After turning on the eddy current brake, allow the disk to spin for a while to build up to a fair velocity. THen bring up a magnet to the disk. The disk's spinning will begin to decelerate, and will eventually come to a halt because of the induction of eddy currents by the magnetic field. If the magnet is not strong enough, the eddy currents that appear in opposition to the motion of the disk will not have an effect on its motion. The magnetron magnets with 4 cm pole pieces are quite satisfactory, and the "monster magnet" in Cabinet D gives spectacular results if you are astute enough (or strong enough) to get the keeper off.