  Recently W. J. Kim, M. Brown-Hayes, D. A. R. Dalvit, J. H. Brownell, and R.
Onofrio [Phys. Rev. A, v.78, 036102(R) (2008)] performed electrostatic
calibrations for a plane plate above a centimeter-size spherical lens at
separations down to 20-30 nm and observed "anomalous behavior". It was found
that the gradient of the electrostatic force does not depend on separation as
predicted on the basis of a pure Coulombian contribution. Some hypotheses which
could potentially explain the deviation from the expected behavior were
considered, and qualitative arguments in favor of the influence of patch
surface potentials were presented. We demonstrate that for the large lenses at
separations of a few tens nanometers from the plate, the electrostatic force
law used by the authors is not applicable due to possible deviations of the
mechanically polished and ground lens surface from a perfect spherical shape. A
model is proposed which explains the observed "anomalous behavior" using the
standard Coulombian force.
