Science fact - 17

Never Ending Battery

For over 170 years, the Oxford Electric Bell (also known as the Clarendon Dry Pile) has been chiming almost continuously, the composition of its power source uncertain. 

The Oxford Electric Bell is an experimental electric bell that was set up in 1840 and which has run almost continuously ever since, apart from occasional short interruptions caused by high humidity. It was "one of the first pieces" purchased for a collection of apparatus by clergyman and physicist Robert Walker. It is usually located in the foyer of the Clarendon Laboratory at the University of Oxford, England, but as of December 2009 it has been moved into an adjacent corridor due to construction work, and is still ringing, though inaudibly, because it is behind two layers of glass.

The Design:

The experiment consists of two brass bells, each positioned beneath a dry pile (a form of battery), the pair of piles connected in series. A metal sphere approximately 4 mm in diameter is suspended between the piles, and rings the bells by means of electrostatic force. As the clapper touches one bell, it is charged by one pile, and then electrostatically repelled, being attracted to the other bell. On hitting the other bell, the process repeats. The use of electrostatic forces means that while high voltage is required to create motion, only a tiny amount of charge is carried from one bell to the other, which is why the piles have been able to last since the apparatus was set up. Its oscillation frequency is 2 hertz.

What is most interesting, and mysterious, about the apparatus is the internal composition of the 'dry pile' batteries. It is known that they have been coated with molten sulphur to prevent effects from atmospheric moisture and it is thought that they may be Zamboni piles.

At one point this sort of device played an important role in distinguishing between two different theories of electrical action: the theory of contact tension (an obsolete scientific theory based on then-prevailing electrostatic principles) and the theory of chemical action. The Oxford Electric Bell does not demonstrate perpetual motion. The bell will eventually stop when the dry piles are depleted of charge – that is, if the clapper does not wear out first.