Abstract
IN a letter to the Physical Review1 (September 15 issue), Dr J. R Pellam and R. B. Scott, of the United States Bureau of Standards, give the results, of their experimental observations on the quantitative behaviour of the velocity of second sound in paramagnetically cooled liquid helium II in the temperature range below 1° K. The helium was cooled by the demagnetization method using hydrated iron-ammonium-alum crystals immersed in the liquid. The remainder of the bath contained the apparatus for the generation and detection of the second sound by the pulse method, and the velocity of the second sound was determined from oscillo-graphic observations of the transit time. Accurate determinations of the temperatures of the liquid helium corresponding to the velocity values obtained could not be made, but it was established that the second sound velocity increases markedly with decrease of temperature below 1° K. reaching nearly twice its value for 1° K. at the lowest temperature attained. As the helium bath warmed up, the velocity minimum of 18·4 m./sec., observed originally by Peshkov2, just above 1° K., was verified.
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References
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Velocity of Second Sound in Liquid Helium II. Nature 164, 1042 (1949). https://doi.org/10.1038/1641042a0
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DOI: https://doi.org/10.1038/1641042a0