Ampere's Circuital Law states that the line integral of magnetic field $\vec{B}$ around any closed loop (called an Amperian loop) is equal to ${\mu }_0$ times the total steady current $I$ passing through the surface enclosed by the loop. $\oint \vec{B}\cdot d\vec{l}={\mu }_0{I}_{enclosed}$ This law is particularly useful for calculating magnetic fields of symmetric distributions like long wires, solenoids, and toroids.