Automatic

An automatic is a mechanical watch with a weighted rotor on the back of the movement. As your arm moves the rotor spins, and through a gear train it winds the mainspring, the coiled ribbon of steel that stores the energy. An escapement releases that energy in beats, usually 21,600 or 28,800 per hour. That works out to 6 or 8 ticks a second, which is why the seconds hand appears to sweep rather than step.

Power reserve is the number to check. Most run 38 to 45 hours off a full wind, so a watch taken off on Friday evening has stopped by Sunday. Newer designs reach 70 to 80 hours and survive a weekend. Accuracy for an untuned movement usually lands between minus 10 and plus 30 seconds a day, against minus 4 to plus 6 for a COSC chronometer. Rate changes with position, so resting a fast watch dial up overnight can pull it back.

Maintenance is the cost. Oils dry and pivots wear, so a service every five to ten years is not optional if you want the watch to last. Expect it to run from a couple of hundred dollars to well over a thousand depending on the movement. Magnetic fields from laptop speakers and bag clasps can make a mechanical watch gain minutes a day, though a demagnetizer fixes that in seconds.