At a meeting of the French Academy in 1700, Bernoulli demonstrated that swirling mercury in an evacuated flask generates light. He emphasized that this 'barometer light' 'has not been explained since its discovery about 30 years ago' by Picard. Here we revisit this phenomenon and find that the repetitive emission of light from mercury moving over glass is accompanied by the collective picosecond transfer of large numbers of electrons. When brought into contact with mercury, the glass acquires a net charge. This charge separation provides a force which, in our experiment in a rotating flask, drags mercury against gravity in the direction of the motion of the glass. Eventually the edge of the mercury slips relative to the glass, accompanied by a picosecond electrical discharge and a flash of light. This repetitive build-up and discharge of static electricity thus gives rise to stick-slip motion. The statistics of the intervals between events and their respective magnitudes are history-dependent and are not yet understood.
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