Welcome back to Watch and Learn, Playboy.com's Q&A on all things watch-related. There were a few questions regarding the first installment "Timeless Timepieces" so let's get right to a couple.
"I don't understand what you mean by 'thinnest movement'. What is a movement?"
A movement, also called a caliber, is equivalent to the engine and transmission of a car. In very basic terms, a mechanical watch tells time by precisely controlling the unwinding of a spring through a series of gears and a lever. The ticking you hear is the sound of the impact made as the lever rocks back and forth and it's forked end strikes one gear's teeth. The ratio of the gears are calculated so they rotate each of the watch's hands at the proper pace to display the correct time. The more precise a watch's parts are made, the more accurately the watch tells time. Tolerances must be tight and the movement needs to run flawlessly because an error of just .0001% will cause a watch to be off 6 to 8 seconds a day. There's much more to it, and other issues like proper lubrication are important, but this basic description will do for now.
"What are the Piaget 9P's and 12P's measurements?"
The manual winding caliber 9P is a mere 2.0 mm thick by 20.5 mm in diameter. Slimmer watches slip discreetly under a cuff. The perception was that the thinnest watches were made by the best manufacturers. The 9P was not only the thinnest in-house movement made when it was introduced in 1957, it was relatively robust when compared to competing brand's ultra-thin offerings.
The self winding (called automatic) caliber 12P is a scant 2.3 mm tall and 28.1 mm wide. Automatic wristwatches were first available in the late 1920's. Their popularity grew rapidly over the next 3 decades. Thin was in and public demand for lower profile watches was immense. Various self winding systems were patented in this era and the late 1950's saw the release of the microrotor - the 12P being one of the best. Shown in the picture above, the gold semicircle (the microrotor) spins on a central pivot (called an arbor). This causes a series of reduction gears to rotate, which winds the mainspring that powers the watch. Basically, an automatic winding system is a convenient replacement for winding a watch by hand via the crown.
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