Dilatations in Rochelle Salt

The inverse piezoelectric effect of Rochelle salt has been studied by means of mechanical dilatometers. The dilatation of a crystal along a direction 45° to the $b$ and $c$ axes and 90° to the $a$ axis, caused by an electric field impressed in a given direction along the $a$ axis, has been found, in general, not to be the same as the dilatation caused by an equal field impressed in the opposite direction. This indicates a permanent electric polarization within the crystal. A prolonged application of voltage along the $a$ axis slowly changes this "permanent" polarization, which gives rise to an inverse piezoelectric fatigue effect, and is associated with the fatigue effect as observed by Valasek. Oscillograms showing the rate of deformation of a Rochelle salt crystal were taken. The forms of these curves, for small distortions of the crystal or small applied fields along the $a$ axis, are in approximate agreement with the exponential type as required by the theory of Schulwas-Sorokin and Posnov. The relaxation times varied from zero at the Curie points to a maximum approximately midway between the critical temperatures. The thermal expansion of Rochelle salt along the three crystallographic axes was determined through the upper critical temperature. No discontinuities were observed, but a change in the rate of expansion occurred at the Curie point for the $b$ and $c$ axes.