What is a Chronograph

A chronograph is a watch complication that functions as a stopwatch alongside the standard timekeeping display. It can be started, stopped, and reset independently of the running time without affecting the accuracy of the base movement. The word comes from the Greek chronos (time) and grapho (to write).

The Basic Mechanism

A chronograph adds a secondary timing system on top of the base movement. The standard hour, minute, and seconds hands continue running normally. The chronograph system adds a central seconds hand (the chronograph hand), a minutes counter (usually a subdial at 3 or 12 o'clock), and sometimes an hours counter (subdial at 6 or 9 o'clock).

Three pushers or buttons control the chronograph:

• Start/Stop (typically at 2 o'clock): engages and disengages the chronograph mechanism
• Reset (typically at 4 o'clock): returns all chronograph hands to zero

In some designs, particularly older or simpler ones, the start/stop and reset functions share a single pusher (monopusher chronograph). Two-button chronographs allow the user to stop the timing, read the elapsed time, and then reset, or to stop and restart for cumulative timing.

Column Wheel vs. Cam-Actuated

There are two primary mechanisms for controlling the start, stop, and reset functions.

The column wheel is a small notched wheel that rotates by one position each time the start/stop pusher is pressed. The columns (raised pillars) and slots on the wheel engage with levers that activate or deactivate the coupling and braking mechanisms. Column wheel chronographs are generally considered smoother to operate because the wheel provides positive, defined states. The pusher feel is crisp, and the transitions between start, stop, and reset are precise. Column wheel chronographs are more expensive to manufacture because the wheel and its associated levers require tight tolerances.

The cam-actuated (also called coulisse lever) system uses shaped cams and levers instead of a column wheel. The cams are simpler to manufacture and require less precision, making cam-actuated chronographs less expensive to produce. The tradeoff is that the pusher feel is typically less refined, and the engagement can be less crisp. The ETA 7750, one of the most widely used chronograph calibers, uses a cam-actuated system.

The Coupling Mechanism

The coupling mechanism connects and disconnects the chronograph gear train from the base movement. There are two types.

A horizontal clutch (also called a lateral clutch) uses a swinging gear that meshes with the running seconds wheel when the chronograph is started. The drawback is that engagement can occasionally cause a slight jump or stutter of the chronograph seconds hand because the teeth must mesh while the wheel is in motion.

A vertical clutch (also called a friction clutch) uses two flat discs pressed together by spring pressure, similar to a car's clutch plate. The chronograph disc is always in contact with the driven disc but is held apart by the clutch mechanism. When the chronograph starts, the discs are pressed together and the friction between them transmits the rotation. Because the surfaces are always in proximity, engagement is instantaneous with no hand jump. Vertical clutches are mechanically more complex and more expensive but produce smoother operation.

The Brake and Reset

When the chronograph is stopped, a brake mechanism holds the chronograph seconds wheel in place so the elapsed time can be read. The brake is a simple spring-loaded lever that presses against the edge of the chronograph wheel, preventing it from rotating.

When the reset pusher is pressed, a heart-shaped cam (called a heart piece) attached to each chronograph hand's wheel is struck by a hammer lever. The heart-piece cam is shaped so that regardless of the hand's position when the hammer strikes, the cam rotates to a specific orientation that corresponds to the zero position of the hand. This is a reliable and elegant solution to returning the hands to zero from any position.

Flyback Chronograph

A flyback chronograph allows the user to reset and immediately restart the chronograph with a single press of the reset pusher, without first stopping it. In a standard chronograph, the sequence for timing consecutive events is: stop, reset, start (three pusher actions). In a flyback, pressing reset while the chronograph is running resets the hands to zero and immediately restarts timing (one pusher action).

This was originally developed for aviation, where pilots needed to time successive navigation legs without pausing. The mechanical implementation requires a modified reset mechanism that briefly lifts the coupling, resets the heart pieces, and re-engages the coupling in a single action.

Rattrapante (Split-Seconds)

A rattrapante chronograph has two chronograph seconds hands, one stacked on top of the other. Both hands start together when the chronograph is started. The rattrapante pusher (usually at 10 o'clock) stops one hand while the other continues. This allows the user to time two concurrent events that started together but end at different times, such as two runners in a race.

The rattrapante mechanism uses a clamp (the rattrapante brake) that grips the shaft of the split hand while allowing the other hand's shaft to continue rotating freely inside it. Pressing the rattrapante pusher again releases the clamp, and the stopped hand snaps forward to rejoin the running hand. Rattrapante chronographs are among the most mechanically complex complications in watchmaking.

Energy Consumption

A running chronograph draws additional energy from the mainspring. The chronograph gear train, coupling mechanism, and counter wheels all consume power when engaged. Most chronograph movements are designed with sufficient mainspring capacity to maintain full power reserve whether the chronograph is running or not, but some show a slight reduction in power reserve when the chronograph is in use.

Running the chronograph continuously does not damage the movement, but it does increase the load on the coupling mechanism and may slightly accelerate lubricant degradation at those contact points. Servicing a chronograph is more complex and more expensive than servicing a simple three-hand movement because of the additional components.