Tourbillon

A tourbillon is a rotating cage, or carriage, in which the escapement and the balance of a watch are mounted, so that the oscillating parts are turned continuously rather than staying in whatever position the watch is held. The British Museum holds a pocket chronometer (museum number 1958,1201.1848) recorded as made by John Arnold in London in 1774, with the movement credited to Abraham Louis Breguet, who converted the watch in Paris between January 1808 and May 1809. The museum's curatorial comments record that Breguet "added his newly invented revolving carriage, called a tourbillon, which constantly changed the position of the escapement and balance to minimize timekeeping errors caused by changes in the watch's position".

Breguet's own archive dates the patent precisely. Its page is headed "Patent granted for a completely new type of regulator called the 'Tourbillon'" and gives the date as 26 June 1801, expressed in the French Republican calendar as 7 Messidor, year IX, with a term of ten years. The same page describes the construction as "mounting the escapement assembly in a mobile cage that rotates on itself" and the intention as "designed to compensate for the operating errors and fluctuations caused by the effects of Earth's gravity". Those are Breguet's own words about its own patent, and the page does not name the office that granted it.

What this Codex does not do: state a benefit. Neither source gives a figure for how much a tourbillon changes a watch's rate, and no test result is quoted on either page. The purpose above is the maker's stated intention and a museum's description of it, not a measured outcome.