The London firm that made the pair of brass bells set up by an Oxford clergyman in 1840 wrote that the batteries behind them would normally last only three or four years. The clapper, 4 millimeters across, is still crossing between the two bells twice a second, and nobody has opened the batteries to see exactly what is inside.

Somewhere in a corridor of the Clarendon Laboratory at Oxford, behind a double layer of glass, a tiny metal ball has been ticking back and forth for almost two centuries. 

It has kept this up through the invention of the telephone, two world wars, the moon landings, and the entire history of the smartphone, and it shows no sign of stopping. The people who built it thought it would be dead within a few years. 

It is a small object with a stubborn set of questions attached to it and the most interesting one is the one nobody will answer.

What the thing actually is

Strip away the mystique and it is a simple-looking gadget. The Oxford Electric Bell is a pair of small brass bells, one sitting beneath each of two batteries, with a light metal ball hanging between them on a thread. That ball, the clapper, is about 4 millimeters across, smaller than a pea.

It was set up in 1840 by Robert Walker, an Oxford clergyman who also taught physics at the university. The bells and the batteries behind them were built by the London instrument-making firm Watkins and Hill.

The mechanism is easy to picture. The two batteries are wired so the bells carry opposite charges. The clapper is pulled toward one bell, touches it, picks up its charge, then gets pushed away to the other bell, touches that one, and swaps charge again. Dr Robert Taylor, speaking to the BBC, put it plainly: “As it moves back and forth, what happens is the little lead bell touches the two bells either side. And it charges and discharges continuously.” That swing happens about twice a second, and it has been happening, more or less without pause, since before the telegraph reached most of Britain.

Why the makers were wrong

When the bell was built, Watkins and Hill said how long they expected it to run. As the firm wrote, “The residual electrical power sufficient to keep up the ringing of the bells seldom lasts for longer than three or four years.”

The bell is now past 185 years, and by one estimate the clapper has crossed between the bells something like 10 billion times.

The trick, as far as anyone can tell, is that the bell asks for almost nothing. It runs on very high voltage but a tiny current, on the order of one nanoamp per swing of the ball. A nanoamp is a billionth of an amp. Moving that four-millimeter ball back and forth barely draws on the charge stored in the batteries, which is why something the makers thought would be spent in a few winters is still, faintly, working. What limits it now is probably not the charge running out but the moving parts wearing down.

The one question nobody will answer

This brings us to the part we keep coming back to. For a device this old and this well-known, you would expect somebody to have taken it apart by now to see how the batteries are built. Nobody has, and that is the whole point.

The batteries are sealed inside a coating of sulfur, and cracking that open would end the experiment. So the exact recipe stays a guess, an informed one, but a guess. The closest thing to an authoritative account is a 1984 paper by A. J. Croft, a former Clarendon researcher. Croft was careful about how little is actually confirmed: “What the piles are made of is not known with certainty, but it is clear that the outer coating is of sulphur, and this seals in the cells and the electrolyte.”

His best guess points back to an earlier design. “Piles similar to this were made by Zamboni, whose batteries were constituted of about 2,000 pairs of discs of tin foil glued to paper impregnated with zinc sulphate and coated on the other side with manganese dioxide.” Croft is describing what a battery like this one was probably made of, based on what its makers had to work with, not reporting the results of taking one apart. As far as we can tell, no such teardown exists, because the moment you do it, you no longer have the world’s longest-running battery. You have a broken novelty and an answer nobody can enjoy.

What it does and doesn’t prove

It is tempting to file this under perpetual motion, the fantasy of a machine that runs forever on nothing. That is the wrong drawer. The bell is not making energy. It is spending, very slowly, a store of charge that was sealed up in 1840, and the reason it lasts is that it spends so little. A nanoamp at a time buys you a very long run, but it is still a run toward empty.

What physicists actually take from it is more modest, and to our mind more interesting than a free-energy fairy tale. It shows how durable a well-sealed battery can be when almost nothing is drawn from it, and it is a reminder that a 19th-century instrument, built by hand and closed up with molten sulfur, can outlast nearly everything built to study it. The lesson is about restraint. Draw next to no power and a battery will keep a small thing moving for longer than even its makers could imagine.