Download or read book Third Sound in Unsaturated Films of Superfluid Helium written by James Cavender Fraser and published by . This book was released on 1969 with total page 250 pages. Available in PDF, EPUB and Kindle. Book excerpt: The velocity of third sound in unsaturated films of superfluid helium has been measured at several temperatures between 1.12K and 2.05K. Helium films from 4 to over 100 atomic layers were adsorbed on glass substrates. The third sound waves were generated thermally. They were detected by sensing the very small temperature oscillations which accompany the hydrodynamic wave. The velocity was measured by the time of flight of sine wave pulses whose frequency ranged from 500 Hz to 5 KHz. The velocity was found to be less than that predicted by Atkins' hydrodynamic theory of third sound in saturated helium films which is based on the thermodynamic properties of the bulk liquid. Atkins' equations have been modified to take into account a variation of the thermodynamic properties of the film with thickness. An analysis of the velocity data has yielded the average superfluid density as a function of film thickness. Reduction of the superfluid fraction has been compared to the Ginzburg-Pitaevskii theory of superfluidity and a quantity which can yield values of the characteristic healing length of the theory has been determined from the data. The presence of a solid or normal fluid layer has been taken into account. (Author).