A terrestrial population of room-temperature millicharged particles can arise if they make up a dark matter subcomponent or if they are light enough to be produced in cosmic ray air showers. In this talk, I'll show that a simple electrified shell (e.g., a Van de Graaff generator) acts as an efficient accumulator for such particles, parametrically enhancing their local density by many orders of magnitude. Cavendish tests of Coulomb's Law, performed since the late 18th century, function as both accumulators and detectors of this overdensity. Reinterpretations of past experiments thus provide some of the strongest bounds on a terrestrial millicharge population. I'll also propose surrounding such a Cavendish test with an additional charged shell, which can enable sensitivity to the irreducible density generated by cosmic rays. Using decades-old technology, this dedicated setup can outperform future accelerator searches for sub-GeV millicharged particles.