Vacuum potential and electric field pattern in the ρ−z plane near the centre of a thin circular disk of radius b as sketched in Fig. 7a. The disk potential outside ρ = a is V_in = −10 V while the potential V_out in the centre ρ < a varies as stated above each panel. In all cases, a = 0.02 b. Numerically integrated electric field lines are plotted in white. The 0 V equipotential is shown in thick red; the potential is zero also at infinity. Black curves indicate equipotentials at every integer value (in volts), with the background colour further highlighting the potential.