Rüchardt's experiment for measuring γ=c_{p}/c_{v} in gases 

The
apparatus (figure 1) allows to measure the ratio γ=Cp/Cv
for air, using a modern version of the Rüchardt method, where Cp=specific
heat at constant pressure and Cv=specific
heat at constant volume.
With this apparatus one simultaneously measures the changes of three thermodynamic variables (pressure, volume and temperature) during the fast volume oscillations induced in a mass of air by a piston sliding with low friction (adiabatic transformation). Volume is measured by a motion sensor (sonar), pressure is measured by a pressure sensor (barometer), temperature is measured by a special sensor that monitors the resistance of a tungsten wire . In
adiabatic regime, where the Poisson relation holds (PV^{γ}=cost),
the γ value may be obtained not only, as in classic Rüchardt
method, from the measured
oscillation period, but also from direct interpolation of the
PV plot.
For small changes in pressure dP and volume dV the Poisson relation is dP= (γP/V)dV A small displacement x of the piston from the equilibrium position, produces a volume change dV = xA (where A is the piston cross section) and a restoring force : F
=A dP =  A^{2} x (γ P/V).
The free piston oscillations are therefore a (damped) harmonic motion (figure 2). A plot (figure 3) of the reduced variables dP/P_{0} vs. dV/V_{0}, is a straight line with slope  γ. The dynamic may be varied by changing the value of the oscillating mass m (by loading the piston with metal rings) or the effective air volume V (by inserting pingpong balls into the vessel ). see: A new MBLversion of the Rüchardt experiment Am. J. Phys. 69, (2001) 
Figure 1 Figure 2 Figure 3 