Skin effect refers to a certain frequency of AC current through the same conductor, the current density is not uniformly distributed in all the cross section of the conductor, it will focus on the surface of the conductor, that is, the density of the surface of the conductor, the density of small, the image is called: "skin effect". Skin effect is usually measured by the penetration depth of the current. The smaller the penetration depth is, the more significant the skin effect is.