Duality

Maxwell's equations (1.1-1.4) are invariant under the substitutions:

$$\left\{ \begin{array}{cc} \underline{\mathcal{E}} \rightarrow \un... ..._e \rightarrow \rho_m\space , & \rho_m \rightarrow -\rho_e \end{array} \right .$$ (1.58)

The duality relation (1.58) may be restated by saying that Maxwell's equations are unchanged when each electric quantity is replaced by the corresponding magnetic quantity, and each magnetic quantity by the opposite of the corresponding electric quantity.