Human Sweat Electrolyte Powered Watch

Quartz watches require very little power for normal operation and no-one likes replacing batteries in watches. Either you spend a surprising amount of money or risk compromising the sealing of the watch. So, let's power the watch with a much larger if less sophisticated battery.

Many people have taken a couple of nails, plunged them into a lemon or potato and used the output to run a clock. These kits are common <link>. They rely on the redox mid points of two metals. This is usually copper (~+0.3V) and the zinc coating of a galvanized nail (~-0.7V). When inserted into a fruit/vegetable/tuber, the salty liquid allows movement of ions between the two dissimilar metals. This will rapidly reach an equilibrium unless you allow electrons to move from one to the other, this is where you can make the electron flow do work such as powering a clock.

Now, for a watch, lets replace the lemon/potato with a slightly sweaty human wrist. This is essentially the same thing, a vaguely acidic ion rich aqueous solution. The watch back is striped copper/zinc with insulation between the stripes. Each pair is ~1V so 2/3 pairs would probably be optimal to avoid a lossy boost circuit. Now when wearing your watch and it gets a bit clammy, at least you know it's doing something useful.