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THE JUMPING HOUR

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1. Definition and principle

The jumping hour is a display complication in which the hour indication does not progress continuously, as does a Hand hours display, but changes instantaneously at the start of each hour. The hour is generally displayed as a digit visible in an Aperture / Window that “jumps” from one value to the next at the exact moment the hour changes. This type of display belongs to the category of display complications. It does not alter the movement’s chronometric precision, but radically transforms the way time is read. The jumping hour is sometimes combined with jumping minutes to form a fully digital-type display, achieved mechanically. It can also be combined with the Retrograde display of the minutes to form the so-called “jumping hours retrograde minutes” complication.

2. Aperture display

The Aperture / Window is the opening made in the Dial through which the hour digit is visible. Its shape is generally rectangular or oval, of sufficient dimensions so that the digit is legible without ambiguity, including during the transitional situations of the jump. The Disc bearing the hour digits rotates beneath the Dial, and only one hour is visible at a time through the Aperture / Window. This arrangement gives the watch an aesthetic very different from Hand Display: the Dial can be cleared of any large Hand, and the time is read directly as a digit rather than interpreted from the angular position of a Hand. Legibility is thus immediate, even for those unfamiliar with reading an analog watch. The Aperture / Window can be positioned anywhere on the Dial according to design choices, although it is generally placed in the upper position (at 12 o’clock) or to the side.

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3. The hour Disc

The time indication is carried by a rotating disc, on which the twelve hour numerals, from 1 to 12, are engraved or printed. This disc completes a full rotation in twelve hours, corresponding to a thirty-degree movement per hour. It is driven by the Motion work, by means of a specific cam mechanism comparable to that of a retrograde display. The rotation of the disc is not continuous, but discontinuous: the disc remains still for almost the entire hour, then suddenly advances by one position — thirty degrees — at the moment the hour changes. This jerky rotational movement is made possible by the jumping hour mechanism. The disc itself must be sufficiently rigid and balanced to withstand the repeated shocks of the hourly jumps as well as those caused by the wearer of the watch.

4. The jumping hour mechanism.

The Motion work drives a wheel on which a Cam shaped like a snail (Snail (cam)) is riveted concentrically. A feeler fitted with a lever carrying a toothed sector travels around the circumference of the Cam. Each hour, when the feeler drops from the largest radius of the Cam to the smallest, the toothed sector of the lever quickly drives the Train (wheel train) of the jumping hour Display. The multiplication ratio of the Train (wheel train) is calculated so as to produce a rotation of the hour Disc of 30°. In order to ensure proper positioning of the hour Disc in its Aperture / Window, a twelve-tooth Star (wheel) is fixed to it, both securely and concentrically. It is positioned by a Neck chain (sautoir), generally adjustable in order to finely set the position of the hour numerals in their Aperture / Window.

5. Variants: jumping minutes and Retrograde minutes

The principle of the jumping hour can be extended to minutes to create a fully digital and mechanical display, in which both indications change by jumps. In this configuration, the minute disc completes one revolution in sixty minutes and jumps from one position to the next every sixty seconds. The combination of jumping hours and jumping minutes constitutes the so-called “mechanical digital” display. A variant combines the jumping hour with a retrograde minute display: theRetrograde display of the minutes progresses over an arc then returns to zero, while the hour jumps at the moment of the return. This coupling naturally synchronizes the return of the minute hand with the hour jump.

6. Historical realizations

Time display via aperture appeared as early as the 17th century in so-called “secret” or “surprise” watches, where the time indication was concealed behind shutters or openings. The jumping hour principle itself, with disc and jumper spring, developed in the 18th century, particularly in high-complication pocket watches. In the 19th century, Swiss and French watchmakers produced jumping hour and minute watches intended for a clientele seeking novelty in time reading. In the 20th century, the jumping hour saw renewed interest with the rise of electronic digital display watches, to which it responded by offering a mechanical alternative to the same type of reading. Several fine watchmaking houses have since incorporated this complication into collector’s pieces, sometimes combined with the retrograde minute and/or the retrograde second.

7. Design Difficulties and Critical Points

The design of a jumping hour mechanism raises several specific technical challenges. The first is managing the energy required for the jump. The spring of the jumper must accumulate enough energy to ensure a decisive and rapid jump, while not excessively drawing on the energy available during the charging phase. A jump that is too slow would cause two digits to appear simultaneously in the Aperture / Window, impairing readability. The second challenge is the precision of the Disc’s positioning after the jump. The third challenge is the mechanism’s robustness against shocks caused by the wearer. The quality of the Materials used and the precision of the adjustments are therefore decisive for the long-term reliability of the mechanism. Coordination with the rest of the Train (wheel train) must be carefully ensured, as the release of the jumper causes a brief discontinuity in the charge of the Barrel, which may momentarily affect the regularity of the balance-hairspring if the masses involved are not correctly calculated. Finally, when the jumping hour is combined with a jumping minute or a Retrograde minute, the coordination of the jumps must be perfect.