A foudroyante hand reads fractional seconds by spinning fast (one rotation per second) on a graduated subdial. The subdial divisions equal the escapement's beat rate per second: at 28,800 vph (8 beats/sec) the dial reads 1/8-sec increments; at 36,000 vph (10 beats/sec, e.g. El Primero-rate), 1/10-sec increments. The mechanism is essentially a small chronograph branch geared off the escape wheel: each beat advances the foudroyante one step.
Foudroyante mechanisms were used in 18th- and 19th-century pocket-watch chronographs to read fractional times in horse racing, marine chronometer trials, and scientific timing. The complication faded as the standard chronograph (with continuous sweep and minute counters) became dominant. Modern revivals include the Jaquet Droz Grande Seconde line, the Maurice Lacroix Masterpiece Le Chronographe, and most prominently the F.P. Journe Centigraphe Souverain (2008), which uses three subdials reading 1/100s, 1-second, and 10-minute totals.
