New Method for Measuring the Absolute Viscosity of Liquid Helium II

A new type of static viscometer for use in liquid helium II has been developed in which the inner cylinder is suspended within a rigidly fixed, coaxial, outer cylinder by a servomechanically controlled magnetic bearing of the type used by Beams and others in vacuum ultracentrifuges. The inner cylinder is accelerated electromagnetically and the absolute viscosity of the medium is determined from the deceleration observed when it is permitted to coast freely. Bearing friction is negligible, and there is no inherent lower limit to the speed of the rotor. Reproducible viscosity values were obtained in gaseous helium. In liquid helium II damping proportional to $\frac{1}{\mathrm{TS}}$ caused by thermomechanical flow, induced by eddy-current heating, was observed. This effect was reduced considerably by elaborate precautions to isolate the apparatus from ground vibrations; further improvement would require more favorable location of the apparatus. At higher speeds, the anomalous behavior reported elsewhere was again observed. The absolute viscosity was found to increase discontinuously to a higher value as the speed was increased. The observation is reproducible qualitatively, but, because of vibrations, not quantitatively. Attention is called to a possible analogy between this phenomenon and the observation of stable vorticity in water.