The Famous Lemon-Battery Bulb Experiment...
W. Beaty 1995

ERROR CORRECTED: A single lemon battery cannot light a flashlight bulb

Some gradeschool science books contain "experiments" which do not work. The prism experiment in my list is one of them. Another is the "lemon battery" or "potato battery" used to run a flashlight bulb. If you stick some copper and zinc into a single lemon, this "battery" does create a small voltage. Touch your lemon-cell to the wires of a loudspeaker or headphones and you'll hear a clicking sound. Connect it to an old-style panel meter (a voltmeter or milliamp-meter; the kind with the moving needle,) and your lemon can make the meter needle move. Three or four lemon-cells connected in series can run an LCD digital clock or light up a red LED Light Emitting Diode. (If you try the digital clock or LED, remember that polarity is important, and if it doesn't work, try reversing the connections.)

HOWEVER... the lemon's electrical output is far too feeble to light up a standard flashlight bulb. Same with motors, buzzers, etc. The lemon battery is far too weak. The experiment described in the books doesn't work.

How can I be certain? All those books say one thing, and I'm just one person who says differently. Doesn't the majority rule? No, because science is based on reality staying the same, and Nature ignores what humans vote upon. It doesn't matter how many books say that lemon batteries can light a flashlight bulb. Nature can't be fooled.

Let's look at a real world example: I stick a fairly wide copper strip and a similar zinc strip into a lemon. (This works much better than copper pennies or zinc nails.) Clean the strips with sandpaper beforehand. First use the strips to tear up the inside of the lemon, then insert the metal strips very close together to give best results. The area of each "battery plate" is around 1 inch square. Measured voltage: 0.91V. Measured short-circuit current: two milliamps (0.002 Amps) immediately decreasing to a constant half a milliamp (0.0005 amps.) What does this mean? Well, a typical flashlight bulb draws an ENTIRE QUARTER OF AN AMPERE when lit. Not a half-milliamp, but 250 milliamps or 0.250 Amps. To light up a normal flashlight bulb, you'd need 500 lemons wired in parallel! 0.2500amps / 0.0005amps = 500 lemons.

However, there are specialized light bulbs which draw very tiny currents. Maybe the experiments in the books weren't talking about a standard flashlight bulb? (Most of them never say. But I'll give them the benefit of the doubt, although perhaps I shouldn't.) From Radio Shack we can get a #272-1139 incandescent bulb which only draws around fifteen milliamps (0.015 amps) at 0.7 volts when lit very dimly in a darkened room. This is the most sensitive incandescent bulb I've ever encountered. To light this bulb we only need 0.0150A/0.0005A = 30 lemons wired in parallel. THIRTY LEMONS. And the bulb is so dim that you can't see the glow unless the room is dark. But wasn't the lemon's electric current higher at the start? 0.002 amps, not 0.0005 amps? Yes, so with only TEN LEMONS wired in parallel, maybe we could cause the special hyper-sensitive light bulb to blink on for a second or two before going dark.

This still translates into "the experiment doesn't work." One single lemon cannot light up any sort of incandescent bulb. At best we can use several lemons to light an LED.

If a science book contains the lemon battery bulb-lightning experiment, it means that the author never performed the experiment to see if it works. LOTS of books and websites say that a single lemon can light a flashlight bulb. Every single one of these is wrong. The mistake is like a kind of infection. If you aren't careful, then your science website can catch a disease!

Can't we build a larger lemon-juice battery in a jar which will light a small bulb? Yes, but your battery needs to be fairly large; much larger than a couple of metal parts stuck into a lemon. At the very least you'll need a jar for the juice, plus some sheets of copper and zinc several inches wide. If you don't have that special Radio Shack bulb, then you'll need more than one lemon-juice jar hooked in series to make the 1.5 volts needed by a standard flashlight bulb. (I'll try building one of these and report back about how large the copper and zinc plates must be.)

If you really want to light up a small lightbulb, why not build an ELECTRIC GENERATOR instead?

How to cheat!

There is a secret way to make a lemon-cell light up an incandescent bulb. You have to cheat. Buy yourself a "super capacitor" or "memory backup capacitor" via mail-order surplus. They cost a few dollars. You want a value between 0.1 farad and 0.5 farads. Try one of these suppliers: To light a bulb, first build a lemon battery and connect it to the terminals of the supercapacitor. (Me, I use alligator clip-leads bought from Radio Shack.) Wait for a few minutes. Now connect your flashlight bulb to the supercapacitor terminals and it should light brightly for a few seconds. (If not, then remove the bulb and try connecting your lemon cell to the capacitor for 15 minutes to make sure the capacitor gathers enough energy.) The capacitor slowly collects electrical energy from the lemon battery, then it dumps that energy into the flashlight bulb over a very short time. You can even use this trick to let your lemon battery run a low-voltage buzzer or turn a small motor (look for "solar cell motors" from various mail order suppliers or Radio Shack.) As with the bulb, you must charge up the capacitor for many minutes, then use it to run your bulb or motor for a few seconds.

It's not an ideal experiment, and it's hard to explain how capacitors work. But it's easier than trying to connect thirty lemon-cells in parallel!


(Natural philoso-fiction?)

Created and maintained by Bill Beaty.
Mail me at: [my email address is my website addr preceded by billb atsign].
View My Stats