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Further studies of the threshold‐of‐cavitation noise in liquid helium have been made with an ultrasonic plane‐wave system. The behavior of this threshold was found to have a similarity to that of the specific heat in the λ region. Both above and below the λ temperature, the threshold was very insensitive to variations of static pressure and unchanged by addition of solid air or the imposition of electric fields. These results are difficult to explain on the basis of conventional theories of nucleation, but possibly indicate that nucleation involves the aggregation of submicroscopic structures.

Visible cavitation has been obtained readily in liquid helium using a barium titanate cylinder resonant at about 46 kc/sec. The nature of the cavitation changes in various temperature regimes, and these changes are reflected by changes in the threshold, as measured by the driving voltage of the transducer. Below about 2.13°K, the threshold for visible cavitation is an order of magnitude higher than the threshold for noise; but between 2.13°K and the λ point the visibility threshold drops and becomes comparable to the noise threshold. Static pressure has a much greater effect on the visibility threshold than on the noise threshold. The results are discussed and the conclusion is reached that the information obtained is more pertinent to the dynamics of the cavities than their nucleation.

Measurements of ultrasonic absorption have been made in potassium as a function of temperature from 2.5° to 77°K at various frequencies between 200 and 400 kcps. The samples are polycrystalline wires in which Young‐modulus‐type waves are propagated and the attenuation is measured using a standard pulse‐echo or a resonance‐decay technique. The results show the presence of a large component of attenuation, presumably due to dislocation motion, which has a broad amplitude maximum in the region between 25° and 40°K. At temperatures below 20°K, the attenuation due to electron‐lattice interaction becomes dominant and its main features (amplitude, temperature, and frequency dependence) are discussed in relation to the values predicted by the free‐electron theories. [Work supported by the U. S. Office of Naval Research.]