outer electric potential, $$\psi$$

https://doi.org/10.1351/goldbook.O04349
electric potential outside the phase concerned. For a conducting sphere of excess charge Q and radius r in vacuum ψ = Q 4 π ɛ 0 r.
Source:
Green Book, 2nd ed., p. 59 (https://dev.goldbook.iupac.org/files/pdf/green_book_2ed.pdf)