State Ampere's circuital law.

Ampere's circuital law states that the line integral of magnetic field $\vec{B}$ around any closed path/loop in vacuum is equal to $\mu_{0}$ times the total current $I_{\text{enclosed}}$ threading through the area bounded by the path: $\oint \vec{B} \cdot d\vec{l} = \mu_{0} I_{\text{enclosed}}$