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".
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The volt is the SI unit of electric potential and electric potential difference: , one joule per coulomb. A positive electric charge of moved through a potential increase of gains of potential energy; the electric field does of work. [ref].