Kitchen salt in old 35 mm photo film containers, make it damp with a little water and a home made battery is made, a lot easier and better than the lemon battery.
Less cells are needed and a steadier current is available to experiment with.
Copper 2.5 mm² wire or flat strip
galvanised or plated woodscrews
old 35 mm film containers
some wires with aligator clips
led of any colour
A duration test is being carried out now, to see how long the battery will last. Start 28 June 2009 at 2000 hrs.
The electrodes will eventually corrode and partially disolve in the damp salt solution, the current will decrease and the led will stop illuminating.
1 JULY 2009 Update. The led was ON for 36 hours and then basically OFF.
By moving the elctrodes the LED illuminated again for a couple of hours.
Problem here is corrosion too, at the electrodes.
By making them as flat strips a better current can be acquired because of a larger surface area exposed to the salt.
As an experiment it is still fun to try it for a school project.