The dependence of switching rate on the crystal thickness has been measured by Merz and explained by him in terms of a surface layer which has a low dielectric constant, and is about 10−4 cm thick. While not explicitly stated in Merz' original arguments, the layer must have a reversible polarization. If a layer without a reversible polarization is assumed, instead, and the discontinuity of the normal component of polarization at the interface between the layer and the bulk is taken into account, then a much thinner layer (∼1 atomic thickness) will explain the thickness dependence. This layer can be taken to be very lossy, so that it has a relaxation time (for the disappearance of electric fields) short compared to the switching time, and yet the layer will still be completely effective in slowing down domain wall motion.

In the presence of an opposing field the polarization of tetragonal BaTiO3 reverses itself. Devonshire's phenomenological treatment shows that in a sufficiently large opposing field the original direction of polarization becomes unstable. The values of commonly observed coercive fields are much smaller than this, and the polarization actually reverses while the initial state is still metastable. The observed coercive field and the shape of the audio-frequency hysteresis loop can, however, be explained in terms of the field dependence of the switching rate that was measured by Merz in pulse experiments. Measurements by Drougard, Funk, and Young, of the small signal conductivity, taken while traversing the hysteresis loop, are also explained in terms of Merz's rate. The absence of an appreciable inductive component in the small signal conductivity measurements indicates that variations in the applied field produce corresponding variations in the switching current with a time lag which is at most a few times 10−8 second and possibly much less. The increases in dielectric constant measured during switching remain unexplained.

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