The circulation around any path that encloses the wire is equal to \(\mu_{0}i\). This is an example of Ampère’s law, which was discovered by the French physicist André-Marie Ampère (pronounced “ahm-pair”) in 1826:
An Amperian loop is simply a closed path in space; the circle in Figure 19-20 is an example. The subscript “through” reminds us that the right-hand side of Equation 19-13 should only include current that passes through the interior of the Amperian loop.