  An unresolved issue in the theory of liquid crystals is the molecular basis
of the electroclinic effect in the smectic-A phase. Recent x-ray scattering
experiments suggest that, in a class of siloxane-containing liquid crystals, an
electric field changes a state of disordered molecular tilt in random
directions into a state of ordered tilt in one direction. To investigate this
issue, we measure the optical tilt and birefringence of these liquid crystals
as functions of field and temperature, and we develop a theory for the
distribution of molecular orientations under a field. Comparison of theory and
experiment confirms that these materials have a disordered distribution of
molecular tilt directions that is aligned by an electric field, giving a large
electroclinic effect. It also shows that the net dipole moment of a correlated
volume of molecules, a key parameter in the theory, scales as a power law near
the smectic-A--smectic-C transition.
