Feature: The Split-Second Chronograph

Doppelchronograph, rattrapante, split-second—you’ve probably heard of one if not all of these terms, and maybe you even know that they all mean the same thing. But what exactly is a split-second chronograph, and how on Earth do they work?

To understand what a split-second chronograph does and how it does it, first it would be useful to appreciate what’s going on inside your average, garden variety chronograph as well. And that’s easier said than done, because despite its prevalence, the chronograph complication is ridiculously complicated.

With just as many additional parts as your average perpetual calendar if not more, the chronograph is like a parasite leaching off of a watch’s beating heart; I mean, it quite literally saps power from the mainspring as it attaches itself to the drive of the running seconds, teeth meshing together via levers controlled by the column wheel to propel the chronograph hands around the dial.

Once the demand is met, the chronograph wheels unmesh and the brakes are applied, and when it’s time to reset, levers force cams affixed to the reverse of the chronograph wheels back to zero. It’s a coordinated dance of parts fine-tuned to offer a balance of force, feel and efficiency, both in the power they sap and the space they take up.

All this to capture a moment in time. From the starting press of the pusher to the final reset, the chronograph makes use of over a hundred and fifty parts to enable a display of on-demand time. Even with the advent of microprocessors, to envision a mechanical solution that accommodates the need for discreet timekeeping is, quite frankly, brain melting.

But what’s really going to bake your noodle is, what do you do when you need to record a split time? If you don’t know what a split time is, it’s this: say you’re timing a racer completing multiple laps, and each time they pass the start-finish line, you’re required to record their lap time. You can’t stop the chronograph because then the next lap, and indeed the entire session, won’t be recorded. What you want is to keep the chronograph running and note the elapsed time as every lap passes. This is known as a split time.

You could do this manually, glancing at the chronograph as your racer zooms by, but as solutions go, it’s a bit agricultural. Ideally your attention will be focussed on the track, not the watch, to guarantee your accuracy. This is where the split-second chronograph comes in.

What better watch to understand the split-second chronograph with than the Patek Philippe Grand Complications 5004P. You could say it was overkill with the addition of the perpetual calendar, which, for our purposes today, is completely redundant, but thanks to the incredible Lemania-based calibre CHR 27-70 Q, we can get a view of what might ordinarily be hidden beneath an automatic winding mechanism.

You can see all the usual hallmarks of the chronograph within the CHR 27-70 Q—the springs, the levers, the gears, the wheels—but there’s a new layer on top of it all, and there’s no prizes for guessing what that does. Given the general complexity of the chronograph au naturel, it’s astonishing to think that the addition of just one extra hand could require so much more—until you understand the level of complication that’s going on here.

You see, the clutch that meshes the chronograph to the running seconds creates one level of on-demand timekeeping; the split-second mechanism, requiring a hand to stop at will and catch up again, requires another. This is Inception levels of mechanical interference here, and it needs a whole lot of brain hurt to make sense of it.

So here it goes: with the chronograph running, a direct drive is taken from the chronograph second hand to turn a split-second chronograph hand at the same time. That would be easy enough if that were the only thing going on, but of course, it isn’t; press the third pusher, and another wheel, similar to the column wheel, is rotated.

This does two things: firstly, it allows two sprung levers to push inwards, clamping the split-second wheel stationary to stop the split-second hand; secondly, it engages a sprung, octopus-like wheel in between the teeth of the split-second wheel—but more on that in a moment.

At this point you might be thinking that simply jamming the split-second hand—driven by the primary chronograph second hand, remember—would just seize the whole mechanism. That’s a reasonable assumption, and it would, were it not for the way the chronograph second hand provides drive to the split-second chronograph hand. Were they simply connected by a rigid shaft, they would indeed seize—but they’re not.

This is how it works: the drive from the chronograph second hand turns a cam—similar to the cams that reset the chronograph—which has a little notch in it. In that notch sits a ruby roller on a lever, which itself is connected to the split-second wheel and held under tension with a spring. When the split-second wheel is free to turn, the roller sits in the notch of the cam and receives the drive from the chronograph seconds, synchronously turning the split-second hand. When the split-second wheel is clamped still, the cam continues to turn, with the sprung lever allowing the ruby roller to simply roll out of the notch, going around the cam over and over again until the brakes are released.

And when they are released, the spring in the roller lever forces the split-second wheel to follow the shape of the cam—again, just like the reset cam in the main chronograph—sending it back to its little notch, synchronising the split-second hand neatly back under the main chronograph hand. And the little octopus-like wheel mentioned earlier? That gives the split-second wheel a little nudge on its way.

There’s a reason the split-second chronograph is considered one of the grand complications. It’s layered, it’s complex, it’s ingenious—and it’s incredibly hard to make. All those little moments happening in a space barely a few millimetres across requires a God-like level of knowledge not just to design, but to manufacture and build as well. After all, if you’re going to perform the split-second chronograph inception, you need imagination. And skill. Lots and lots of skill.

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