The Jaeger-LeCoultre (or LeCoultre & Cie as it was marketed in North America in the mid twentieth century) Atmos is a unique member of the horological pantheon. The 259-serial pictured above is from my personal collection and is an excellent example of how the design of the Atmos contributes to the clock’s longevity; having left the factory in the mid 60s.

What makes an Atmos clock unique among the class of pendulum-driven mantle clocks is that it does not need to be manually wound. Like all mechanical timepieces, the Atmos is driven by a mainspring. However, unlike most clocks of its class and construction, the Atmos’ mainspring is wound by the expansion and contraction of a volatile liquid, ethyl chloride, inside of a hermetically sealed bellows system.

As the surrounding temperature rises the ethyl chloride vaporizes, compressing a spiral spring. When the temperature falls the vapor condenses back into a liquid, allowing the spring to return to its resting state. The constant motion generated by the compression and expansion of this spring drives the bellows system and subsequently winds the clock’s mainspring.

A temperature change of only one degree can sufficiently power the clock for up to two days.

While the Atmos’ construction is unique among many clocks today, clocks powered by changes in atmospheric pressure date all the way back to the early 17th century. The inventor of the mechanism, Cornelis Drebbel, built as many as 18 atmospherically-driven clocks that we know about today.

If you ever get a chance to inspect an Atmos clock in person or add one to your collection, I highly recommend it. The smooth motion of the pendulum and openness of the mechanism it drives is mesmerizing.