THE hydrodynamic properties of liquid helium-II present a complex picture. In thermally induced flow the superfluid exhibits a well defined critical velocity above which non-linear frictional forces appear. 1 These have been explained in terms of turbulence in the superfluid. In isothermal flow the observed behaviour is more complex. In very narrow channels (diameter < 1 micron) the flow rate is only slightly pressure dependent, while in wider channels complex non-linear effects have been reported by Allen and Misener 2 and Atkins. 3 The possibility of turbulence in the superfluid was suggested but no clear demonstration was forthcoming.
About two years ago we made some experiments on gravitational flow in a system consisting of a capillary in series with a 'Vycor' glass filter 4 and found that the flow rates depend upon the 'history' of the experiment in a highly complex manner. The need for a systematic investigation of isothermal flow in wide capillaries thus appeared desirable and the first series of experiments indeed showed that the rates of flow vary with the initial conditions of the experiment. 5
The present paper is concerned with detailed measurements of gravitational flow in wide capillaries over a wide range of temperatures below the lambda-point. It is shown that, besides the normal viscosity and the mutual friction, 6 it is necessary to introduce a third mechanism involving turbulence in the superfluid in order to explain the observed behaviour.