Skin Effect

In ac circuits, the current density is greater near the outer surface of the conductor. The current tends to crowd toward the outer surface.
This is called skin effect. A longitudinal element of the conductor near the center of the axis is surrounded by more lines of magnetic force than near the rim. This results in an increase in inductance toward the center. The decreased area of conductance causes an apparent increase in resistance. At 60 hertz, the phenomenon is negligible in copper sizes of #2 AWG and smaller and aluminum sizes #1/0 AWG and smaller. As conductor sizes increase, the effect becomes more significant.