In a corridor off the foyer of the Clarendon Laboratory in Oxford there is a glass case, and inside it a bell has been ringing since 1840. Two small brass bells hang under two tall columns, and between them a metal ball about four millimetres across swings back and forth, touching one bell and then the other, twice a second. You cannot hear it. It sits behind two layers of glass and the sound is too faint to get out. It was one of the first pieces of apparatus bought by the clergyman and physicist Robert Walker, and it may have been built as early as 1825. Apart from short pauses when the air gets too humid, it has not stopped.
Nobody knows exactly what is inside the columns. They are batteries, almost certainly Zamboni piles, stacks of thousands of thin metal and paper discs sealed in molten sulfur to keep the damp out. The only way to find out for sure would be to open them, and opening them would end the experiment. So the question has been left alone, which seems right.
The obvious question is how a battery lasts 186 years. It is not perpetual motion, and the bell will stop one day. But I wanted to know how close to nothing it is running on, so I worked it out.
The bell runs on static electricity. The two columns charge the two bells to opposite signs. The ball touches one bell, picks up a little of its charge, gets pushed away by it, gets pulled across to the other bell, dumps that charge, picks up the opposite charge, and goes back. Each crossing carries a tiny parcel of charge from one side to the other. That parcel is the bell’s entire running cost.
How big is the parcel? A metal ball four millimetres across has a capacitance of about 0.2 picofarads on its own, and somewhat more when it sits close to the bells. Call it between 0.2 and 1 picofarad. Zamboni piles run in the kilovolt range, so call the swing between the bells one to two thousand volts. Multiply, and each crossing moves something like a billionth of a coulomb. Two crossings a second makes a current of somewhere between 0.4 and 4 billionths of an amp.
That number can be checked, and it holds. The Zamboni pile article gives these as kilovolt devices with current output in the nanoampere range, which is exactly where the arithmetic lands. A second check: two rings a second since 1840 comes to about 11.7 billion rings. The published figure is roughly ten billion, which is close enough given the humid pauses and that the published figure is a round number.
Now add it all up. Over 186 years, at that current, the bell has moved somewhere between two and twenty-four coulombs. An ordinary AA battery holds about nine thousand. So two centuries of ringing has used between three hundredths and three tenths of one percent of the charge in a single AA. Put that charge through a small flashlight bulb and it would light for somewhere between four and forty seconds.
I expected the energy to be tiny too. It is not. Energy is charge times voltage, and the bell’s voltage is about a thousand times that of a household battery. Run the same numbers for energy and the bell has spent between two and forty-seven kilojoules since 1840, which is somewhere around one AA battery’s worth. Roughly a flashlight battery of energy, spent over two centuries.
That difference is why it still rings. A battery does not wear out by running out of energy in the abstract. It wears out by running out of metal. Every coulomb that passes through a cell eats a fixed weight of zinc, about a third of a milligram, because each unit of charge is a chemical reaction that happened. In a tall stack every coulomb passes through every disc, so each disc pays the full bill. The bell’s bill, over 186 years, is somewhere between one and eight milligrams of zinc per disc. Measured in energy, it has used about a battery. Measured in metal, which is what actually runs out, it has used almost none, because it moves so little charge at such a high voltage.
There is something fitting in that. This kind of device was part of an old argument about where a battery’s push comes from: whether it is the mere contact of two different metals, or a chemical reaction slowly eating them. If it were mere contact, a bell like this could ring forever, and that would be perpetual motion. The answer turned out to be chemistry, and chemistry is why this bell will one day go quiet. The same fact that lets it last so long is the fact that guarantees it will end.
When it stops, it will be because the charge on the two sides has finally evened out, or because the little ball has worn through after billions of strikes, whichever comes first. That could be next year or centuries away. Meanwhile it rings, twice a second, behind glass where you can watch it but not hear it, on a charge budget so small that one AA’s worth of charge would last it tens to hundreds of thousands of years.
I keep coming back to how little it asks. Most things that last a long time are made to be strong. This one lasted by being frugal, moving almost nothing and doing it patiently, at a thousand volts and a whisper of current, for longer than anyone who built it could have imagined.