- 1 medium size lemon or lime
- About 4 in. wire with insulation removed, #12 or #18 works just fine
- 1 steel nail, #6 or 8 is ok
- 1 zinc plated nail, #6 or 8 is ok
- Small piece of sand paper
- Knife or wire pliers to remove insulation
- A voltmeter that can read tenths of a volt, but nothing fancy beyond that.
The lemon battery project requires almost no advance setup. Just gather the above material, strip the insulation off the wire and use the sand paper on the wire and nail ends just before performing the experiment.
Lightly sand the end of the wire and nail. Without letting any part of the nail touch the wire, insert both 'terminals' in the lemon about 1 to 1 1/2 inches deep and as close together as you can get them.
Note - if they touch, our battery will be 'shorted' and no voltage difference will be shown on the meter. If that happens, just pick a new spot on the lemon and try again.
Turn the voltmeter on to a DC volt setting. If you have a battery handy (AAA, AA, C or D is fine), use it to show the class that the meter is really working. The meter will show 1.2 to 1.5 volts if new, depending on which you size you used, and possibly less if it is an old one. Let them know that it is this voltage difference that makes the light bulb in a flashlight come on, as well as the lights inside a car, or their car's headlights at night when they need them.
Note - if the meter shows a minus sign "-" in front of the number, just switch the meter leads (black and red wires) around so that the black wire touches the other end of the battery.
Go ahead and touch one of the meter leads to the nail and the other to the wire. What happens? The reading you see may be different from one lemon to another, and from one trial to the next. This is because the voltage difference we see depends on how far apart the terminals are, how well we make contact with the meter leads, how much and how strong the juice (our Electrolyte) is in each lemon, as well as other things that we cannot easily control in this experiment.
The important thing to note is that there is definitely a voltage difference.
Replace the copper wire with the zinc nail. Touch a meter lead to each nail as shown in the photo. What happened with the lemon battery this time? Same voltage? Higher? Lower?
This time replace the steel nail with the copper wire. Zinc and Copper make great battery terminals. Can you see why?
What Just Happened?
Two dissimilar metals are immersed in the lemon's juice, which acts as the electrolyte. The nails and wire act as the cathode (+ terminal) and anode (- terminal), and similar chemical reactions take place when the voltmeter is hooked up. Ions flow through the electrolyte and electrons flow through the wire.
If the terminals in our experiment are not connected to the meter, no voltage potential can be read. Likewise, if the two metals in the lemon are the same, the chemical reactions do not occur, no ions flow in the electrolyte and no voltage potential is generated ... in other words, nothing happens. If we do use the right metals for our terminals, and we connect the voltmeter, we will get a voltage reading.
One question may remain ... why can't we turn a light bulb on with this battery? The answer is, even if we connected several lemon batteries together (in series) to get the same voltage as in a D cell battery, the current we can get out of a lemon battery is just too small to light the bulb. But it is fun to try!
A fruit battery doesn't generate enough power to actually light a bulb, so a meter is needed to see the effect.