  The principal fact that the pyroelectric vector $\Pb^0$ typical of polar
crystals is invisible in equilibrium has been verified. Nevertheless, the
general formula for $\Pb^0$ as a definite bulk quantity associated only with
the inversion properties of a given charge distribution in the bulk has been
obtained. Its application to ferroelectric perovskites BaTiO$_3$, PbTiO$_3$,
and KNbO$_3$ is discussed.
