Jumping Hour

The jumping hour didn't originate on the wrist: it appeared around 1650 in nocturnal clocks, where a disc rotated behind an opening to avoid reading hands in darkness. Josef Pallweber formalized it in 1883 by applying it to pocket watches with two windows — one for hours, one for minutes — and from 1890 the minutes disc sat permanently alongside the hours disc. Audemars Piguet brought the complication to the wrist in 1921.

The mechanical principle is simple to describe, extremely difficult to execute. An hour disc rotates slowly, hidden under the dial, accumulating energy through a jumping spring. When the minutes reach 60, the accumulated energy discharges entirely in an instant: the disc advances by one exact step and the new number appears in the window. The problem is bounce: if the mechanism doesn't hold the disc with absolute precision, the hour oscillates or overshoots the correct position. For this reason many calibers adopt a fly governor — a small device with palettes that spins freely and acts as a pneumatic brake — or a star with a return spring that locks the disc in the correct position without dampening the jump.

The energy peak at the moment of the jump is the critical point of the entire drive train. In the Chopard L.U.C Quattro Spirit, mechanical separation between the hour train and the minute train serves precisely to isolate that peak, preventing it from propagating to the minute hand. In certain calibers this peak is actively exploited: in the MeisterSinger module of the Bell Hora Edition Notre Dame, the energy of the jump powers a hammer that strikes the sound plate, transforming the complication into a passing strike.

Significant variants exist. The montre à guichet — Cartier's Tank à Guichets is the classical reference — shows hours and minutes each in its own rectangular opening, with the minutes disc scrolling continuously. The wandering hour is a poetically older form: the number travels along a circular arc as the minutes advance, disappears to the right and reappears to the left with the next value. The retrograde jumping hour combines the instantaneous jump with the return to zero of the minute hand, as in the Reservoir Mark II. The GMT jumping hour — also called caller-style — applies the same principle to the second time zone, with the 24-hour hand advancing in jumps rather than rotating continuously.

At the bench I've stripped down enough of these calibers to know that regulating the jumping spring is the most delicate work: too much tension and the fly governor can't brake, too little and the jump is sluggish or even incomplete. It's not a complication you regulate quickly.