Answer
The electromagnet is ac.
Work Step by Step
The ac magnetic field continually changes the magnetic field in the copper ring. This induces a current in the copper ring, turning it into an electromagnet. The ring's field continually opposes the original field produced by the iron-core electromagnet, and the ring is continually repelled by it. The ring hovers at a position at which the repulsive force just balances the copper ring's weight.
