The behavior of a steady‐state argon arc plasma flowing through a cold 1‐cm circular copper pipe at pressures from 0.1 to 1 atm and gas flow rate of 2600 ml/min is studied by microwave cavity techniques. The high electron density (ne>1016cm−3) and large density gradients in the arc preclude the usual simple interpretation of data. At this density it is however possible to interpret the cavity resonant frequency as a measure of the ``diameter'' of the arc column. Experimental results show the dependence of this diameter on pressure, arc current, and the presence of contaminant gases in concentrations as small as 10 ppm. A simple explanation of the influence of diatomic contaminants is given which attributes column contraction to enhanced thermal conductivity brought about by diffusion of dissociation energy. Dilation of column caused by krypton is attributed to enhanced electron concentration in the cooler regions of the plasma due to krypton's lower ionization potential. Calculations of thermal conductivity of argon‐contaminant mixtures are shown to correlate well with the experimental results. The microwave technique appears in a new role as monitor and judge of the plasma behavior.