In order power a lightbulb, it needs to be provided with both
voltage and current. As you've already checked the voltage across the
lemons by using a voltmeter, you know that your setup is correct. The
important thing now is that you need to get more current. (You may
recall that power = voltage * current, so you'll need to increase the
current in order to get enough Watts of power to light the bulb.)
You've got the right idea by using cells of lemon batteries
together. By connecting more of these cells in parallel, you can
increase the current. (By using more lemons in series in each cell, you
can increase the voltage.) In order to check how much current you're
getting, you'll need to use an ammeter. Your voltmeter probably has a
setting which measures milliamps, and this will work for measuring
current as well.
If it turns out that you're not getting enough current then you'll
probably need to use even more lemons. The only other thing that may
help beyond that is if you squish the lemons first so that they're
really juicy inside. This might help to increase the current that you
get from each one.
If the ammeter shows that you are getting enough current to run
your lightbulb, then the problem is most likely to be in the lightbulb,
not the battery. Make sure that you're attaching the wires to the
lightbulb correctly, and check that the bulb isn't dead using a regular
battery. If the lightbulb still won't work, you should be able to power
a small digital watch using several lemons.
If you still can't get it to work, some other experiments along the
same line of thought include touching your tongue to a wire attached to
only one lemon battery. Because the lemon produces such a small
current, this won't hurt you, but you should be able to feel a tingle.
Also, you should be able to hear a crackle inside the lemon if you put
your ear up to it. (I've never actually tried this myself, but I've
heard that it does work.)
Beyond that, I really don't know what to say. Good luck!
(published on 10/22/2007)