And we won't find out how or why until it finally goes out.

In a world increasingly obsessed with energy-efficient and long-lasting batteries, a nondescript bell at the University of Oxford's Clarendon Laboratory has been gently chiming since 1840 as one of the world's longest-running battery-powered science experiments.

Shortly after America declared independence, electricity was still a largely unknown factor and was considered to have sources in animals, lightning, static, and metals. Alessandro Volta invented the "voltaic pile" in 1800, an electrostatic battery created by layering ingredients like copper, zinc, and cardboard soaked in salty brine or vinegar.

Volta showcased his device around for public and private presentations, a form of scientific entertainment popular at the time. In a matter of months, William Nicholson and Anthony Carlisle used the mechanism to divide water into its basic components – hydrogen and oxygen – ushering in the new branch of science called electrochemistry.

Dry pile batteries of the day consisted 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. The piles, of course, are not dry, but contain the right amount of water to provide the electrolyte without causing a short-circuit," explained AJ Croft, a former researcher at the Clarendon Laboratory.

The Oxford Electric Bell

DavidCWG; CC 3.0

Reverend Robert Walker purchased the Oxford Electric Bell in 1840, which wound up on display in Oxford, where he was a professor.

The bell didn't begin its life as an experiment but as a novelty manufactured by London scientific instrument makers Watkins and Hill. Before the study of physics had its name, Reverend Robert Walker, Reader in Experimental Philosophy, purchased the device in 1840, which wound up on display in Oxford, where he was a professor.

The batteries in Oxford are also sealed within an outer coating believed to be sulfur, which ends up making them look more like a pair of candles whose interior composition is otherwise still a mystery, though scientists have some ideas. These batteries are called dry piles due to their resemblance to those constructed by Italian priest Giuseppe Zamboni.

Zamboni's electrostatic batteries were constructed from discs of various metals whose combinations were assembled in stacks, sometimes several thousand layers thick, and then either compressed in a glass tube or stacked between glass rods and dipped in molten sulfur or pitch as insulation.

The Oxford Electric Bell's construction consists of a tiny 0.15-inch (4 millimeters) clapper that oscillates perpetually between the lips of two bells through an electrostatic force sustained by the ringing. When the clapper strikes one bell, it acquires a charge from the corresponding pile and is electrostatically repelled, sending the clapper toward the other bell, where it picks up a charge from that bell's respective pile, and so on, cycling continuously.

The experiment

"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," Croft wrote in a 1984 paper published in the European Journal of Physics.

"The residual electric power sufficient to keep up the ringing of the bells seldom lasts longer than three or four years ... it is a pretty apparatus but alas very transient in its working power," the makers themselves penned in a letter.

And yet, in a glass display case and under a domed cloche or cover, the Oxford Electric Bell has been chiming steadily all this time, but the voltage left in the battery is so low that the ringing is now too faint for the human ear to hear. Instead, the metal clapper hangs from a fine wire, appearing to tremble back and forth between the two bells, pulling what Croft says is one nano ampere each time it oscillates between the bells' surfaces, which is a minuscule amount of current.

"And it charges and discharges continuously," Robert Taylor explained in an interview with BBC Radio Oxford. "A small amount of charge trickles between the two ends and the only loss, basically, is the resistance of air."

It is suspected that the battery's efficiency and longevity are in part due to the only loss being through finite air resistance for the tiny lead clapper charging and discharging as it swings in the tight space between the extremely close bells, "and occasionally that it slows down if the humidity is too high," Taylor concluded.

Solving the mystery

Dry pile batteries are far from being simple novelties. During World War II, infrared telescopes were powered using them as portable, low-current electricity sources. Croft wrote that an Oxford physicist, inspired by the bell, looked up a recipe for a similar battery from the era to power the telescopes.

Croft theorizes that "the clapper seems more likely to wear out than the electro-chemical energy. At that point, we might finally learn what's been powering the thing."

With the increased expense of battery materials and the overly slow ramp-up of global mining operations amidst increasing technological demand, old-fashioned and alternative battery chemistries have recently had a habit of resurfacing, from sodium-metal-halide to water-based redox flow.

Science doesn't yet know what exactly is inside the Oxford Electric Bell and, thus, how those ingredients have contributed to its longevity or how it might inspire tomorrow's electric battery chemistries. And we won't until it finally stops.

"It'll keep going for about another five or 10 years possibly, given the fact that it has actually slowed down noticeably in the last 40 years," said Taylor. "The reason it'll stop, of course, it'll run out of energy. All batteries eventually run out of energy."

Even if it is 200 years later.