A constant current that flows through a conductor results in a constant current density in the conductor. The current distributes evenly over the conductor cross section. Alternating currents however, do concentrate on the surface of a conductor which leads to an increased effective resistance.
In this document the standard relations between frequency, resistance and skin depth are shown. To check this relation, the increasing resistance is measured from 1 kHz to 1 MHz using the Bode 100.