Skin Effect

Quick Answer

Alternating current does not use a conductor evenly. The changing magnetic field inside the metal opposes current hardest along the axis and pushes it toward the surface, and the effect strengthens with frequency. Less of the cross-section carries current, so the effective resistance rises above the DC value.

The mechanism is induction working against itself. Current in a wire sets up a magnetic field, an alternating current sets up a changing one, and a changing field inside the metal induces eddy currents that oppose the flow most strongly at the centre, where the enclosed flux is greatest. What is left is a current concentrated in a surface layer, and that layer thins as the square root of frequency.

The consequences are practical rather than exotic. In copper at mains frequency the layer runs to about nine millimetres, so ordinary wiring never notices. At a megahertz it is about sixty-five micrometres, and a solid conductor is then mostly dead weight carrying nothing. That is why litz wire exists, its many thin strands separately insulated and woven so each takes its turn at the surface; why RF conductors are silver plated, the surface carrying nearly all the current; and why heavy feeders are sometimes tubing rather than rod.

It also means a resistance read off a meter is the wrong figure for AC work. Resistivity and geometry give the DC value, and the AC value climbs above it as frequency rises. A neighbouring conductor makes matters worse, crowding the current further to one side. That is a separate mechanism called proximity effect, and it is why a tightly wound coil or a bundled cable loses more than a lone wire of the same size.