Skin effect
Skin effect is the tendency of an alternating electric current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....

 (AC) to distribute itself within a conductor with the current density being largest near the surface of the conductor, decreasing at greater depths. In other words, the electric current flows mainly at the "skin" of the conductor, at an average depth called the skin depth. The skin effect causes the effective resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...

 of the conductor to increase at higher frequencies where the skin depth is smaller, thus reducing the effective cross-section of the conductor.
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