Piezo-Electric Activity of Rochelle Salt under Various Conditions

Electrical Properties of Rochelle Salt Crystal are analogous to the magnetic properties of iron, the dielectric displacement $D$ and polarization $P$ varying with the electric field $E$ in the same general manner as $B$ and $I$ vary with $H$ for iron, and showing an electric hysteresis with loops distorted by an amount corresponding to the permanent polarization $P_0$, whose value is about 30 e.s.u./${\mathrm{cm}.\mathrm{}}^3$ but varies for different crystals. The dielectric constant ($\kappa =\frac{\mathrm{dD}}{\mathrm{dE}}$) was measured from — 70° to 30° $C$. and found to be surprisingly large, increasing from about 50 at — 70° to a maximum of about 1,000 near 0°. The modulus of piezo-electric activity for shearing stresses ($\delta$) varies with temperature, — 70° to 40° $C$., in a very similar manner, increasing from less than ${10}^{-6\mathrm{}}$ at — 70° to a maximum of about ${10}^{-4\mathrm{}}$ at 0°. The ratio $\frac{\delta }{\kappa }$ varied with the electrode material, being greater for tin foil than for mercury electrodes. The difference may be due to the alcohol used in shellacking the tin-foil electrodes on. There are other indications that $\delta$ and $\kappa$ are related. The variation of $\delta$ with humidity is such as can be accounted for by the decrease in the dielectric constant of the outer layer as a result of dehydration. The change of polarization produced by pressure as measured by the change in the hysteresis loop agrees with the value found directly from the piezo-electric response, as required by Lord Kelvin's theory. Also fatigue effects on $\delta$ produced by temporarily applied fields are traceable to fatigue in the polarization. The electrical conductivity below 45° is less than 5 × ${10}^{-9\mathrm{}}$ mhos/${\mathrm{cm}.\mathrm{}}^3$ but from 43° to 57° increases rapidly to 5 × ${10}^{-4\mathrm{}}$.