The Skin Effect in Electrical Conductors

A thick copper wire carrying direct current uses its entire cross-section to conduct electricity, but the same wire carrying high-frequency alternating current behaves very differently: the current crowds increasingly toward the outer surface of the wire, leaving the core doing very little of the work, a phenomenon called the skin effect. The underlying cause is electromagnetic, not a property of the copper itself. Any alternating current generates a constantly changing magnetic field around and within the conductor, and that changing field, in turn, induces small opposing electrical currents, called eddy currents, within the conductor's own body. These induced currents are strongest at the center of the wire, where the changing magnetic field is most concentrated, and their effect is to oppose the flow of the original current specifically in that central region, effectively pushing the current outward toward the surface. The higher the frequency of the alternating current, the faster the magnetic field changes, the stronger the induced opposing currents become, and the more severely the current gets confined to a thin outer layer, or "skin," of the conductor. At sufficiently high frequencies, so little current flows through the center that a solid wire's core becomes almost electrically useless, wasting the material and increasing the effective resistance of the wire beyond what its full cross-sectional area alone would predict. Engineers address this in several ways: for high-frequency applications, cables are sometimes built as a bundle of many thin, individually insulated strands rather than a single solid conductor, since this arrangement gives the current far more total surface area to travel along relative to the same amount of copper, substantially reducing the resistance penalty the skin effect would otherwise impose.