Dielectric Anomalies in Rochelle Salt Crystals

Abstract

Residual charge and fatigue effects for Rochelle salt condenser.—The throw of a ballistic galvanometer connected to a condenser with Rochelle salt crystal as dielectric depends on the time of charging and the time of discharging. It increases to a maximum as the charging time is increased to 2 sec. (for crystal plates.15 cm. thick) and then decreases slowly, reaching for a time of 24 hours a value about half the original. The decrease of the throw for.03 sec. charging, due to fatiguing for 24 hours at 100 volts, was found to be nearly independent of the temperature from -15°C to +20°C. The first increase is probably due to residual charges which are easily displaced to a more or less definite limit and which account for the high dielectric constant in this temperature range. The subsequent decrease is due to a fatigue effect which may be likened to an electrolytic polarization of the internal displacement current. These effects are associated with the water of crystallization, since dessication decreases them while previous soaking in alcohol accelerates them. To explain the limited temperature range of the effects it is supposed that at -20°C there is a loosening of the water ions enough to permit a slight motion of about ${10}^{-9\mathrm{}}$ cm with reference to the Rochelle salt molecules, while at temperatures above +25°C further loosening allows electrolytic conduction to increase greatly.