  Smoluchowski (1914) is generally credited with having proposed the explanation that prevents their operation. He replaced the demon by a physical device, in this case a gentle spring that presses the trapdoor against the side of the partition. The combined spring and trapdoor is supposed to act as a valve. The trapdoor is held shut if a molecule collides from the left, but is opened by a fast collision from the right, so that a pressure difference develops. However, the spring is a system with its own kinetic and potential energy. The collisions will transfer energy to the spring, making it oscillate. When the internal energy of the spring matches the temperature of the gas, it is flapping back and forth, effectively randomly. It becomes as likely as not to be spontaneously open when a molecule arrives from the left and to be swinging shut when a molecule arrives from the right. This balance means pressure or temperature differences are no more likely to occur than they would spontaneously if the hole was simply left open. If the internal energy of the spring is out of equilibrium with the gas, it will be heated (or cooled) by the collisions with the molecules until equilibrium is reached.