Your phone probably needs a charge before dinner. Your TV remote might go a year or two before you’re hunting for AAAs. Meanwhile, in a corridor off the foyer of the Clarendon Laboratory at the University of Oxford, a small brass bell has been ringing on the same batteries since Queen Victoria was a newlywed.

It’s known as the Oxford Electric Bell, and Guinness World Records lists its power source as the world’s most durable battery. The strangest part is that, after more than 180 years, nobody can say for certain what that battery is made of.

A novelty that refused to stop

The device was made by Watkins and Hill, a London scientific instrument firm. It was bought by Robert Walker, a clergyman and physics professor at Oxford, who attached a handwritten note reading “Set up in 1840.” Oxford’s physics department has said a second label may date the device as far back as 1825.

Nobody bought it expecting a record. Bells like this were sold as demonstration pieces, and a description from the period noted that the power keeping one ringing seldom lasted more than three or four years. This one just kept going.

Today it sits behind two layers of glass and still rings about twice every second, though you can’t hear it. The charge is now so weak that the clapper barely taps the bells, and the sound doesn’t make it through the glass.

How it works

The setup is simple. There are two small bells, each sitting under a tall column called a dry pile, with a tiny metal clapper hanging between them. When the clapper touches one bell, it picks up a charge from that pile, gets pushed away, and swings over to the other bell. There it picks up the opposite charge and gets pushed back again. That’s the whole cycle, repeated roughly 10 billion times since 1840.

The secret to its long life is how little power it uses. The bell runs on static electricity, which needs a high voltage to get things moving but only moves a tiny amount of charge each swing. By one estimate, each swing now draws about 1 nanoamp, which is a billionth of an amp. For comparison, a smartphone running a video can draw roughly a billion times that.

It isn’t perpetual motion, despite what you’ll sometimes read online. Every tap loses a little energy, and the bell will stop once the two piles have evened out their charge, or once the clapper wears out, whichever happens first.

The mystery inside

Dry piles were one of the earliest kinds of battery. The type most people associate with them was developed by the Italian physicist Giuseppe Zamboni in the early 1800s. His versions were made from alternating layers of tin foil and paper coated with zinc sulfate and manganese dioxide, sometimes thousands of layers stacked into a column.

The Oxford piles are thought to be Zamboni-style, but nobody can confirm it. In a 1984 paper in the European Journal of Physics, Oxford researcher A.J. Croft explained that the exact contents are unknown, though the outer coating is clearly sulfur, which seals everything in.

The obvious fix would be to cut one open and look. But the researchers’ worry is that opening it would ruin the experiment of seeing how long it lasts. So the answer will probably have to wait until the bell finally goes quiet on its own.

It still has its off days

“Almost nonstop” is the honest way to put it. The bell doesn’t love damp weather, and Oxford has plenty of that. When humidity climbs, the clapper can slow down or stop entirely, then start again by itself once the air dries out. Nobody needs to give it a push.

The bell has also had at least one useful side effect. According to Croft, it inspired an Oxford physicist during World War II to dig up an old recipe for a similar battery, which was then used to power infrared telescopes for the British military.

What it says about the battery in your pocket

It’s tempting to ask why modern batteries can’t last like this. The short answer is that they’re built for a completely different job. A phone or laptop battery has to deliver a lot of current, fast, and be recharged hundreds of times. Every charge cycle wears down its chemistry a little, which is why most phone batteries noticeably lose capacity after a few years.

The Oxford bell does the opposite. It barely asks anything of its battery, never recharges it, and sits in a quiet case where nothing bumps it. Engineers already use the same basic trade-off in devices that sip tiny amounts of power, like smoke detectors and some medical implants, where a battery can last a decade or more.

So the bell isn’t a lost secret that could fix your phone. What it shows is how far a battery can go when the device it powers asks for almost nothing. Smithsonian put it simply: the mystery will likely stay unsolved until the battery dies or the mechanism breaks. Until then, somewhere in Oxford, it’s still ringing twice a second, too quietly for anyone to hear.