The flow of superfluid in an annulus is considered on the basis of free-energy arguments. Two critical velocities appear: One is equivalent to comparing the free energy of the fluid with the free energy of solid-body rotation; the other is equivalent to making the free energy of the fluid zero and corresponds to Feynman's criterion.

• Received 13 July 1966

DOI:https://doi.org/10.1103/PhysRevLett.17.747