Eddy-Current Braking

Drop two metal plates through a magnet and one falls in slow motion while the other drops normally; the only difference is a few slots cut in the second. As a solid plate crosses the field, the flux through it changes, and by Faraday's law that drives swirling loops of current inside the metal, the eddy currents. By Lenz's law their own magnetic force opposes the motion that made them, so the plate is braked: the drag is $F = (A^2/R)\,B'(y)^2\, v$, proportional to the speed, to the square of the field gradient, and inversely to the plate's resistance. Cut slots in the plate and you break the loops, raising the resistance and almost switching the brake off, so the slotted plate sails through. This is exactly how the brakes on trains and roller coasters and the damping in sensitive balances work, and why the heat ends up in the metal rather than in worn pads.

Figure 1. Eddy-current braking. Top: a solid and a slotted conducting plate released together fall through a magnetic-field band; the eddy current loops appear as the flux changes, braking the solid plate. Bottom: the two speeds versus time, the solid plate stalling in the field while the slotted one keeps accelerating. Method: closed-form eddy drag F = (A^2/R) B'(y)^2 v. Source: Halliday, Resnick and Walker, Fundamentals of Physics, Ch. 30.

field B1.4

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WHAT TO TRY

Watch the speed plot as the plates enter the band: the solid (blue) curve flattens or dips as it is braked, while the slotted (green) curve keeps rising almost like free fall.
Turn the field up: the solid plate is braked harder (the drag scales as $B^2$) and falls further behind, while the slotted plate barely notices.
Turn the field to zero: both plates free-fall together, their curves on top of each other, the proof that the brake is purely magnetic.
Notice the eddy loops glow brightest at the edges of the field band, where the gradient $B'(y)$ is steepest, and fade at the centre where the field is flat.