Quick answer

A real Jaeger-LeCoultre Atmos has no winding crown, no key, and no battery. It winds itself from changes in air temperature and air pressure. A clock that takes a battery, ticks once per second, or comes with a key is not an Atmos. Most items sold as a fake Jaeger LeCoultre Atmos are quartz clocks in an Atmos-shaped case or decorative torsion clocks built in China. True copies of the movement are rare. The mechanism is hard to build.

How the real clock works

Jean-Leon Reutter built the first Atmos in 1928. Jaeger-LeCoultre bought the rights in the 1930s and put its own name on the clock.

A sealed capsule inside the movement holds gas. A change of 1 degree Celsius in room temperature expands or contracts the gas. The capsule moves, pulls a chain, and winds the mainspring. One degree Celsius gives about 48 hours of running time. The balance wheel beats twice per minute.

Checks that settle the question

  1. No crown. The Atmos has no stem and no winder on any standard model.
  2. No second hand. The Atmos Classique shows hours and minutes.
  3. Chain, capsule, mainspring. Look through the glass. A real clock shows all three.
  4. Tick rate. One tick per second means quartz. The Atmos beat is two per minute.
  5. Dial printing. "Jaeger-LeCoultre" sits in a thin serif face. Copies show thick type, wrong spacing, and a missing hyphen.
  6. Case back. Genuine clocks carry a serial number, a reference number, and a Swiss origin mark. Painted or stamped numbers with uneven depth point to a copy.
  7. Glass and plating. Bubbles in the glass and soft plating show up on low-cost cases.

Serial numbers and model names

Numbers appear on the case back and on the movement. Jaeger-LeCoultre keeps records and can confirm a clock against its archive. Model names to check: Atmos Classique, Atmos Transparente, Atmos Regulateur, Atmos Du Millenaire, Atmos Marqueterie, Atmos Astronomique. A listing with no model name and no number gives you nothing to verify.

What copies get wrong

The gas capsule is the hard part. Copies use a solid rod or a fixed metal can that does not move. Copies also skip the chain and drive the hands with a quartz module in the base. Weight is another tell. A real Atmos is a heavy clock with thick brass and glass. A copy feels light and the base rings hollow when you tap it.

Prices and risk

Used Atmos clocks sell at auction from about $1,000 to $4,000. Marqueterie and limited models go higher. A listing at $150 with free shipping is a quartz clock in a brass case. Service on a real Atmos costs several hundred dollars. Ask for the service record before you buy.