Let's face it, there are a lot of well-engineered and beautiful inventions that you wouldn't exactly call useful (here's a good example). In watchmaking, the pinnacle expression of such a creation has got to be the tourbillon.
Developed in the late 18th century by Abraham-Louis Breguet, the tourbillon's original purpose was to counter the effects of gravity on the escapement of a pocket watch. Because it sat upright in your pocket all day, the spring was stretched unnaturally by gravity, so putting it in a gently spinning cage prevented deformation and inaccuracy over time. But now we wear our watches on our wrists and this just isn't a problem anymore.
That hasn't stopped a cult of the tourbillon from developing though. It's a beautiful thing to behold and has that charm that only old-world mechanics have. Traditionally the rotating cage makes a full revolution every 60 seconds (though not always) and it often doubles as the running seconds hand.
Modern makers have continued to push the tourbillon to the extremes, though most acknowledge that it's purely an exercise in watchmaking prowess rather than an actual push for accuracy. Watches like the Duomètre à Sphérotourbillon from Jaeger-LeCoultre, the MikrotourbillonS from TAG Heuer, and the Thomas Preacher Flying Triple Axis add varying speeds, extra axes of rotation, and other complications to make the visual display even more impressive. Others like artisan watchmaker Haldimann have stripped it down to basic purity in the H1. The modern master of the tourbillon is Greubel Forsey, who makes exceptional multi-tourbillon watches like the Invention Piece 2.
No matter the complexity, you need a tourbillon in your wristwatch about as much you need a muscle controlled Iron Man repulsor. Their both cool, they both represent something impressive, and they're both completely unnecessary.
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