In this paper the small motions of a perfectly elastic dielectric medium, electrically polarizable, subject to external uniform magnetic field are considered. For the problem to be determined, the Maxwell-Hertz ordinary electromagnetic equations must be supplemented by an equation which the analogy with the Ohm’s law for moving electric conductors may suggest. This equation is different from that which some Authors by other assumptions have written, but it is included in the more general Voigt's constitutive relation. Supposing that the magnetic field gives rise to dissymmetry of the internal stress, elastic and piezoelectric stress, the dynamical equations are determined and the case in which the potential of these stresses exist is considered.