Entrainment of Second Sound in Steady-State Counterflowing Normal and Superfluid Helium II

The velocity dependence of second sound in liquid helium was measured as a function of the relative velocity (${\stackrel{\ensuremath{\rightarrow}}{\mathrm{v}}}_{n}\ensuremath{-}{\stackrel{\ensuremath{\rightarrow}}{\mathrm{v}}}_{s}$), where ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{v}}}_{n}$ is the normal fluid velocity and ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{v}}}_{s}$ is the superfluid velocity. The experimental technique consisted of measuring the time of flight of a second-sound pulse in steady-state counterflowing normal and superfluid for subcritical velocities. The results are in agreement with the theoretical predictions by Khalatnikov.