The most important effect of skin effect on the impedance of a single wire is the increase of the wire's resistance, and consequent losses. The effective resistance due to a current confined near the surface of a large conductor (much thicker than δ) can be solved as if the current flowed uniformly through a layer of thickness δ based on the DC resistivity of that material. The effective cross-sectional area is approximately equal to δ times the conductor's circumference. Thus a long cylindrical conductor such as a wire, having a diameter D large compared to δ, has a resistance approximately that of a hollow tube with wall thickness δ carrying direct current. The AC resistance of a wire of length ℓ and resistivity ρ ρ is: R ≈ ≈ ℓ ℓ ρ ρ π π ( D − − δ δ ) δ δ ≈ ≈ ℓ ℓ ρ ρ π π D δ δ