A comparison of theory and experiment for photo-ionization cross-sections II. Sodium and the alkali metals

Abstract
An experimental study has been made of the period and logarithmic decrement of a single disk and of a pile of equally spaced disks performing torsional oscillations in liquid helium II. For small amplitudes (less than about 0$\cdot $1 radian), the decay of amplitude is exponential, and from the solution of the Navier-Stokes equation deduced in the appendix, values of the viscosity were deduced from the results obtained with the single disk, and of the density of the normal component of helium II from the results from the pile of disks; the values found are in good agreement with earlier work. For larger amplitudes, the logarithmic decrement of both systems increases considerably with amplitude, and for the pile of disks the period also increases with amplitude. From the increase of period, it is concluded that the superfluid component is dragged more and more with the disk system at higher velocities, while the increase of decrement is interpreted as being due to additional frictional forces associated with the dragging of the superfluid component. The mutual frictional force proposed by Gorter & Mellink proves inadequate to explain the observed effects.

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