Investigations of Pulsed Second Sound in Liquid HeliumII

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Abstract
Pulse methods are applied to the case of second sound in liquid helium II. Both transmitter and receiver consist of thermal elements and d.c. pulses are used throughout. The advantages of pulsing are enumerated for the particular case of second sound. The analagous electrical circuit is formulated for the geometry involved. Data is recorded photographically and examples for several cases presented. Velocity of second sound is given as a function of temperature; both the high temperature and low temperature ranges are extended past those of previous investigators employing standing wave methods. Attenuation of these d.c. pulses is given as a function of temperature. The "filtering in time" inherent to the pulse method permits the investigation of several forms of coupling between second sound and ordinary sound. The previously known conversion between second sound and classical sound in helium vapor is examined quantitatively. In addition, conversion of second sound to first sound within the liquid is induced by exceeding critical intensities and observed by means of a classical detector. A third conversion, probably occurring upon reflection from a boundary, is illustrated.

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