Skin effect

Skin effect is the tendency of high-frequency current to crowd into the thin outer 'skin' of a conductor, abandoning the interior. At DC, current spreads evenly across a wire's whole cross-section; but as frequency rises, the wire's own magnetic field pushes the current outward, until at gigahertz frequencies it flows in a layer thinner than a sheet of paper. The depth at which the current has fallen to about 37% of its surface value is called the skin depth — a few micrometres in copper at 1 GHz.

The consequence is that a conductor's effective resistance rises with frequency, because the current is squeezed into a smaller and smaller area. This is why high-frequency cables are often silver-plated (only the surface matters), why audio cable claims about 'pure copper cores' are mostly marketing, and why power engineers bundle stranded Litz wire or hollow conductors. Skin effect is a major source of loss in transmission lines and is a key reason a long cable rounds off sharp digital edges.