Invisible Power 

Demonstration effect: press the power switch, you can see on the core set of metal ring was held fast.
Scientific principle: after the ventilation with alternating current coil, and produces an alternating magnetic field, on the core set of small metal ring produces eddy currents in alternating magnetic field, with eddy current metal ring is to produce an eddy current magnetic field.Eddy current magnetic field direction is the opposite of coil magnetic field direction, mutually exclusive, so a small metal ring was invisible force holds up.