An electrometer measures small electric potential differences or electric charges while drawing very little electric current. The conducting-pail experiment in electric charge uses a voltage response to infer electric charge with a fixed effective capacitance.
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This can be used to detect the ionization of air by radiation , see e.g. youtu.be/CZ7DoLLwW04?t=76 from Video "Ions produced by radiation carry a current by Institute of Physics".