Measurement of bulk moduli and ratio of specific heats of gases using Ruchardt's experiment

Three factors are discussed which affect the measurement of bulk modulus and ratio of specific heats of a gas in Ruchardt's experiment: damping, departure from adiabatic conditions, and molecular interactions. The experiment is most readily interpreted at small volumes of gas, where the oscillations are of one kind (intermediate between adiabatic and isothermal extremes) and the effect of damping is negligible. For larger volumes of gas a striking effect is observed, namely amplitude dependence of the oscillations: An initial set of intermediate oscillations is followed, as the amplitude decreases due to damping, by oscillations of lower frequency. The bulk moduli deduced from the latter are equal to isothermal values for monatomic and diatomic gases, but not for polyatomic gases. The amplitude, below which "isothermal" oscillations occur, increases with the volume of gas. Results for bulk moduli and ratio of specific heats are presented for eighteen gases with atomicity ranging from 1 to 12. (C) 2000 American Association of Physics Teachers.