DIY · Card 36 · Engineering · Electricity
Two potatoes, two nails and two scraps of copper. Wire them in a row and a clock with no battery starts ticking, powered by metal slowly dissolving inside a vegetable.
01The brief
Build a battery from potatoes and make a clock run on it.
0.9 V
from each potato with zinc and copper, whatever size the potato is
5,000 Ω
or so of resistance inside a raw potato cell, which keeps its current tiny
1800
the year Alessandro Volta built the first battery, from zinc, copper and salty cloth
A potato clock looks like magic: a vegetable with two bits of metal in it, and a clock that ticks. In fact it is a real battery, built on the same principle as every battery since the first one in 1800. Two different metals, something wet and salty between them, and a wire to let electrons flow.
The surprise is where the energy comes from. Not the potato, which only carries charge between the metals, but the zinc coating on the nail, which slowly dissolves to give up its electrons. It is the same kind of energy hiding in the chemistry of the fizz-powered boat, and the current it makes is the same current that spins the homopolar motor.
02Kit
Two potatoes from the kitchen and a few bits from a hardware shop.
Tick off what you have
0 of 5 ready
03Method
Push in the metals, link the potatoes in series, connect the clock, and measure what you made.
Step 1 of 8 · Minute 0
You need two potatoes, two galvanised (zinc-coated) nails, two short pieces of thick copper wire or two copper coins, three leads with crocodile clips, and a small LCD clock with its battery taken out.
The clock stays blank.
Why
A loose clip, or the clock connected the wrong way round.
Fix
Check every clip is on bare metal, then swap the two leads at the clock.
It worked yesterday but not today.
Why
The metals have corroded, or the potato has dried out where they touch.
Fix
Push the metals into fresh potato, or scrub them clean and use new spots.
The voltage is far below 0.9 V.
Why
The nail isn’t galvanised, or the metals touch inside the potato.
Fix
Use dull grey zinc-coated nails, and keep the two metals a few centimetres apart.
04Chemistry and physics
Metals differ in how tightly they hold on to their outermost electrons. Zinc lets go easily; copper holds on harder. Put both into the potato’s juice, which is full of dissolved salts and acids, and zinc atoms start to leave the nail as ions, leaving two electrons each behind. Join the metals with a wire and those electrons rush through it to the copper, where hydrogen ions from the juice collect them and bubble off as hydrogen.
Every potato pushes with about 0.9 volts, set by the two metals. But charge moves through a potato only slowly, so each one also has thousands of ohms of resistance inside it. Add potatoes in a row and both add up. Build a circuit and find out what you can run:
Voltage of the row
1.80 V
Voltage at the device
1.71 V
Current
8.6 μA
Inside resistance
10.0 kΩ
It runs · The clock needs only a few millionths of an amp, which even a potato can supply. Each potato adds its voltage to the row.
Because each electron in the wire was given up by a zinc atom, the zinc is the battery’s real fuel, and you can count how long it would last. A gram of zinc holds about 2,950 coulombs of charge. A clock sipping a few millionths of an amp would take years to use it up. The potato, drying out and going soft in a week or two, gives up long before. Compare them:
Bars on a logarithmic scale
Your clock stops after about
14 days
The potato gives out long before the zinc does. It is not the fuel: it is just the wet, salty go-between that lets the zinc give its electrons away. The energy comes from the metal.
The circuit uses typical classroom figures: 0.9 V and about 5 kΩ per raw potato, ten times less resistance boiled. Real potatoes vary a lot; a cheap multimeter will tell you what yours do.
05The big idea
A battery is
slow rust.
When iron rusts, its atoms give up electrons to oxygen, and the energy leaks away as a little heat. A battery runs a reaction like that, but splits it in two: the metal gives up its electrons in one place, and they are collected somewhere else.
The wire in between is a detour, and the detour is where we make the electrons work. A potato battery, a torch battery and the battery in a car are all controlled corrosion, with a clock or a light bulb in the middle.
06The math
1 · Cells in series
Eₙ = n E, rₙ = n r
n cells in a row add their voltages E, and their internal resistances r.
2 · The current
I = n E ⁄ (n r + R)
Through a device of resistance R, the internal resistance takes its share too.
3 · Faraday
Q = 2 F × m ⁄ 65.4
Each zinc atom gives two electrons. F = 96,485 coulombs per mole of electrons.
Two potatoes give 1.8 V behind 10 kΩ. An LCD clock behaves like a resistance of about 200 kΩ, so I = 1.8 ⁄ 210,000 ≈ 8.6 μA, and about 1.7 V reaches the clock: plenty. A buzzer of 1 kΩ gets I = 1.8 ⁄ 11,000 ≈ 0.16 mA, but only 0.16 V across it. The potatoes’ own resistance swallows almost everything.
