When a massive body made of conductive material moves through a magnetic field, eddy currents are induced. According to Lenz's law, the body is then subjected to a force which is opposed to the cause of the eddy currents, i.e. the motion of the pendulum. The braking action increases with the strength of the magnetic field. If slits are cut into the body, this reduces the generation of the eddy currents.
Demonstrative experiment due to large instruments
Easy and sturdy setup