Mechanical Principles › Complications › Measurement of Short Time Intervals

THE SPLIT-SECONDS CHRONOGRAPH

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1. Definition and names

The split-seconds chronograph, also called split-seconds chronograph in English, is a variant of the two-pusher chronograph fitted with an additional hand — the split-seconds hand — superimposed on the main seconds hand (of the chronograph). These two hands share the same axis and start together when the chronograph is activated. They can be momentarily separated on command, allowing an intermediate time to be recorded while continuing to measure the total elapsed time.

The term “split-seconds” refers to both the function and the additional hand. It expresses the action of “catching up”: once stopped to read an intermediate time, the split-seconds hand is recalled to rejoin the main hand, catching up on the lead it accumulated during the reading. This recall is instantaneous; the split-seconds hand rejoins the main hand in a single jump.

The split-seconds chronograph is fundamentally different from the flyback chronograph. The flyback improves the efficiency of successive measurements by eliminating the dead time between two measurements. The split-seconds function, on the other hand, allows the simultaneous measurement of two intervals sharing the same start but different ends. The two functions meet distinct needs and are not interchangeable.

2. The operating principle

The operation of the split-seconds chronograph relies on three distinct phases. In the first phase, common running, the two hands — the main hand and the split-seconds hand — rotate superimposed, indistinguishable from one another. The chronograph has been started by the first pusher, exactly like a standard chronograph.

In the second phase, known as the dissociation phase, the third pusher — the split-seconds pusher — is activated. The split-seconds hand is stopped at the position reached. The main hand continues its rotation, measuring the total time. The user can read the intermediate time displayed by the split-seconds hand, while the total measurement continues.

In the third phase, known as the recall phase, the split-seconds pusher is activated a second time. The split-seconds hand is released and instantly rejoins the main hand in a jump. The two hands are once again superimposed and continue together. This recall is the most spectacular and demanding mechanical feature of the complication.

It is possible to press the split-seconds Pusher several times during the same measurement, thus recording as many intermediate Times as necessary. Each press stops and then resets the split-seconds Hand. The two Hands never diverge beyond the duration elapsed since the last separation, as the reset is always immediate upon the second press.

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3. The split-seconds Hand and its Axis

The split-seconds Hand and the main chronograph seconds Hand strictly share the same rotation Axis. This coaxiality is a fundamental mechanical constraint: the two Hands must rotate around the same geometric center, so as to be perfectly superimposed during common running phases.

The Axis of the split-seconds Hand is hollow: it takes the form of a tube through which the pivot of the Chronograph Hand (main Hand) passes and into which the Axis of the main Hand is inserted. This construction allows each Hand to rotate independently of the other around the same central Axis. The precision of the fit between the tube and the Axis is critical: any radial Set results in a visible misalignment between the two Hands.

The split-seconds Hand is generally positioned slightly below the Chronograph Hand. In some Calibres, both Hands are identical in shape and color, making it impossible to distinguish them when superimposed. In others, the split-seconds Hand is a different color, making it easier to read intermediate Times.

4. The split-seconds locking mechanism

4.1 The split-seconds clamp

The stopping of the split-seconds hand when the third pusher is operated is ensured by a dedicated component, commonly called the split-seconds clamp. This component consists of two elastic Steel branches, attached to a lever controlled by the split-seconds pusher. When the pusher is pressed, the two branches tighten around the split-seconds wheel plate (whose pivot carries the split-seconds hand), instantly immobilizing it.

The clamping force of the split-seconds clamp must be carefully calibrated. Insufficient clamping does not guarantee a clean stop of the hand, which may continue to rotate slightly due to inertia or friction with the chronograph wheel. Excessive clamping risks deforming the plate’s teeth or generating parasitic friction during the common running phase, disrupting the chronograph’s accuracy.

The design of the split-seconds clamp is one of the most delicate parts of the split-seconds mechanism. Its geometry must ensure identical clamping at each actuation, regardless of the hand’s angular position at the moment of stopping. The repeatability of the immobilization directly determines the reliability of reading intermediate times.

4.2 The reset mechanism

The reset of the split-seconds hand to the main hand is ensured by the split-seconds lever spring specific to the split-seconds hand. This spring instantly propels the split-seconds wheel in rotation until the split-seconds hand rejoins the main hand.

The speed of the reset is determined by the energy stored in the split-seconds lever spring and by the Mass of the split-seconds hand. The reset must be fast enough to appear instantaneous to the observer, but not too abrupt so as not to cause an overshoot — the split-seconds hand passing beyond the main hand before coming back. This overshoot, known as bounce or overload, indicates a split-seconds lever spring that is too strong.

Some high-quality Calibres incorporate a speed-limiting or reset-damping device, in the form of a light Brake lever. This device ensures that the reset always occurs at the same speed, regardless of the angular gap between the two hands at the time of reset. A reset from a large gap and a reset from a small gap must produce the same visual behavior.

5. The third pusher (split-seconds pusher)

The split-seconds chronograph necessarily is a chronograph with three pushers. The first pusher starts and stops the chronograph, as in the standard two-pusher chronograph. The second pusher performs the reset to zero, when the chronograph is stopped. The third pusher, specific to the split-seconds function, controls the separation and the return of the split-seconds hand.

The position of the third pusher varies depending on the calibres and manufacturing traditions. It is most often placed at 8 or 10 o’clock in the case middle, opposite the two main pushers positioned at 2 and 4 o’clock. In certain architectures, it is integrated into the crown or into a dual-function pusher. Its position must allow precise and unambiguous actuation, without risk of confusion with the first two pushers.

