The Atmos clock is one of the essential timepieces made by Jaeger-LeCoultre, and, at least among horological enthusiasts, it’s also probably the single best-known clock in the world. The first prototype was built in 1928, by an engineer named Jean-Léon Reutter (a resident of Neûchatel). The Atmos was not the first clock to be wound by changes in the atmosphere around it; clocks powered by temperature or air pressure changes go back to the 17th century, when the very first known was invented by Cornelius Drebbel. (Drebbel, an impressive polymath, is remembered today for inventing the first navigable submarine, but he also invented an early working air conditioner, different types of thermometers, one of the first compound microscopes, and, of all things, a chicken egg incubator.) The oldest pre-Atmos clock still in existence is the so-called Beverly Clock, which has been running since 1864 more or less continuously without ever having been wound manually.
The first prototype of the Atmos clock from 1928 is now in the Jaeger-LeCoultre museum in Le Sentier, and interestingly enough, it did not react to temperature changes, as modern Atmos clocks do, but rather, to changes in air pressure. Inside it was a tube partially filled with mercury, and the height of the column of mercury varied with air pressure, working in the same way as a mercury barometer.
The actual production Atmos clocks, however, did use temperature change to wind the mainspring, using a bellows with a combination of mercury and ammonia. In 1939, Jaeger-LeCoultre began using ethyl chloride instead (I’ve always assumed this was due to toxicity concerns, but the bellows is a closed system and it may be that the change was motivated by a desire for greater winding efficiency as well). The use of ethyl chloride has continued down to the present day; ethyl chloride is technically a central nervous system depressant but only in concentrations far higher than you’d ever be exposed to if your Atmos sprang a leak. The picture up above shows Atmos clocks at JLC Le Sentier being “run in” over a period of weeks so their rate can be checked; Atmos clocks can be very, very accurate but due to the delicacy of the oscillator system, it can take some time for them to settle down to a steady rate.
Here we can see the guts of an Atmos clock; the bellows has been removed and you can see the winding system, naked to the light of day. As temperature changes, the ethyl chloride inside expands and contracts (ethyl chloride’s boiling point is a cool 54.09 degrees Fahrenheit/12.27 degrees Celsius) and it’s sensitive enough to temperature changes that a one degree temperature change is enough to provide two days of power reserve. This is partly due to the sensitivity of the gas itself to temperature, but it’s also thanks to the extremely low energy required to keep the balance swinging. As the gas expands, it causes the bellows to expand, which pulls on the chain attached to the back plate, which tightens the mainspring. Winding only occurs when the bellows is expanding, not when it contracts.
The Atmos clock uses something called a torsion pendulum to keep time. The balance is a pretty massive circle of metal, suspended from a thin wire, and it makes a full cycle of one swing to the left, and one to the right, each minute. his extremely slow period is part of the reason such relatively small temperature changes can keep it wound; very little energy is expended, as there are no pivots, and the major causes of energy loss are air friction, and the physical interaction of the escapement with the balance. The suspension wire does the same thing that a balance spring does in a watch, and like a balance spring, if it’s affected too much by temperature changes the clock will not run accurately. In Atmos clocks, the suspension wire is made of an alloy called Elinvar (so named because its elasticity is invariable – more or less – at typical ambient temperatures for watches and clocks). Elinvar, along with Invar, was discovered by Charles Guillaume, whose work we mentioned in our earlier story today on Jaeger-LeCoultre’s tourbillons.
This cabinet is a very important one for the manufacturing of Atmos clocks: it’s the chamber where Elinvar wire is pre-aged. The basic formula for Elinvar is a matter of public record, but its reaction to temperature changes is a factor of grain size and orientation as well, both of which are determined by how the metal is worked. Such trade secrets could make the difference between mediocre and excellent performance of a timepiece, whether a watch or clock. The elasticity of a newly made spring, or suspension wire in this case, can vary significantly over time, due to structural changes in the metal as it flexes and expands; artificially accelerating the aging process helps prevent unwanted variability in rate.
There is something really wonderful about seeing these archaic, high precision machines, in their neat, quiet ranks, coming slowly to life. The Atmos clock is a delicate device but it makes the dynamic relationship between a mechanical timekeeper and its environment clearly visible as well. In most cases, variations in temperature only have the potential to upset timekeeping but the Atmos takes one of the oldest enemies of precision timekeeping and makes it a virtually inexhaustible source of power, and its regal autonomy gives it real personality. At an entry level price of only $6,750, you get an almost unbelievable amount of character and history, and the Atmos passes the acid test of any timepiece – making you happier you bought it as you learn more about it, instead of feeling like you were suckered – with flying colors.
See all the Atmos clocks in the current collection right here, and check out our Value Proposition coverage of the Atmos here. Read Part I of our visit to JLC in Le Sentier, and the company’s first tourbillons, here.
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