Oxford Electric Bell

A pair of bells in Oxford's Clarendon Laboratory have been ringing, powered by a dry pile battery, since 1840. The sound is too faint to hear.

Context

The Oxford Electric Bell is kept in a glass case in the Clarendon Laboratory, part of the University of Oxford's physics department. Two bells sit side by side with a small metal ball hanging between them. The ball swings back and forth, touching each bell in turn, and each time it touches it picks up electric charge from one bell and is pushed to the other.

The power source is a dry pile, an early type of battery made of many stacked layers, often credited to the Italian priest and physicist Giuseppe Zamboni. Such a battery gives only a very small current but can last a very long time. The device is thought to have been set up in 1840, and it has been running almost without a break since then, although the exact origin and maker are not fully documented.

The sound is far too quiet to hear through the glass, but the movement of the ball can be seen. The bell has rung billions of times by some counts.

It is not truly perpetual motion. The pile gradually uses up its chemical reserves, and scientists expect it will eventually stop, though nobody can say precisely when.