  We describe a Casimir apparatus based on a differential force measurement
between a Au-coated sphere and a planar slab divided in two regions, one of
which is made of high-resistivity (dielectric) Si, and the other of Au. The
crucial feature of the setup is a semi-transparent plane parallel conducting
over-layer, covering both regions. The setup offers two important advantages
over existing Casimir setups. On one hand it leads to a large amplification of
the difference between the Drude and the plasma prescriptions that are
currently used to compute the thermal Casimir force. On the other hand, thanks
to the screening power of the over-layer, it is in principle immune from
electrostatic forces caused by potential patches on the plates surfaces, that
plague present large distance Casimir experiments. If a semi-transparent
conductive over-layer with identical patch structure over the Au-Si regions of
the plate can be manufactured, similar to the opaque over-layers used in recent
searches of non-newtonian gravitational forces based on the isoelectronic
technique, the way will be paved for a clear observation of the thermal Casimir
force up to separations of several microns, and an unambiguous discrimination
between the Drude and the plasma prescriptions.
