  We report on measurements of the Casimir force in a sphere-plane geometry
using a cryogenic force microscope to move the force probe in situ over
different materials. We show how the electrostatic environment of the
interacting surfaces plays an important role in weak force measurements and can
overcome the Casimir force at large distance. After minimizing these parasitic
forces, we measure the Casimir force between a gold-coated sphere and either a
gold-coated or a heavily doped silicon surface in the 100-400 nm distance
range. We compare the experimental data with theoretical predictions and
discuss the consequence of a systematic error in the scanner calibration on the
agreement between experiment and theory. The relative force over the two
surfaces compares favorably with theory at short distance, showing that this
Casimir force experiment is sensitive to the dielectric properties of the
interacting surfaces.
