How does the Oxford Electric Bell work, and why is it considered one of the strangest experiments in the world?

 In an Oxford lab, a tiny metal sphere strikes a brass bell. It has repeated this exact motion 10 billion times since 1840, powered by a battery scientists can't even open.

Known as the Oxford Electric Bell, this quiet, ceaseless chiming is one of the longest-running scientific experiments in existence. It operates on the principles of electrostatic attraction. The mechanical setup is remarkably elegant:

• The Power Source: Above each of the two brass bells sits a "dry pile," an early type of battery.
• The Clapper: A tiny metal sphere hangs suspended by a thread precisely between the two bells.
• The Motion: The dry piles generate a high voltage but an extremely low current. The clapper is electrostatically attracted to the first bell. Upon striking the brass, the clapper absorbs a tiny electrical charge and is immediately repelled. It swings across the gap to the second bell, strikes it, transfers the charge, and is repelled back.

This pendulum-like cycle repeats at a frequency of about 2 hertz.

The reason the bell is considered one of the strangest experiments in the world lies in its sheer endurance and the mystery surrounding its batteries. The batteries are "Zamboni piles," which typically consist of thousands of alternating discs of silver foil, zinc foil, and paper. However, the exact chemical composition of the Oxford piles remains completely unknown. The exterior of the batteries is coated in a thick, solid layer of melted sulfur to seal out atmospheric moisture, hiding the internal components from view.

Because cutting open the sulfur casing would destroy the experiment, scientists are forced to wait until the battery finally dies to find out exactly what is inside. The batteries have lasted for nearly two centuries because the system draws a practically imperceptible amount of current—only a fraction of a nanoampere with each swing of the clapper. In fact, physicists suspect that the mechanical clapper will eventually wear out and break before the electrochemical energy inside the dry piles actually runs dry.

Today, the entire apparatus is encased behind a double layer of thick glass to muffle the constant ringing. It holds the Guinness World Record for the world's most durable battery, quietly chiming away as a fascinating relic of 19th-century science.