The Oxford Electric Bell or Clarendon Dry Pile 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 located in a corridor adjacent to the foyer of the Clarendon Laboratory at the University of Oxford, England, and is still ringing, though inaudibly, because it is behind two layers of glass.
“The Clarendon Dry Pile was purchased by Robert Walker (Professor of physics 1839 – 1865) and bears the label in his handwriting “Set up in 1840,” though a later note indicates that it may have been constructed some 15 years earlier. It consists of two voltaic “dry-piles,” covered with an insulating layer of sulphur, connected in series and, at their lower ends, to two bells. Between the bells is suspended a metal sphere about 4mm in diameter which is attracted alternately by the bells and transfers charge from one to the other. The frequency of its oscillation is about 2Hz; so far the bells have been rung of the order of 10 billion times.
The internal construction of the piles themselves remains a matter for conjecture, but records of similar popular curiosities of the period, e.g. Zamboni piles, indicate that they are probably of alternate layers of metal foil and paper coated with manganese dioxide.
Some published reports of the Pile unfortunately refer to it as an example of perpetual motion but the Guinness Book of Records has it under the ‘world’s most durable battery’ delivering ‘ceaseless tintinnabulation.’ It is seen but not heard as the ringing is muffled, in the ground floor display cabinet near the main entrance of the Clarendon Laboratory.”(University of Oxford, Department of Physics)
The experiment consists of two brass bells, each positioned beneath a dry pile, 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.
The exact composition of the dry piles is unknown, but 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 if the clapper does not wear out first. (Wikipedia)