Abstract
The fundamental ideas of third sound are surveyed, and the historical importance of third sound as a technique for the investigation of helium-film systems is emphasized through the discussion of several examples. The desire to study 3He–4He mixture films has led to the development of new methods for detecting third sound and these are discussed. We then describe some of the interesting properties of 3He–4He mixture films that have been discovered by third-sound techniques and by recent preliminary third-sound and nuclear magnetic resonance investigations of the mixture films conducted in this laboratory.

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