We have modeled the generation of a voltage signal in noncontact potentiometric scanning force microscopy for a locally enhanced potential due to an isolated patch of electrostatic charge on an insulating surface. Both the distribution of the charge and the dielectric nature of the insulator is taken into account. When the charge is assumed to disperse ohmically on the surface, a complex time-dependent decay in the signal occurs. The profile of the decay predicted by this assumption has a shape similar to that observed experimentally in submicron scale contact charging experiments. ©1998 American Institute of Physics.