  We use a single ion as an movable electric field sensor with accuracies on
the order of a few V/m. For this, we compensate undesired static electric
fields in a planar RF trap and characterize the static fields over an extended
region along the trap axis. We observe a strong buildup of stray charges around
the loading region on the trap resulting in an electric field of up to 1.3 kV/m
at the ion position. We also find that the profile of the stray field remains
constant over a time span of a few months.
