A chronograph is a watch that tells the time and also measures short intervals with at least one extra hand. That hand starts, stops and returns to zero at the press of a pusher, without disturbing the running of the watch. Three parts share the work: a control system, a clutch and a reset device.
Three presses, three commands
Press once and the hand sets off. Press again and it stops. The reset pusher sends it back to twelve. According to the Fondation Haute Horlogerie, these commands pass through a control system made of pushers, levers and a control wheel. That system drives a clutch, which connects the chronograph to the watch's gear train or separates it.
On the wrist the gesture feels simple. Inside the movement it sets off a small choreography of levers that must act in a precise order, every single time.
The column wheel, the traditional conductor
The column wheel coordinates the phases of the chronograph: start, stop and return to zero. It carries a ring of small pillars. With each press it advances one step, and the lever tips either drop between two columns or rest on top of them. Open the caseback of a chronograph that has one and it is often the first part the eye finds, with its polished cap.
The same source notes that in standard-quality chronographs this wheel is replaced by a cam system, which the Foundation calls "more rational". A cam is a shaped part whose profile guides the levers. It performs the same control function with a construction that is simpler to produce.
The clutch: horizontal or vertical
The Fondation Haute Horlogerie describes two families. The horizontal clutch, also called lateral, brings toothed wheels together side by side to transmit motion. The vertical clutch couples parts stacked on the same axis. Either way, the principle holds: as long as the clutch is open, the chronograph hand stays still while the watch keeps ticking.
Hammers and hearts: back to zero
To bring the hands back to their starting point, "hammers" press on "reset hearts" fixed to the chronograph hand arbors. The heart lives up to its name: its shape means that wherever the hammer strikes, it turns the heart to the same position. That profile brings the hand exactly back to twelve, whatever time was measured.
Reading the counters
The large chronograph seconds hand usually sits in the centre, where it is easiest to read. It moves in step with the balance and hairspring: the Foundation gives the example of 6 jumps per second at 3 Hz, or 10 at 5 Hz. Counters then add up the minutes and sometimes the hours measured.
The minute counter can move in three ways: "dragging", slowly and continuously; "semi-instantaneous", pushed by a finger from about the 58th second; or "instantaneous", jumping at once as the seconds hand passes 60. Watching this counter as the seconds hand completes its lap is a good way to identify a watch's construction without opening it.
Integrated or modular
The mechanism can be built into the movement or added on top as a module, also called a "plate". The Foundation describes the integrated solution as nobler and more costly to develop; the module allows a single base movement to be fitted with different complications.
A little history
In 1816 the Parisian Louis Moinet built his "compteur de tierces", which displayed sixtieths of a second. In 1821 Nicolas Mathieu Rieussec presented, at a horse race, a device that left a drop of ink on a rotating dial: hence the word "chronograph", "time writing". The first modern version, with start, stop and reset, was born around 1861 in the Vallée de Joux, made by Henri-Ferréol Piguet. 1969 brought the first automatic chronographs, including the Chronomatic and Zenith's El Primero (Fondation Haute Horlogerie).
Frequently asked questions
Why does the chronograph seconds hand move in small jumps?
Because it follows the rhythm of the balance and hairspring: 6 jumps per second at 3 Hz, 10 at 5 Hz, according to the Fondation Haute Horlogerie.
Is a chronograph necessarily accurate?
The Fondation Haute Horlogerie states that the accuracy of recorded times is only guaranteed if the watch has met the criteria of the chronometer label. Our article chronograph or chronometer explains the difference.
Further reading
The Chronograph module of the School takes this mechanism apart piece by piece, and the Flyback chronograph module shows how a measurement restarts with a single press. To understand what drives the seconds hand, read our article on the balance wheel, then join the Décalibre School.