Density of He II in Superfluid Flow

The density of He II in supercritical, superfluid flow has been measured by a sensitive capacitance technique in the temperature range from 1.3 to 1.8 K. The average density of the liquid decreased with increasing superfluid velocity. The largest fractional density change observed was $\frac{\Delta \overline{\rho }}{\rho }=-9\mathrm{}\times {10}^{-6\mathrm{}}$ at a velocity of 7.75 cm/sec. The results can be accounted for adequately by the two-fluid, thermohydrodynamic equations of motion utilizing the expressions for the nonlinear, superfluid dissipation force and the mutual friction force.