Atmos Clock by Jaeger-LeCoultre
Invented by Jean-Léon Reutter in 1928 and improved by LeCoultre, the Atmos winds itself from tiny swings in room temperature and almost never needs a hand on it.
Reutter's almost perpetual clock
The Atmos was designed by the engineer Jean-Léon Reutter, who took out a French patent on it in 1928 and a Swiss patent in 1931, according to the British Museum. Royal Museums Greenwich also dates its development to 1928, while the collector reference Antique Clock Guy places Reutter's unsold prototypes a year earlier, in 1927.
The first clocks were made in Paris. The British Museum names the maker as the Société de Radiologie, starting in 1930, while Antique Clock Guy says a department of the Compagnie Générale de Radio began making and selling them in 1929, with Reutter in charge. These early clocks are now known as the Atmos I.
How the Atmos works
The Atmos does not run on air pressure, despite its name. Inside is a sealed capsule, a kind of metal bellows, containing ethyl chloride. As the room warms the capsule expands, and as it cools the capsule contracts, and that movement winds the mainspring. The British Museum's example winds only as the temperature falls.
The rest of the clock is designed to use as little power as possible. Instead of a swinging pendulum it has a balance that twists slowly back and forth on a fine suspension spring made of Elinvar, an alloy chosen because temperature barely affects it. The British Museum's record repeats the claim that a change of just one degree Celsius could keep the clock going for 120 hours; Antique Clock Guy gives about two days.
Clockmakers had chased the idea of a self-winding clock long before Reutter. The British Museum points to an earlier clock made for James Cox, now in the Victoria and Albert Museum, that drew its power from changes in barometric pressure, and notes that it was not truly perpetual. The Atmos is not either: both draw their energy from their surroundings.
LeCoultre takes over
Sources tell the LeCoultre story differently. The British Museum says Jacques-David LeCoultre saw a Reutter clock in 1936 and improved the design. Antique Clock Guy gives an earlier timeline: LeCoultre agreed in 1932 to develop movements, began delivering them in mid-1933, took over all production from the Compagnie Générale de Radio in 1935, and announced its new Atmos in 1936.
Production was small at first. Antique Clock Guy says only about 1,000 to 2,000 movements a year were made in 1933 and 1934, that the original Atmos I was dropped by November 1936, and that full production of the new Atmos II only got going in mid-1939. Each clock could take about a month to make, followed by about five weeks of testing and adjustment.
Jaeger-LeCoultre still makes the Atmos and says it was brought to market in the 1930s. The company also notes that the clock became known as the President's Clock because it was the Swiss Confederation's official gift.
How to identify and date an Atmos
Look for the serial number on the movement. An Atmos in the British Museum, which Jaeger-LeCoultre dated to the first half of 1947, has no maker's name anywhere; its only marking is the serial number 125425 on the front movement plate. Royal Museums Greenwich holds an Atmos Classic, model 5800, serial number 437226, made in 1976. Early Reutter-era clocks often carry a plate marked Reutter Brevet.
Antique Clock Guy groups serial numbers by model, though it warns that ranges overlap. Atmos II and III clocks, made from 1936 to late 1955, fall roughly between 4,000 and 59,999. The Atmos IV runs from 60,000 to 69,999. Models V to VIII share the range from 70,000 to 599,999, and the last of the original Reutter-design clocks ended in late 1983. Later clocks with the calibre 540 and newer movements start at 600,000.
See also: other clocks made for a mantel or shelf on OnlineClock.app.
Questions people ask
How do I tell how old my Atmos clock is?
Find the serial number on the movement and compare it with a published model range. Numbers below 60,000 point to the Atmos II or III of 1936 to 1955, and numbers above 600,000 point to clocks made after 1983.
Does an Atmos clock need winding?
No. It winds itself from small changes in room temperature, which make a sealed capsule of ethyl chloride expand and contract and wind the mainspring.
Is the Atmos still made?
Yes. Jaeger-LeCoultre still makes Atmos clocks, which it describes as almost perpetual.