Single‐fringe interferograms are presented of 4‐mil Cu wires exploded at 20 kV into argon at ambient pressures of 1/8, 1/16, and 1/32 atm. Features discernible include a compressive‐head shock wave, arc plasma, a weak plasma wave and the expanding metal wire. On the basis of certain plausible assumptions it is seen that the arc plasma has a temperature of about 2.5 eV; but its leading edge, a region not in thermal equilibrium, has electron temperatures ∼ 102eV and is the boundary of an electron‐driven shock wave.