  Experimental observations of the change in the polarization charge of
pyroelectric crystals of LiNbO3, LiTaO3 and CsNO3, as the temperature of the
crystal is changed from about 300K to a lower limit of 4.2K, are described. It
was found that the rate of change of the polarization charge slowed down
considerably and, perhaps, reached a zero rate of change. The onset of this
saturation of the polarization charge occurred at low temperatures, above the
temperature of liquid helium, which were different for each of the three types
of crystal.
