line crossing through the point and zero. Incremental permeability is the slope of the B-H curve at a given point.

Since the curve is non-linear, that should tell you something about the permeability of that material (it's not constant).

Initial permeability is the slope near zero.

Since B and H are vector fields, in general, $\mu$ is a matrix (2nd rank tensor), but for anisotropic materials it is a scalar.

If you calculate the ratio in T/(A/m) then you'll get $\mu$ in H/m. It may be preferably to express it as the relative permeability $\mu_r$ = $\frac{\mu}{\mu_0}$, where $\mu_0$ is defined as $4\pi\times10^{-7}$ H/m.

• Is it possible to find Effective permeability or absolute permeability from initial permeability. Whether the Effective or absolute value is greater or lesser than the initial permeability? – Kasi Jul 1 '14 at 6:53
• Initial permeability at low frequency is the highest - how it drops off from there is material-dependent. – Spehro Pefhany Jul 1 '14 at 11:39