:<math> \tan \delta_m = \mu^{\prime\prime}_r / \mu^{\prime}_r </math>
Three special media are frequently encountered in electromagnetic problems :Â Â {| class="wikitable"|-! scope="col"| Medium ! scope="col"| \epsilon_r! scope="col"| \mu_r! scope="col"| \sigma ! scope="col"| \sigma_m |-! scope="row"| Free Space| 1.0| 1.0| 0.0 | 0.0 |-! scope="row"| Perfect Electric Conductor (PEC)| 1.0| 1.0| ∞ | 0.0 |-! scope="row"| Perfect Magnetic Conductor (PMC)| 1.0| 1.0| 0.0 | ∞ |}Â Â
* '''Vacuum''' or '''Free Space''': ε<sub>r</sub> = μ<sub>r</sub> = 1 and σ = σ<sub>m</sub> = 0