  A Mach-Zehnder interferometer with absorbing Fabry-P\'{e}rot cavities is used
to measure the optical phase differences upon rotation around a vertical axis.
The details of the expected experimental results are described based on the
idea that a conducting medium can introduce phase differences due to changes in
material conductivity that are expected to occur when a medium moves with
respect to a reference frame. Experimental details and results are discussed.
