Mutual friction in a heat current in liquid helium II. II. Experiments on transient effects

Abstract

It was shown in part I that, when helium II is carrying a steady heat current on which is superimposed a second-sound wave, the mutual friction acting between the two fluids (the Gorter-Mellink force) is of the form G(v$_{s}$ - v$_{n}$), where (v$_{s}$ - v$_{n}$) is the instantaneous relative velocity between the fluids, and the factor G is proportional to the square of the time average of this relative velocity. The present paper describes some experimental studies that have been made of the manner in which G changes when the heat current in a wide ($\sim $2 mm) channel is suddenly changed from one steady value to another; the changes in G have been observed as changes in the attenuation of second sound, and, where possible, as changes in the temperature gradient in the helium. It has been found, for example, that, when a steady super-critical heat current is suddenly switched on in initially undisturbed helium, G rises to its equilibrium value only after a delay time which is of the order of 1s, and that, when the heat current is removed, a non-zero value of G persists for at least 30s. The results indicate that the Gorter-Mellink force is probably associated with turbulence in the superfluid. It is suggested that the force may therefore be due fundamentally to the presence in the superfluid of motions for which curl v$_{s}\neq $ 0, and it is recalled that experimental evidence in favour of this view has been provided by the recent discovery (Hall & Vinen 1956a) that a mutual friction acts in helium that is simply in a state of uniform rotation.