  In this paper we make use of the concept of conserved geometric phase and of
group velocity,in conjunction with the representation
theory\cite{Dirac,Dirac-letture}, in order to derive some relevant physical
quantities for the description of the dielectric and magnetic response of
crystalline materials. As an application of the model, we derive the expression
of the macroscopic dipole moment per unit volume, and the expression of the
current induced by a uniform static external electromagnetic field.
