40 PULSE HEIGHTFIG. 2, Coincidence counting rate in a small pulse-height interval as a function of pulse height. Pulse-height interval was 2.5 pulse-height divisions.
No abstract
The difference in the angular momentum of liquid helium between an initial state in equilibrium with a steadily rotating cylindrical container and a final state with the container at rest has been measured both in He I and He II. If we denote this quantity by L and the corresponding value for solid-body rotation of the liquid by L c , then L/L c~ 1 for all temperatures and speeds used in the He I phase. In the He II region, however, and for speeds less than about 1 rad/sec, L/L c decreases monotonically from unity with decreasing initial angular velocity of the containing vessel apparently approaching the ratio p n /p as the initial angular velocity approaches zero. The results are compared with the Onsager-Feynman vortex model for He II and it is argued that concordance only can be obtained provided macroscopic rotational states of long lifetime can be shown to exist in this theory.The suspicion that Landau's criterion holds only in a restricted range of small velocities is strengthened by an experiment due to Osborne 4 who, from studies on the shape of the free-equilibrium surface in rotating helium, showed that both the normal and superfluid components move classically for speeds ranging from 4 to 70 cm/sec. It is true that the motions in the two experiments differ-in the former the fluid is accelerated whereas in the latter it is not. However, Donnelly et al., b in a variation on Osborne's experiment, showed that the two components also moved together during acceleration.The situation appeared to us sufficiently interesting to attempt an experiment of a somewhat different nature than those we have quoted. This consists of a direct measurement of the angular momentum possessed by a mass of He II at a given temperature which has been allowed to come into equilibrium with a cylindrical vessel rotating at a known constant angular velocity. Hall 6 has recently reported some results of a somewhat similar method. However, our results which are rather more extensive are for a simply connected volume of liquid, whereas Hall used a much more complicated geometry in which the liquid helium was confined to the annular regions in a pile of disks. METHOD OF MEASUREMENTThe method selected is, in principle, as follows. The cylindrical container / (Fig. 1) which we shall call simply "the bucket," is suspended by a long rigid rod F from a torsion fiber B attached to the shaft of a motor A. A mirror C is attached to the rod so that a spot of light from an external lamp can be reflected to indicate on a scale (or recording camera) the instantaneous angular position of the bucket. A small cylinder of copper D, fixed to the rod and situated between the pole pieces of an electromagnet E, provides a means of stopping the rotation of the suspended system at will by turning on the magnet current. 4 D.
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