Low frequency sinusoidal currents applied to the coil causing circulating currents to devlop and reduce the overall flux density, which would require an increase in current amplitude to compensate for the drop in flux density.

Small eddy currents in the core material that result in a concentration of flux density at the center of the core material.

High frequency sinusoidal currents applied to the coil causing a greater amplitude of current required to produce more flux density.

Even distribution of flux density across the core material due to negligible eddy currents in the core material.