In 1951, Koehler and later, in 1972 Flammersfeld introduced a trick in the original Rüchardt setup, to increase the number of oscillations that are limited by the unavoidable friction-damping and gas leak (through the piston-tube seal): they made a thin hole on the tube (at half-height) and provided a gas-feeding pump to keep the pressure inside the vessel constant. By properly trimming the gas inlet flux (through a throttling valve) they obtained the following result: during the oscillations the piston is pushed-up by the gas overpressure until it crosses the hole position; then the gas leakage through the hole reduces the pressure, and the piston falls back. The force acting onto the piston varies at a rate that is regulated by the piston oscillation frequency leading to forced oscillation; fine adjustment of the throttle valve allows to achieve maximum amplitude at resonance.