How Do Fruit Powered Clocks Work Using Lemons and Oranges As Batteries?

“Natural energy” is a favorite buzz-phrase of hucksters and kooks pushing everything from arthritis cures to communication with the dead.

There seems to be this idea that “natural energy” is everywhere, to be plucked out of the air by such magic trinkets as copper bracelets (for arthritis) or by those crystal amulets that you wear around your neck or fondle in your pocket to ward off what supposedly less sophisticated societies would call “evil spirits.”

If any of these things provided one-thousandth of the energy that their boosters expend in peddling them, we’d never have to burn coal or petroleum again.

As far as fruits and vegetables are concerned, their only “natural energy” is in the form of the calories that you get by eating them, the energy that you gain when you metabolize, or “burn,” the food, just as you can release energy by burning a piece of coal. Eating coal, however, doesn’t work because our bodies have no mechanism for digesting and metabolizing it, that is, for extracting its chemical energy.

Oranges and lemons contain precious little food energy, as you might guess from the fact that they don’t burn worth a damn (except for the oils in the rind). Even if you could convert all its nutritional energy into electricity instead of muscle power, the fifteen calories in a lemon would keep a 7½watt night-light burning for only about two hours.

Other than that, the only way to get useful energy out of a lemon would be to drop it from a tall building.

Does the fruit clock actually work? Amazingly, it does. It will run for weeks or months with its wires thrust into a fruit or vegetable, almost any fruit or vegetable. “Potato-powered” clocks are quite popular, presumably because there’s nothing quite so dumb and lifeless as a potato, and getting energy out of it appeals to people’s sense of the ridiculous.

Here’s how the veggie clocks work.

The wires that you thrust into the fruit are made of two different metals, usually copper and zinc. Together with the fruit juices in between, these two metals make a genuine electric battery (more properly called a voltaic cell, but we’ll call it what everybody else does). All it takes to make a battery is two different metals with some sort of electricity-conducting liquid in between.

You know that an electric current is a flow of electrons going from one place to another, through a wire, through a lightbulb, through a motor or in this case through an electronic digital clock. The question is, How do you entice electrons into traveling from one place to another so they can run a clock along the way?

A battery induces electrons to travel because it contains two different kinds of atoms that hold on to their electrons with different degrees of tightness. For example, copper atoms hug their electrons more tightly than zinc atoms do. So if you give zinc’s electrons a chance, they’ll leave home and migrate to the copper, where they feel more wanted.

Clever humans that we are, we offer the electrons only one route from the zinc to the copper: through our digital clock. If they want to get to the copper, they’ll simply have to force their way through our clock, operating it as they go.

Then why is the fruit or vegetable necessary? The juice inside it is what chemists call an electrolyte: a liquid that conducts electricity. It completes the circuit of electrons, restoring them and their charges to the zinc, which would otherwise quickly become so depleted of electrons that the whole process would stop.

So where does the “natural energy” actually come from? It’s inherent in the constitution of the zinc and copper atoms, in their natural difference of electron-holding powers.

A battery is so easy to make that at least one may have been built by the Parthians, a people who lived two thousand years ago in what is now Iraq. In 1938 a German archaeologist described a small clay jar from that period, then in the National Museum in Baghdad.

The jar contained an iron rod inside a copper cylinder; one needed only to fill it with fruit juice (or wine) for it to have enough kick to power an ancient Parthian digital wristwatch.

Okay, so nobody really knows what it was used for. If indeed it was a battery. If it wasn’t a hoax.