Skin effect is the tendency of an
alternating electric current (AC) to become distributed within a
conductor such that the
current density is largest near the surface of the conductor, and decreases with greater depths in the conductor. The electric current flows mainly at the "skin" of the conductor, between the outer surface and a level called the
skin depth. The skin effect causes the effective
resistance of the conductor to increase at higher
frequencies where the skin depth is smaller, thus reducing the effective cross-section of the conductor. The skin effect is due to opposing
eddy currents induced by the changing
magnetic field resulting from the alternating current. At 60
Hz in copper, the skin depth is about 8.5 mm. At high frequencies the skin depth becomes much smaller. Increased AC resistance due to the skin effect can be mitigated by using specially woven
litz wire. Because the interior of a large conductor carries so little of the current, tubular conductors such as pipe can be used to save weight and cost.