The viscosity of liquid helium

It is a well-known fact that liquid helium changes from one form of liquid to another at a temperature of 2.19º K, under its own vapour pressure at that temperature. The liquid above this temperature is spoken of as helium I, and below it, as helium II. This temperature is known as the λ point. The apparent difference between these two forms of the liquid is that helium I visibly boils as it is being evaporated in a way similar to ordinary liquids, but immediately this temperature is passed boiling stops and the liquid appears to be absolutely quiescent, although the temperature is progressively reduced by continued evaporation. Many properties of these two forms of liquid helium have been studied, but apparently up to the present the viscosity has not been determined. In order to keep the liquid helium at a given temperature below 4.2º K, its boiling point, it is necessary to allow it to boil under reduced pressure, consequently the formation of bubbles excludes any possibility of using a capillary viscosimeter method. The necessity of operating the liquefying system under reduced pressure also allows small amounts of impurity to leak in; these traces of impurity condense over the liquid helium and drop as fine particles through the liquid. This has to be considered in designing the apparatus.