A detent escapement, also called a chronometer escapement, is the escapement fitted to marine chronometers. It is detached: the balance wheel swings freely for most of its oscillation and is pushed only on every second swing. The design stayed in use until mechanical chronometers went out of use in the 1970s.
How does a detent escapement work?
The escapement leaves the balance wheel alone for most of each oscillation. That freedom is what makes it a detached escapement. The balance receives a push on only every second swing, so it gets one impulse per full cycle. The return swing passes with no push at all. The design also has a weakness. Unlike the lever escapement, it cannot start itself. If a knock stops the balance, the balance stays stopped.
Where it comes from
Pierre Le Roy invented an early version in 1748. The first detent escapement that worked well was designed by John Arnold around 1775. Thomas Earnshaw changed the design in 1780, and his employer, Wright, patented it in 1783. In 1804 the Board of Longitude had Earnshaw and John Roger Arnold each make a scale model of the spring detent escapement to see how it worked. Royal Museums Greenwich notes that who truly invented the escapement is still disputed.
Why is it rare in wristwatches?
Because it cannot start itself, watches came to use the self-starting lever escapement instead. The detent escapement is also known for being delicate and temperamental. Those qualities make it hard to shrink down for a wristwatch. Some makers have taken on the challenge. Urban Jürgensen announced a miniaturised detent escapement in 2011. At Baselworld 2014, Christophe Claret showed the Maestoso, a watch with a working detent escapement.
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