Ampère-Maxwell Law

Theorem
Let $\mathbf B$ be a magnetic field due to a steady current $I$ flowing through a wire $s$.

Then:
 * $\oint \mathbf B \cdot \rd \mathbf l = \mu_0 I$

where:
 * the line integral is taken around a closed path
 * $\d \mathbf l$ is an infinitesimal vector associated with $s$
 * $\mu_0$ denotes the vacuum permeability.

That is, the line integral of $\mathbf B$ through the area enclosed by the closed path is equal to $\mu_0 I$.