Third Sound in Liquid Helium Films

Abstract

The existence of "third sound," a surface wave on thin films of superfluid helium, has been demonstrated experimentally. The velocity and attenuation of the waves have been measured as functions of excitation frequency, film thickness, and temperature. In the main, the results agree with earlier predictions and yield new information about the forces which cause the formation of helium films. Preliminary measurements on third sound in moving films and on surface-tension ripples on bulk liquid helium are also described.