The chainreactor is a working OU device but gravity is a quite disappointing force. The track displayed above uses centrifugal force to pull the chain of metal balls out of the center of the magnetic field.
From the center (true the axle) an extra ball is inserted, it is attracted by the magnets and smashes into the chain. The outwards tube the last ball is fired into is curved so that some momentum is transferred onto the wheel. The ball is then spiraled inwards making it accelerate beyond the speed of the wheel. It now curves back into the axle (not clearly shown in image) and the process continues until the setup explodes.