Rotor

A rotor is a weighted mass pivoted inside the movement so that ordinary arm movement turns it, and that turning winds the mainspring. The category it creates is covered in this Codex under automatic. John Harwood and H. Cutts's British patent GB 218,487, "Improvements relating to wrist watches", filed 7 July 1923 and published 10 July 1924, describes "watches wound automatically by a weight oscillated by the movement of the wearer", in which "overwinding is prevented by frictionally connecting the weight A to a plate D pivoted on the same spindle".

Harwood's US patent 1,576,120, "Wrist watch and other watch", filed 8 October 1923 and granted 9 March 1926, is more explicit: "the winding of the main spring is effected by an oscillating weight pivoted within the watch, the oscillation of this weight when the watch is being worn turning a plate or member frictionally connected to the weight".

The patent vocabulary has not changed in a century. Montres Breguet SA's EP 4 296 788 B1, filed 21 June 2022, is still written around an "oscillating mass" that "drives the first wheel to rotate"; the word "rotor" does not appear in it. No standards body or metrology institute read for this entry defines the word.

What this Codex does not do: repeat the customary attributions without a record. The claim that Abraham-Louis Perrelet built a self-winding mechanism in the 1770s is widely circulated, but no museum or archive record confirming it was retrieved for this entry, so it is not stated here as fact.