The recently discovered technique of streak interferometry was applied to study 4 mil Cu wires exploded into argon at reduced pressures. Typical interferograms at 1/16 atm show an intensely luminous, peripheral arc formed in an annulus several mm from the wire. hen filters are used to diminish the diffuse light from the glow, clear fringes can be reduceed in the entire glowing region. Near the tip, measured fringeshifts are negative indicating the presence of electrons. No shock wave is seen. During an interval of about 1 microsec fringe-shifts near the periphery of the expanding glow change to positive values and a compressional shock wave can be seen to separate and propagate ahead. Estimates obtained from approximate interferogram reductions indicate electron densities as high as 10 to the 18th power/cc in the annular region of the arc. A sequence of interferograms at pressures 1/16 to 1 atm is presented and implications for the mechanism of shock production are discussed.