The Relation between the Mechanical and Piezo-Electrical Properties of a Rochelle Salt Crystal

Stress-strain curves for a Rochelle salt crystal for 16 to 36°C.—A crystal cut so as to give maximum piezo-electric response under compression, was mounted so that a known force was applied to the center of a brass block resting on the top, and the corresponding strain was measured to within ${10}^{-6\mathrm{}}$mm by means of an optical extensometer described by S. R. Williams. Variation with piezo-electric charge. When the load is applied, the initial strain is less than the final strain by an amount proportional to the initial piezo-electric charge, the difference varying with the temperature and reaching a maximum for 22.25°, the temperature of maximum piezo-electric response. For a stress of 2225 gm/${\mathrm{cm}}^2$, the strain with the charge is less than the "no charge" strain by an amount 0.7 × ${10}^3$ per coulomb per ${\mathrm{cm}}^2$; for this stress the initial and final strains were 9.5 and 13.4 × ${10}^{-5\mathrm{}}$, the corresponding moduli being 2.32 and 1.62, both times ${10}^{10}$ dynes. The final strain is independent of temperature; it is not proportional to stress, being 7.5 × ${10}^{-5\mathrm{}}$ for 1 kg/${\mathrm{cm}}^2$ and 12.5 × ${10}^{-5\mathrm{}}$ for 2 kg/${\mathrm{cm}}^2$.