A solid copper pendulum is set into motion between the poles of an electromagnet. The magnets induce eddy currents in the copper which oppose the motion of the pendulum. The pendulum quickly slows to a stop.
When a copper pendulum with strips cut into it is swung between the same magnets, it is not slowed nearly as much as the solid pendulum. This is because the cuts in the copper prevent large eddy currents from forming.
Source: MIT TechTV
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