An eddy current brake slows a moving conductor without touching it. A magnet (permanent or electromagnet) sits near a moving metal disc, drum, or rail. As the conductor sweeps through the magnetic field, Faraday's law induces circulating currents — eddies — inside the metal. Those currents create their own magnetic field that opposes the motion (Lenz's law). The result is a drag force that dissipates kinetic energy as heat in the conductor itself. No pads, no wear, no dust.

Where you'll find them:

• Roller coasters — fin brakes on Intamin and B&M rides use copper fins passing between neodymium magnets. Reliable, no maintenance, and they can't fail to a released state.
• Truck retarders — Telma-style driveshaft brakes on heavy trucks and buses save the friction brakes on long downhill grades.
• High-speed rail — ICE 3 and Shinkansen use eddy current rail brakes that induce currents directly in the running rail.
• Gym equipment — the resistance knob on a spin bike moves magnets closer to or farther from the flywheel.
• Precision balances and analog meters — a small aluminum vane in a magnetic field damps needle oscillations.

The key behavior: braking force is proportional to velocity. Fast → strong braking. Slow → weak braking. Stopped → zero force. This is a fundamental limitation: an eddy current brake cannot hold a vehicle stationary. It's a retarder, not a parking brake. Every eddy-brake system needs a friction backup for the last few mph and for holding.

Rule of thumb — power dissipation: All kinetic energy removed becomes heat in the conductor. For a 40,000 kg truck slowing from 25 m/s to 15 m/s:

ΔKE = ½ × 40,000 × (25² − 15²) = ½ × 40,000 × 400 = 8 MJ

If that happens over 20 seconds, average dissipation is 400 kW — all going into the brake rotor and surrounding air. This is why heavy-duty eddy brakes have finned rotors, forced-air cooling, or liquid cooling. Overheat the conductor and its resistivity climbs, which reduces braking force right when you need it most.

Design levers: Force scales with B² (magnetic field squared), conductor thickness (up to skin-depth limits), and velocity. It scales inversely with conductor resistivity — which is why copper and aluminum dominate over steel discs. Switching from permanent magnets to electromagnets lets you modulate braking force electrically, at the cost of complexity and coil power.