Piezo-Electric Activity of Rochelle Salt under Various Conditions

Abstract

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./${\mathrm{cm}.}^{3}$ but varies for different crystals. The dielectric constant ($\ensuremath{\kappa}=\frac{\mathrm{dD}}{\mathrm{dE}}$) was measured from --- 70\ifmmode^\circ\else\textdegree\fi{} to 30\ifmmode^\circ\else\textdegree\fi{} $C$. and found to be surprisingly large, increasing from about 50 at --- 70\ifmmode^\circ\else\textdegree\fi{} to a maximum of about 1,000 near 0\ifmmode^\circ\else\textdegree\fi{}. The modulus of piezo-electric activity for shearing stresses ($\ensuremath{\delta}$) varies with temperature, --- 70\ifmmode^\circ\else\textdegree\fi{} to 40\ifmmode^\circ\else\textdegree\fi{} $C$., in a very similar manner, increasing from less than ${10}^{\ensuremath{-}6}$ at --- 70\ifmmode^\circ\else\textdegree\fi{} to a maximum of about ${10}^{\ensuremath{-}4}$ at 0\ifmmode^\circ\else\textdegree\fi{}. The ratio $\frac{\ensuremath{\delta}}{\ensuremath{\kappa}}$ 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 $\ensuremath{\delta}$ and $\ensuremath{\kappa}$ are related. The variation of $\ensuremath{\delta}$ 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 $\ensuremath{\delta}$ produced by temporarily applied fields are traceable to fatigue in the polarization. The electrical conductivity below 45\ifmmode^\circ\else\textdegree\fi{} is less than 5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}9}$ mhos/${\mathrm{cm}.}^{3}$ but from 43\ifmmode^\circ\else\textdegree\fi{} to 57\ifmmode^\circ\else\textdegree\fi{} increases rapidly to 5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$.Optical Properties of Rochelle Salt as Calculated from the Natural Polarization.--- Assuming only one electron is displaced the natural period corresponds to a wavelength of 4.2 \ensuremath{\mu} and the specific rotation for sodium light comes out 10\ifmmode^\circ\else\textdegree\fi{}, the observed value being 22\ifmmode^\circ\else\textdegree\fi{}.1.