The ancestor of the Atmos clock is the famous James Cox perpetual-motion clock c. 1760, now in the Victoria and Albert Museum, London. In 1913 J. E. Reutter considered by what means a perpetual clock based on the expansion of liquids or gases could be made, and in 1926 the first commercial Atmos clock appeared. It had a torsion pendulum driven from a train, powered by a spring wound by the driving mechanism. A sealed U-tube containing liquefied gas in each limb, separated by mercury, has one side immersed in liquid of high specific heat insulated by a vacuum jacket; the other is exposed to the air in the glass dome protecting the clock. A slight rise in temperature expands the gas in the exposed limb, forcing mercury into the other limb insulated from the temperature change, unbalancing the mechanism on its axis. A ratchet wheel and click communicates the motion to the driving spring; reduction of temperature reverses the motion but still winds the spring.