An experimental investigation of the propagation of second sound in uniformly rotating resonators filled with liquid helium II has been made. It is found that in the uniformly rotating liquid the velocity of the second sound is not changed by more than 0·1%, but there is an excess attenuation which is, except near the
λ
point, proportional to the angular velocity
ω
, independent of second-sound amplitude, and independent of frequency in the range 1·5 to 4·5 kc/s. These results are described phenomenologically by a mutual friction force
B
(
ρ
s
ρ
n
/
ρ
)
ω
(v
s
— v
n
) per unit volume in the two-fluid model. The constant
B
is of order unity for second sound propagated at right angles to the axis of rotation; it is smaller by a factor of at least 5 when the second sound is propagated parallel to the axis of rotation. It is suggested that the mutual friction in rotating helium may contain a component perpendicular to (v
s
— v
n
), and that there should be no mutual friction in an irrotational circulation. Experiments to verify these predictions are proposed.
A Note on the Pressure Equation of the Two Fluid Model of Helium II