The ergonomics of the third pusher present a particular design challenge. In professional usage conditions — sports timing, medical use — the user may need to successively and rapidly actuate the split-seconds pusher (splitting then rejoining). The layout of the pushers must make this manipulation natural and free from the risk of error.

6. The split-seconds column wheel

Quality split-seconds chronographs are equipped with a column wheel dedicated to controlling the split-seconds function, in addition to the chronograph’s main column wheel. This split-seconds column wheel sequences the states of the function — joint running, splitting, waiting to rejoin — and drives the split-seconds clamp and the rejoining mechanism.

The split-seconds column wheel generally has a different number of columns from the main column wheel, since the sequences of states it encodes are different. It is driven by the split-seconds pusher via its own mechanism, independent of the main chronograph’s control mechanism.

The presence of two column wheels in the same Calibre — one for the main Chronograph, one for the split-seconds — is an Indicator of the quality and complexity of the design. Some more economical calibres use a split-seconds Cam instead of the split-seconds column wheel, following the same trade-offs described for the standard Chronograph.

Synchronization between the main column wheel and the split-seconds column wheel is a condition for correct operation. The Chronograph reset must necessarily be carried out when both Hands are superimposed, that is, when the split-seconds mechanism is in the common running phase and not in the dissociation phase. A mechanical lock normally prevents resetting during the dissociation phase.

7. The split-seconds wheel and its drive

7.1 Energy transmission to the split-seconds wheel

During the common running phase, the split-seconds heart, integral with the Chronograph runner, drives the split-seconds lever, which is integral with the split-seconds wheel.

7.2 Friction and disturbance of the Chronograph

The presence of the split-seconds hand on the main hand’s axis introduces additional friction in the chronograph train. This friction, even if minimal, consumes part of the energy provided by the base train and may slightly influence the accuracy of the chronograph. The design of the contact surfaces between the solid axis and the tube is optimized to minimize this friction.

During the dissociation phase, the split-seconds clamp immobilizes the tubular axis of the split-seconds mechanism. If the fit between the main axis and the tube is not perfect, the immobilization of the tube can create additional friction on the main axis, slightly disturbing the regularity of the main chronograph’s measurement. This disturbance, if it exists, represents a systematic measurement error characteristic of the calibre in question.

8. Applications and Uses

8.1 Sports Timing

The split-seconds chronograph was historically developed for uses requiring the recording of intermediate times within a continuous measurement. Sports timing is the most natural application: in a race involving several competitors starting together, the split-seconds hand can be stopped as each competitor crosses the finish line, while the main hand continues measuring the total time since the start.

In athletics events, the split-seconds function allows recording a runner’s time at an intermediate point of the course while continuing to time until the finish. In swimming or cycling, it allows recording successive lap times without interrupting the overall measurement. These uses require quick and reliable operation of the third pusher.

8.2 Medical and scientific use

In medicine, the split-seconds chronograph allows measuring the duration of successive and potentially simultaneous physiological phenomena. Measuring reaction time, timing contractions during childbirth, or timing surgical phases are documented historical applications.

9. Mechanical complexity and its position in the hierarchy of complications

The split-seconds chronograph is universally regarded as one of the most difficult mechanical complications to design, manufacture and regulate. Its complexity is significantly greater than that of the standard two-pusher chronograph or the flyback chronograph. A quality split-seconds calibre comprises between 350 and 500 parts, a significant proportion of which are specific to the split-seconds function.

Regulating a split-seconds chronograph is a lengthy and demanding operation, reserved for specialised watchmakers with specific experience. The perfect superposition of the two hands during the common running phase, the sharpness of the split-seconds clamp, the speed and consistency of the catch-up, the simultaneity of the hammers upon reset, and the absence of disturbance to the main chronograph during dissociation are all quality criteria that must be checked and adjusted individually.

The relative rarity of high-quality split-seconds chronograph production, compared to standard chronographs, reflects this complexity. Few manufactures master the in-house production of all split-seconds-specific parts. Genuine split-seconds calibres — with all their parts designed and produced by the manufacture — are references recognised within the profession and among collectors.

10. The split-seconds chronograph in grand complications

The split-seconds chronograph is combined with other complications in fine watchmaking pieces. One of the most frequent associations is the split-seconds chronograph with the perpetual calendar. These pieces represent several years of development and delicate assembly, and constitute technical milestones in the history of the manufactures that produce them.

Some grand complications further incorporate a striking mechanism — minute repeater or Grande sonnerie — with the Split-seconds chronograph. These pieces sometimes count more than 600 parts and represent the limit of what mechanical wristwatch horology has achieved in terms of functional density. Their creation is the culmination of the Swiss grand complication tradition.

11. The Split-seconds chronograph in horological tradition

The Split-seconds chronograph is one of the complications whose history is best documented in horological literature. The first split-seconds pocket watches date back to the second half of the 19th century. Their development coincided with the professionalization of sports timing and the rise of competitive sport in Europe.

The transposition of the split-seconds mechanism into a wristwatch, of dimensions much smaller than those of a pocket watch, constitutes one of the great technical challenges of 20th century horology. The miniaturization of the tubular Axis, the split-seconds clamp and the return spring within the constraints of a 28 to 36 mm Movement required decades of refinement and increased precision in machining techniques.

Today, the Split-seconds chronograph remains a prestige Complication, associated with manufactures whose technical mastery is recognized in the profession. Its presence in a manufacture’s catalogue is a strong signal of horological expertise, on a par with the Grande sonnerie or the Tourbillon. Its production continues to call upon the finest watchmakers and the most precise machining resources available in the profession.