  A simple four-sublattice order-disorder model is developed for description of
phase transitions and dielectric properties of the Rochelle salt crystal. The
model is developed as a generalization of the semimicroscopic Mitsui model. The
symmetry properties of lattice and spatial orientations of effective dipoles
connected with the asymmetric structure units in the elementary cell are taken
into account. The model allows to investigate the temperature and field
behaviour of transverse (besides longitudinal) components of dielectric
susceptibility. The influence of the transverse electric field
$\vec{E}\parallel\vec{b}$ on the phase transition points and spontaneous
polarization is studied.
