Detent escapement

The Seiko Museum Ginza calls it "The detent escapement (chronometer escapement)" and explains the name from the French: "The French word détente means that something is not under pressure, and as the name implies, for the majority of the detent escapement's cycle, the balance can move freely." A detent locks the escape wheel; the balance trips it once per complete cycle and takes a single push, so that "the balance encounters force from the escape wheel only once during the two swings of its cycle, its isochronism is maximized", and with "the pallet and the escape wheel being nearly parallel to each other, the friction was minimal".

Who devised it is contested, and this Codex gives what each source says rather than choosing. The Seiko Museum Ginza credits the French clockmaker Pierre Le Roy in 1748. The Antiquarian Horological Society, a learned society rather than an issuing authority, lists under 1780 "Thomas Earnshaw claims to have been working on a pivoted detent escapement" and under 1782 "John Arnold's patent for a spring detent escapement". Royal Museums Greenwich, in its record for Earnshaw's portrait, states that Earnshaw "devised the spring detent chronometer escapement and his own form of temperature-compensated chronometer balance in 1782," the same year the Society assigns to Arnold's patent.

The dispute was official in its own time. Royal Museums Greenwich holds a "Model of Earnshaw's escapement", commissioned by the Board of Longitude in 1804 so that the spring detent's action could be judged; its box is labelled "Made by THOS. EARNSHAW in 1804 to the order of / The Board of Longitude." The museum records that a comparable model was made by John Roger Arnold, that both men were ultimately awarded £3,000, and that the argument over who invented the escapement has continued to the present day. The escapement's working life was long: the Seiko Museum Ginza states that "Timepieces with detent escapements were used for standard time-keeping until quartz timepieces became the norm in the 1970s".