摘要：
At a microscopic scale, no surface appears perfectly smooth. All materials exhibit surface irregularities and bumps. One measure of surface roughness is the rms (root-mean-square) height h of the surface bumps.
At low frequencies, the depth of penetration of current (the skin depth) exceeds h. Low-frequency currents, therefore, submarine below the surface bumps, unaffected by surface roughness.
High-frequency currents, on the other hand, remain tightly bound to the surface at all times. At frequencies so high that the skin depth δ shrinks to less than the rms bump height h, the current follows the contours of the surface, over hill and down dale, as it flows along the conductor. At these high frequencies, the apparent resistance of the material increases to a value representative of the additional distance over which the current must flow to traverse the contours of the surface.
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