The most power a single potato cell can ever deliver is E² ⁄ 4r = 0.81 ⁄ 20,000 ≈ 40 μW, forty millionths of a watt. And the fuel: one gram of zinc releases 2 × 96,485 ⁄ 65.4 ≈ 2,950 C. At 9 μA, that lasts 2,950 ⁄ 0.000009 ≈ 330 million seconds, about ten years.
07Real life
From remote controls to electric eels, every battery is two different materials and something to carry charge between them.
AA batteries
The alkaline cell in a remote control runs on zinc too. Its zinc powder gives up electrons, which travel round the circuit to a paste of manganese dioxide. The chemistry is tidier and packed far tighter, but the idea is the one in your potato.
Galvanised steel
The zinc coating on your nail is there to stop rust. Zinc corrodes in preference to iron, giving itself up to protect the steel underneath. Ships carry blocks of zinc on their hulls for the same reason: sacrificial metal that dissolves so the hull doesn’t.
Electric fish
Alessandro Volta called his first battery an artificial electric organ, after the electric ray. Electric eels and rays stack thousands of flat cells in series, each making a fraction of a volt, so together they can deliver a shock of hundreds of volts.
Potato lamps
In 2010 a team at the Hebrew University of Jerusalem showed that slices of boiled potato between zinc and copper plates could run a small LED lamp for days. They suggested it as cheap light for places far from any power grid.
Phone batteries
A phone’s lithium-ion cell moves lithium ions instead of zinc, back and forth between two electrodes as it charges and empties. It packs vastly more energy into far less space, and can be refilled hundreds of times.
Fruit and veg
Lemons, apples, pickles and limes all work, because all of them are wet and full of dissolved salts and acids that let charge move. The voltage barely changes from one to another: it is set by the two metals, not the food.
1800
In the 1780s Luigi Galvani made the legs of dead frogs twitch by touching them with two different metals, and concluded that animals contain a special ‘animal electricity’. His friend and rival Alessandro Volta was not convinced. He suspected the electricity came from the two metals, and the frog was only a damp conductor.
In 1800 Volta proved it. He stacked discs of zinc and copper with cloth soaked in salt water between them, and the pile gave a steady current, the first battery. Your potato clock is a Volta pile with a vegetable in place of the cloth. The unit of voltage, the volt, is named after him.
08Is and isn’t
The potato makes the electricity.
The zinc does. Every electron in the wire was given up by a zinc atom dissolving off the nail. The potato’s juice just lets the circuit complete inside; take away the zinc and nothing happens.
Any two nails will work.
You need two different metals, one more eager to give up electrons than the other. Two copper strips or two zinc nails give no voltage at all.
A bigger potato gives more volts.
The voltage is set by the pair of metals, about 0.9 V for zinc and copper, whatever the size. A bigger potato or bigger plates lower the resistance and allow a little more current.
Enough potatoes could charge a phone.
In theory, but you’d need thousands. A raw potato can deliver only a few tens of millionths of a watt; a phone charger supplies several watts. The energy also comes from the zinc, which would soon run out.
09Go further
A
Three in a row
Add a third potato in series. Measure the voltage. Can you now light a red LED, perhaps in the dark?
B
Side by side
Connect two potatoes in parallel instead: copper to copper, zinc to zinc. The voltage stays the same. What happens to the current?
C
Metal swap
Try an aluminium foil strip, an iron nail or a steel paper clip instead of the zinc nail. Which pair gives the most voltage?
D
Food test
Measure a lemon, an apple, a cucumber and a potato with the same metals. Does the voltage change, or only the current?
E
Boiled versus raw
With a grown-up, boil one potato for eight minutes. Compare how long the clock runs on it with a raw one.
10Questions
Push a galvanised nail and a piece of copper into each of two potatoes, a few centimetres apart. Link the nail of the first potato to the copper of the second. Then connect the first potato’s copper to the clock’s + terminal and the second potato’s nail to its − terminal. An LCD clock will start running.
The zinc on the nail gives up electrons more easily than copper. In the potato’s juice, zinc atoms dissolve as ions and leave electrons in the nail. Those electrons flow through the wire to the copper, where they are taken up by hydrogen ions from the juice. The potato is the go-between; the energy comes from the zinc.
With zinc and copper, about 0.9 volts per potato, whatever its size. Two potatoes in series give about 1.8 V, enough for a small clock. But each potato also has thousands of ohms of internal resistance, so the current it can deliver is tiny, well under a milliamp.
An LED needs about 1.8 to 2 volts before it lights at all, and then a fair current. Two raw potatoes only just reach the voltage, and their high resistance lets very little current through. Three or four potatoes in series, boiled first, can make a red LED glow.
Yes. In 2010 researchers at the Hebrew University of Jerusalem found that boiling a potato for a few minutes broke down its internal structure and let it deliver up to ten times more power, for days at a time. The potato still isn’t the fuel; it just gets out of the way.
Safety · A potato battery is far too weak to hurt anyone. A grown-up should push in the nails and handle any boiling. Throw the potatoes away afterwards: metal has dissolved into them, so never eat them.
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