In the late 1920s the young engineer Jean-Leon Reutter experimented with a clock which would not need direct mechanical or electrical intervention to keep it wound, a clock powered only by Perpetual Motion. His design included a device powering the movement independently, using mercury - a substance which would react to the most sensitive changes in temperature and atmospheric conditions, hence the name: "Atmos Clock." These first models used the mercury in glass expansion device. Later, due to dangers in handling and instability, the mercury in the bellows powering the Atmos Clock was changed to a more stable saturated gas, ethyl chloride. Reutter's system was patented in 1928 to later be sold and improved by Jaeger-LeCoultre. The "perpetual" movement is defined as "a movement which continues to function indefinitely without any exterior source of energy.