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Ensembles of anchored cathode spots of a dc mercury vacuum arc have been studied by fast framing and streak photography. From these photographs, several statistical properties of the cathode spots have been determined: distribution functions for their diameters, velocities, and displacements, as well as spot shape and average values for the spot current and its density. The measurements showed that the anchored cathode spots were quasi-stationary. No indications of a microstructure within the individual cathode spots were found at an optical resolution of 0.37 Â¿m. Strong evidence is presented that the dc cathode spot parameter values reported here are typical for a clean mercury surface, and that those reported in the earlier literature are typical for impurity-covered surfaces.