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 (${\overrightarrow{\mathrm{v}}}_n-{\overrightarrow{\mathrm{v}}}_s$), where ${\overrightarrow{\mathrm{v}}}_n$ is the normal fluid velocity and ${\overrightarrow{\mathrm{v}}}_s$ 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.