$
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$
麦克斯韦方程组（介质）
\begin{equation}\ali{
&\div\bvec D = \rho_f\\
&\curl\bvec E = -\pdvTwo{\bvec B}{t}\\
&\div\bvec B = 0\\
&\curl\bvec H = J_f + \pdvTwo{\bvec D}{t}
}\end{equation}
其中电位移矢量 $\bvec D = \epsilon_0 \bvec E + \bvec P$， 磁场强度 $\bvec H = \frac{\bvec B}{\mu_0} - \bvec M$．
在各向同性线性介质中，有 $\bvec P = \chi_E \epsilon_0 \bvec E$， $\bvec M = \chi_B \bvec H$． 代入上式得 $\bvec D = (1 + \chi_E)\epsilon_0\bvec E$ 和 $\bvec H = \frac{\bvec B}{(1 + \chi_B)\mu_0}$．
定义相对介电常数为 $\epsilon_r = 1 + \chi_E$， 相对磁导率为 $\mu_r = 1 + \chi_B$， 则 $\bvec D = \epsilon_r\epsilon_0\bvec E$， $\bvec H = \frac{\bvec B}{\mu_r\mu_0}$，
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—— 小时（项目创始人）