sufficient and all of Breguet's watches used unidirectional winding. Before the invention of the slipping mainspring, automatic watches had a system for locking the weight. Most common, as in the 1780 drawing, when the mainspring was fully wound a lever K was raised that entered a hole N in the weight to prevent it from moving until the mainspring had unwound enough to lower the lever. Different methods were used in side-weight, rotor and center-weight mechanisms. Wrist watches The advent of the wrist watch after World War I led to renewed interest in self-winding mechanisms, and all four types listed above were used: 1922: Léon Leroy, a side-weight system. This was almost certainly derived from Breguet's design. 1923: Harwood, a center-weight system (invented ca 1917). This is discussed below. It is not known if Harwood was influenced by earlier pocket watch designs. 1930: Léon Hatot, Rolls watch, where the whole movement moves sideways. This was probably an independent re-invention. 1931: Rolex, a rotor system with unidirectional winding. Harwood "Bumper" wristwatches: 1923 Invented by John Harwood, a watch repairer from Bolton, England, who took out a UK patent with his financial backer, Harry Cutts, on 7 July 1923, and obtained a corresponding Swiss patent on 16 October 1923. The Harwood system used a pivoting weight which swung as the wearer moved, winding the mainspring. The ratchet mechanism wound the mainspring only when moving in one direction. The weight did not rotate a full 360°; spring bumpers limited its swing to about 180°, to encourage a back and forth motion. This early type of self-winding mechanism is now referred to as a 'hammer' or 'bumper'. Like its 18th-century counterparts, Harwood's watch also had a problem with jerking because "the brass weight hit too sharply against the banking pins as it pivoted". When fully wound, Harwood's watch would run for 12 hours autonomously. It did not have a conventional stem winder, so the hands were moved manually by rotating a bezel around the face of the watch. The watches were first produced with the help of Swiss watch manufacturer Fortis and went on sale in 1928. 30,000 were made before the Harwood Self-Winding Watch Company collapsed in 1931 in the Great Depression. 'Bumper' watches were the first commercially successful automatic watches; they were made by several high grade watch manufacturers during the 1930s and 1940s. Rolex The Rolex Watch Company improved Harwood's design