Editing Permeability (electromagnetism) (section)

== Relative permeability and magnetic susceptibility <span class="anchor" id="relative_permeability"></span> <span class="anchor" id="Relative"></span> ==
Relative permeability, denoted by the symbol <math>\mu_\mathrm{r}</math>, is the ratio of the permeability of a specific medium to the permeability of free space ''μ''<sub>0</sub>:
: <math>\mu_\mathrm{r} = \frac \mu {\mu_0},</math>
where <math>\mu_0 \approx </math> 4{{pi}}&nbsp;×&nbsp;10<sup>−7</sup>&nbsp;H/m is the [[Vacuum permeability|magnetic permeability of free space]].<ref>[https://www.bipm.org/documents/20126/41483022/SI-Brochure-9-EN.pdf The International System of Units], page 132, The ampere. [[The International Bureau of Weights and Measures|BIPM]].</ref> In terms of relative permeability, the [[magnetic susceptibility]] is
: <math>\chi_m = \mu_r - 1.</math>

The number ''χ''<sub>m</sub> is a [[dimensionless quantity]], sometimes called ''volumetric'' or ''bulk'' susceptibility, to distinguish it from ''χ''<sub>p</sub> (''magnetic mass'' or ''specific'' susceptibility) and ''χ''<sub>M</sub> (''molar'' or ''molar mass'' susceptibility).