WRIST WATCHES | Up till recently, I've had the assumption that all watches needed a battery in order to make their little arms tick themselves around. There is however, a quite clever and, dare I say - revolutionary - invention called the automatic watch, which is a mechanical solution that, instead of running on power from a battery, is wound up - much like a vintage wall clock. With the exception that the automatic watches are wound automatically, solely from the momentum of your arm movements! Isn't that swell?! Am I the only one who has been unaware of this?
Seiko 5 Automatic SNK607. Notice the transparent case
back, putting its self-winding mechanism on display.

Unaware Jo presents: the automatic watch!

How it works

Without bogging down into too much technical detail, self-winding is basically achieved by employing an eccentric weight, called a rotor, that gains momentum from your arm movements. From that energy, the rotor is then able to wind up the so called "mainspring" of the watch, which in turn drives the watch arms themselves.

Solar Energy - nice alternative, also for digital watches

G-Shock AWG-100. One of Casio's
many solar powered watches
This aforementioned solution which is entirely mechanical, is only applicable on analogue watches, for natural reasons, but as for digital (as well as analogue) watches, there's another option for a completely green and battery free means of powering up your watch; namely through solar energy!  So, by using solar power cells to powering up your watch, the result is still practically a fully automatically driven watch (...as long as you get outside every now and then.).
While automatic watches are made by virtually all major, well known watch manufacturers including Seiko, Omega, Rolex, Longines, Breitling, TAG Heuer and Oris just to mention a few, solar powering has thus far been most common in Citizen watches with a technology they're calling Eco-Drive. Casio has a similar solution for some of their watches as well.
The solar powered solution is actually an improvement over the mechanical automation, as the automatic watches require their daily movement to keep going. Solar powered watches, on the other hand, can stay alive for up to half a year in a dark drawer (referring to Citizen specs), which means they don't need to be worn every day, and can be charged much more easily as well, simply by refraining from keeping it in a dark place for 6 sustained moths.

Check out which model I chose myself, while learning how to adjust a metal watch band here!

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Thanks for reading!
~theJo