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Time variation of the magnetic field \(\boldsymbol{B}\) or the time variation of the surface \(A\) penetrated by magnetic field \(\boldsymbol{B}\) generates an electric field \(\boldsymbol{E}\) along the edge of the surface \(A\), which can lead to an electric current. This experimental finding is called the law of induction and is mathematically summarized in the fourth Maxwell equation (if \(A\) is kept constant):`\[ \nabla \times \boldsymbol{E} ~=~ - \frac{\partial \boldsymbol{B}}{\partial t} \]`