The Temperature Variation of the Compressibility of Molten Sodium

The present method for the determination of the adiabatic compressibility of molten sodium depends in the first instance upon the measurement of the velocity of propagation of a short train of ultra-high frequency compressional acoustic waves in the material. This figure is then combined with the best available value of the density to yield the compressibility, on the assumption that the rigidity of the liquid is zero. The wave trains are produced and detected by a piezoelectric transducer, and the resultant signal is exhibited on the screen of an oscilloscope whose sweep circuit is time-calibrated to permit the measurement of the transit time of a train over a known path length in the medium. A novel acoustic interferometer is employed, with which small changes in velocity can be measured with high precision. The velocity of compressional waves in molten sodium at the freezing point, 97.6°C, is 252,600±500 cm/sec, and its temperature coefficient remains constant at the value -52.4±0.3 cm/sec °C at least to 545°C. The adiabatic compressibility is 16.85×${10}^{-12\mathrm{}}$ cgs at 97.6°C, and its temperature coefficient increases from 0.012×${10}^{-12\mathrm{}}$ cgs at this temperature to 0.013×${10}^{-12\mathrm{}}$ cgs at 545°C. The relations between these quantities and other elastic and thermal properties of solid and liquid sodium are discussed.