= Gas-efflux test of charge neutrality
A sensitive <charge neutrality> test lets nominally neutral gas escape from an electrically isolated vessel connected to an <electrometer>. A net charge per escaping molecule would leave a measurable opposite charge on the vessel. Removing stray ions and charged particles is essential. Hydrogen and helium measurements constrain charge imbalance despite different particle binding and internal motions, providing evidence for velocity-independent additive charge rather than a charge proportional to relativistic particle energy. The 1960 experiment used this efflux principle. https://doi.org/10.1103/PhysRevLett.5.562[Search for a Small Charge Carried by Molecules (1960), gas-efflux apparatus and hydrogen/helium measurements]{ref}
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