Electromagnetic Damping: A Closer Look at a Contactless Braking System

A magnet oscillating on a spring above a coil is a standard demonstration of contactless electromagnetic braking, normally confirmed by showing that the electromagnetic part of the damping scales as 1/R, the inverse of the coil circuit's resistance. In our version of the experiment the damping did not change, within the 4% run-to-run scatter, between 15 Ω and 2.2 MΩ. Using a US$43 data-acquisition system built around a Raspberry Pi Pico, we trace this null result to the measuring circuit: the coil was closed only through the amplifier's 1 MΩ bias resistor, so about 16 nA flowed and electromagnetic braking was a few millionths of the total damping. A second hidden error was the clock. Mains hum appeared at 63 Hz instead of 50 Hz because the firmware's sampling loop ran slower than assumed, by a factor of 1.261 ± 0.013; the oscillation frequencies in phone videos of the runs give 1.2618 ± 0.0003 independently. With both errors understood, the oscillator's damping, γ = (11.3 ± 0.3) × 10⁻³ s⁻¹ (mean ± SD of seven videos) for spring 1, does not come from the coil and decays faster at large amplitude. We describe a corrected circuit and five checks that would have caught both errors. Data, firmware, hardware files and analysis code: https://doi.org/10.5281/zenodo.23448904