# Science Questions

## How do kinetic watches work?

Sat, 28th Apr 2012

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### Question

Hi Chris

Just listened to your podcast on Circadian Rhythms - excellent as always - which led me to wonder about the kinetic watch on my wrist.

I don't need to wind it nor replace batteries. I understand that this is because my wrist moves about so much during the day that that is enough energy to keep the watch going, but how exactly does that work?

Thanks as always.

Regards

Neil

Dave -   If you have ever taken a cup of coffee or something with some foam on the top and spun the cup, the coffee stays still, but the cup turns around.  This is based on the same principles.

There are some weights inside the watch which are free to move and as you move your wrist around, the weights tend to stay still when your wrist is moving, and so they move around and spin around as you move around, and go through your daily life.  This relative movement between the weight and the watch is connected to a little generator via some gears and cogs, and that charges the battery.  And so, as long as you keep using it and moving around, it will charge the battery up and it should keep going.

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Some watches are able to communicate with frequency standard radio transmitters to get the correct time but I have never heard of this function being built into a self winding watch. syhprum, Mon, 30th Apr 2012

Hmm, I can only assume that the "self-winding" mechanism does not wind up a spring but is used to produce a current to charge a battery or, maybe as the watch seems to stop relatively quickly if not kept active, just a large capacitor. If it regains the correct time I guess it must have a receiver to pick up some time signal (Various Long Wave time signals or, maybe, GPS). Perhaps Neil could say the make and type of watch because I think there must be some advertising blurb that says what it does.

The mechanism to do this is simply an eccentric weight on a central shaft which will respond to your wrist movements (because of its inertia) by pivoting around its central shaft. There may be a mechanical system for creating from this, a rotation in one direction or, if it is to drive a small generator, this could be done electrically by rectifying the voltage output. However, there may be a cleverer way of doing this given the introduction of electronics. If it is mechanical you can usually sense the eccentric weight by feel when you move the watch about.

graham.d, Mon, 30th Apr 2012

I believe in the new-fangled ones, the eccentric weight drives a generator that charges a supercap. The old-fangled ones were entirely mechanical. The eccentric weight wound a spring through a ratchet mechanism.

Of course, assuming you have too much disposable income, and you have several of these things, you have to buy at least one of these -