Skin Resistance of a Transmission-Line Conductor of Polygon Cross Section

If a conductor cross section is any straight-sided polygon that can be circumscribed on a circle, it is found to have the same skin resistance as a conductor whose cross section is this circle. For example, a square wire has the same resistance as a round wire of the same radius, though the square perimeter is 4/π times as great. This "polygon rule" is derived from the "incremental-inductance rule" of the skin effect, published in 1942. It applies equally to inner or outer conductors, though the current distribution is very different. It applies to some unusual shapes that are difficult to compute by any other method.