d
Ienc =
H ⋅ dl
Ampere's Circuital Law is an important law of magnetostatics that enables one to easily calculate the magnetic field generated by a current flowing through a wire.
Normally, to apply this law, we imagine a circle around a wire(s), having a radius equal to that at which we wish to determine the magnetic field. Here Ienc is the net current passing through the interior of that circle, dl is part of the path the circle makes and H is the magnetic field intensity upon that circle. We then take the dot product of H · dl and integrate around the closed loop our circle forms, thus giving us an equation from which we can easily calculate the value of H.
This is also the basis for one of Maxwell's Equations.