The Rüchardt experiment, invented by Eduard Rüchardt, is a famous experiment in thermodynamics, which determines the ratio of the molar heat capacities of a gas, i.e. the ratio of  (heat capacity at constant pressure) and   (heat capacity at constant volume) and is denoted by  (gamma, for ideal gas) or  (kappa, isentropic exponent, for real gas).  It arises because the temperature of a gas changes as pressure changes.
The experiment directly yields the heat capacity ratio or adiabatic index of the gas, which is the ratio of the heat capacity at constant pressure to heat capacity at constant volume. The results are  sometimes also known as the isentropic expansion factor.