XXVIII. The propagation of heat in liquid helium at temperatures below 0·6°k

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Abstract
Experiments are described in which sine waves of heat are propagated in liquid helium at temperatures between 0·25°k and 0·6°k. Although propagation takes place in a closed tube, resonance does not occur, but, by measuring the phase and amplitude of the temperature waves, values are deduced for the velocity and attenuation of the waves at frequencies between 60 c/s and 2 kc/s. The velocity increases with frequency; at 0·5°k it tends to a limit of about 133 m sec−1 at high frequencies, but at 0·25°k it reaches about 190 m sec−l at 2 kc/s and is still increasing. The attenuation per wavelength decreases with increasing frequency being nearly 2π at the lowest frequencies. At temperatures up to 0·4°k it is shown that the mean free path of the phonons must be greater than the tube diameter (2 cm) and that their reflection at the wall must be partly diffuse. The data are quantitatively explained if about one-third of the phonons suffer diffuse reflection and the remainder suffer specular reflection. Above 0·5°k the mean free path is found to be smaller than the tube diameter, being about 0·08 cm at 0·54°k and about 0·04 cm at 0·58°k.

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