Electromechanical Properties of Rochelle Salt at the Lower Curie Point

The electrical characteristics of a.c. excited rochelle salt crystals were measured at temperatures in the neighborhood of the lower Curie point. From the experimental data the coefficients describing the electromechanical (i.e., dielectric, elastic, piezoelectric) properties were calculated. The results can be coordinated by an empirical formula for the thermodynamic potential. The values of the numerical coefficients in this formula as determined from the measurements near the lower Curie point agree with those found from previous studies of the upper Curie point. The numerical agreement indicates that the two Curie points are an outcome of a single physical phenomenon: the reciprocal of the susceptibility is given by the difference of two terms. The first is an expression of the interatomic restoring forces; it is a slowly varying function of the temperature. The second term accounts for the electrostatic interaction between distant parts of the crystal; its numerical value is $\frac{4\pi }{3}$. In rochelle salt the first term happens to be approximately equal to $\frac{4\pi }{3}$, so that (small differences of large numbers!) the susceptibility is a rapidly varying function of the temperature. In this way a change of no more than 5 percent in the magnitude of the intrinsic physical properties of the crystal lattice over a temperature range of 80 degrees can produce all the remarkable electromechanical effects in rochelle salt.