This formula for the increase in AC resistance is accurate only for an isolated wire. For nearby wires, e.g. in a cable or a coil, the AC resistance is also affected by proximity effect, which can cause an additional increase in the AC resistance. The internal impedance per unit length of a segment of round wire is given by: Z int = k ρ ρ 2 π π R J 0 ( k R ) J 1 ( k R ) .