A solid object immersed in the plasma of a high-pressure arc is surrounded by a dark sheath. The sheath is at plasma temperature along one boundary and near thermal equilibrium with the solid object at the other boundary. Because of ion diffusion into the sheath, the latter easily transmits low electron current densities. But to pass high current densities, such as those carried by the electrodes, sheath insulation must be broken down. This breakdown or re-ignition process occurs at the electrodes after current zero every half-cycle of an ac arc. In the mercury arcs studied, re-ignition from current zero involves heating of a channel which extends outward more than ¾ mm from the electrode to a temperature of about 4400°K, which requires the input of about 0.64 J/cm3 to the vapor. Addition of iodides to the vapor may inhibit the re-ignition process by either electrode or vapor-phase processes.