The Reversible Inductivity of Rochelle Salt Crystals and its Relation to Frequency and Temperature

Abstract

Reversible inductivity of Rochelle salt crystals is defined as $k_r=-\frac{\mathrm{dD}}{\mathrm{dE}}$ or the limit of the ratio of the change of induction to the change of electric field in the dielectric as these approach zero. The method of measurement used involved determining the capacity of a condenser with a plate of Rochelle salt crystal, cut normal to the ă-axis, as dielectric, by adjusting the oscillating circuit containing it to resonance with a loosely coupled vacuum tube circuit. Variation with frequency of alternating field was studied up to 4 × ${10}^7$ cycles, for the crystal at 0° C (Fig. 3). Above the natural periods of vibration of the plate, 60,000 and 156,000 cycles, $k_r$ was constant at 70, while below them it was constant at about 112 to 8,000 cycles and then began increasing. The lower frequency values are affected by piezo-electric effects due to mechanical vibration in certain ranges and by polarization effects which increase as the frequency decreases; hence the high frequency values have a simpler physical significance. Variation with temperature, — 80° to 50° C: Curves obtained for frequencies of ${10}^6$, 50,000 (near resonance), and 8,000 cycles (Fig. 4), all show maxima for about — 16° and 24° C. The minimum at about 6° may be associated with the maximum piezo-effect which occurs in this range. Hysteresis loop: The previous results are for an applied electric field of zero. As the field is increased, $k_r$ reaches a maximum, then a minimum, then increases to infinity as conduction begins for 2,700 or 3,000 volts/cm, depending on the direction. Natural polarization may be estimated from the asymmetry of the hysteresis loop. Two samples gave 125 and 175 volts/cm. A comparison of the temperature curves for three samples cut from the same crystal shows marked differences which indicate that the natural polarization is far from uniform even in the same crystal. Relation of $k_r$ to high frequency resistance is probably close. The two properties vary in general in much the same way. This seems to indicate that the energy absorbed in the dielectric is very simply related to the process of doublet formation.