Between our look at the longest prison sentences the other day and the 69-year-old pitch drop experiment finally getting caught on camera last month, reader Justin got curious and wrote in to ask, “What’s the longest experiment that scientists have filled their decades or lifetimes with?”

While the pitch drop gets the nod for longest uninterrupted duration, there are at least two projects that started before it and keep going today, but have had some stops and starts along the way. The older of the two, and grand champion for years-in-progress, is the Oxford Electric Bell, a.k.a. the Clarendon Dry Pile.

The bell, as the name suggests, is an experimental electric bell kept at the University of Oxford’s Clarendon Library. It was built by Watkin and Hill, an instrument-making firm in London, and purchased by Robert Walker, a professor at Oxford. In 1840, he set it ringing. Today the bell still tolls.

The bell is actually two metal bells, with a metal clapper set between them. The clapper is powered by two “dry piles,” an early form of battery. Dry piles were normally composed of alternating strips of metal foil and paper—sometimes hundreds or thousands of layers thick—like electric club sandwiches. A variety of metals could be used, but Watkin and Hill left no record of what their piles were made of.

Scientists are eager to find out just how long the mystery battery can go, and then open it up and find out what it's made of, but the whole thing is a bit of a waiting game. Whatever its makers used, the device has some staying power. Guinness World Records called the bell’s dry piles the “world’s most durable battery,” and for one hundred and seventy three years, minus occasional interruptions, the bell has been ringing.

The clapper oscillates between the two bells at a usual frequency of 2Hz, or two cycles per second, depending on the weather. High humidity can cause the clapper’s movement to slow and even stop, but when the humidity drops the bell can begin again without external intervention. As the clapper strikes and rings one bell, the corresponding dry pile charges and electrostatically repels it. The clapper then swings toward the other bell, and the same thing happens.

Because there’s just little bits of energy being discharged through the process, the drain on the battery—whatever it's made of—is very small, so it can happen again and again and again, causing a continuous ring. If we fudge a little and say that the clapper has had a 2Hz frequency for the entire 173 years, that means it’s made a whopping 10,911,456,000 strikes against those bells.

Eventually, the electrochemical energy of the dry cells will be exhausted and the bell will go quiet. Not knowing what powers the contraption, though, no one is sure when that will happen, and silence instead could come when the clapper or one of the bells wears out. Not that anyone can hear it, anyway: To keep the patrons of the Clarendon Library from going mad from the noise, the bell is kept encased in sound-damping glass.

The second longest-running experiment is an experimental clock (called the Beverly Clock) in New Zealand that's been ticking since 1864 without needing to be wound, and is driven by variations in atmospheric pressure and temperature.