  We report a simple safe and attractive pedagogic demonstration with magnetic
compasses that facilitates an intuitive understanding of the concept that
ferromagnetism and ferroelectricity do not result from dipole-dipole
interactions alone. Phonon dispersion relations were calculated for a
3-dimensional simple-cubic dipole crystal and a 2-dimensional square-lattice
dipole crystal. The latter calculation corresponds to the compass demonstration
that confirm the antiferro ground state structure. A sum rule for the three
optical phonon frequencies is discussed. A mathematical non-analyticity in the
longitudinal optical phonon are also illustrated.
