Quick answer
A fake Atmos 566 is a clock sold under the Jaeger-LeCoultre Atmos name with a quartz or battery movement inside. A real Atmos has no battery, no winding crown, and no power cord. It winds itself from air temperature change through a sealed gas capsule. If the clock has a battery hatch or a key, it is not an Atmos.
The number is a caliber, not a model
Jaeger-LeCoultre numbered Atmos movements in the 500 range. Calibers 540, 560, 561 and 566 appear in different Atmos references across the production years. So "566" is a movement number. It is not a model name in the JLC catalog. I could not verify a reference named "Atmos 566." Listings that use the number as a model name deserve care.
How a real Atmos works
- Jean-Leon Reutter designed the clock in 1928. Jaeger-LeCoultre took over production in 1936.
- A sealed capsule holds gas that expands and contracts with temperature. That motion winds the mainspring.
- Published figures state a 1 degree C change gives about 48 hours of running reserve. JLC service material lists other values for other calibers.
- The torsion pendulum rotates once a minute. That equals one oscillation per minute, about 0.017 Hz.
- No winding, no battery, no cable.
Checklist for a suspected replica
- Open the base. A genuine Atmos has no battery compartment.
- Look for a crown or a winding key hole. Genuine clocks have neither.
- Watch the pendulum for 60 seconds. It should complete one slow turn. A tick, a jump, or a second hand points to quartz.
- Find the bellows. The capsule sits behind the movement as a round drum with a coil spring. Quartz copies leave that space empty or cover it with a plate.
- Read the case back. Genuine clocks carry the JLC name, "Swiss Made," and a caliber stamp such as "Cal. 566."
- Weigh the clock. Brass with gold plate gives a real Atmos a heavy feel. Replicas use thin chromed metal.
- Look for the transport lock. Genuine clocks have a lever that stops the pendulum for shipping.
Price signals
Used Atmos clocks trade from about $500 for plain models into the thousands for rare references. Auction results set the top of the range. A listing that offers a "new Atmos 566" for $200 is a replica.
What this guide does not cover
Atmos calibers changed across decades. Plating, glass, and dial layout differ by year. A serial number is the firm check. Jaeger-LeCoultre and its service centers can match a serial number against production records.