A cylindrical epoxy spacer εr D 3.75 with recessments
is placed between parallel electrodes shaped at the outer parts similar to
Bruce’s profile, but also recessed to reduce the field intensity at the triple point
(gas–solid–electrode interface). Outside the spacer, gas insulation (εr D 1) is
provided, and the whole system is placed within a cylindrical metal pressure
vessel with zero potential, the vessel not being shown.