$\overrightarrow{\underset{\_}{B}}={\mu }_0{\underset{\_}{\mu }}_{\mathrm{r}}\overrightarrow{\underset{\_}{H}}$ or $\overrightarrow{\underset{\_}{B}}={\mu }_0{\underset{\_}{\overrightarrow{\overrightarrow{\mu }}}}_{\text{r}}\overrightarrow{\underset{\_}{H}}$

in a medium where the magnetic flux density vector $\overrightarrow{\underset{\_}{B}}$ and the magnetic field vector $\overrightarrow{\underset{\_}{H}}$ are linearly related, μ₀ being the magnetic constant

NOTE – For an isotropic medium, μ_r is generally expressed as:

${\underset{\_}{\mu }}_{\text{r}}={\mu }_{\text{r}}^{\prime }-\mathrm{j}{\mu }_{\text{r}}^{″}$

where ${\mu }_{\text{r}}^{\prime }$ is the relative real permeability and ${\mu }_{\text{r}}^{″}$ represents the magnetic losses.