A metal ring is placed on the core of a coil carrying alternating current. The ring

Explanation

The changing flux induces a current in the closed ring, and by Lenz's law the ring's field opposes the coil's, so the two repel and the ring jumps. Cutting a slit in the ring stops the induced current from circulating and the effect disappears, which is the standard control. The ring also warms up because of the current flowing in its own resistance.