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An experimental study of unipolar arcs was conducted in a low-pressure mercury rf-generated plasma. The results were found to be consistent with the concept of a sheath mechanism for driving the unipolar arcs. Floating double-probe measurements yielded electron temperatures of ∼2 eV and electron-number densities of ∼1×1011 cm−3. The variation of the unipolar arc current with (1) the electron temperature and density and (2) the metal surface area exposed to the plasma verified the predicted dependence of the arc current on the plasma parameters and the metal surface area. Finally, alternative mechanisms for sustaining the observed arcs by high-frequency rectification were ruled out on the basis of the recorded current waveforms of the unipolar arcs.