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The theory of the thermal boundary layer at the walls of a spherical acoustic resonator is discussed in detail. For gases at low pressures, the temperaturejump effect is found to make a significant contribution to the resonance frequencies of the radial modes but not to their acoustic losses. Experimental results are reported for argon at 273.16 K and pressures between 15 and 248 kPa, and compared with the theory. These were obtained using the four radial modes with lowest frequency of a spherical resonator with a radius of 60".The thermal accommodation coefficient between argon and the aluminium wall of the resonator was found to be (0.84 i 0.05). The results suggest that a determination of the gas constant with a fractional imprecision of 1 x IO-' or better should be possible using a spherical acoustic resonator.

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Microwave resonances have been used to measure the volumetric thermal expansion of a spherical cavity between the temperature of the triple point of water (Tt) and the temperature of the triple point of gallium (Tg). Using the TM 1,1 and TM 1,2 modes, we find 106 [V(Tg)/V(Tt) - 1] = 1418.5 ± 1.0 and 1418.1 ± 0.6, respectively. These results are in agreement with the value 1416.6 ± 1.5 obtained by filling the cavity with mercury and using it as a dilatometer. The microwave measurements are sufficiently accurate that they can be used for primary gas or acoustic thermometry and for measuring the changes in volume standards with temperature, pressure, or time. We have evidence that microwave measurements can be used to determine the volume of a spherical cavity with an uncertainty of the order of 30 ppm and further improvements are likely.