The Enigma of Volta’s “Contact Tension” and the Development of the “Dry Pile”

The collection of the Clarendon Laboratory in Oxford possesses a remarkable instrument that has been ringing almost continuously since it was purchased from Watkins & Hill of London in 1840 (figure 1).1 A tiny brass clapper suspended by a silk fibre moves to and fro between the bells attached to the base of two vertical pillars, known as “dry piles”. These are connected in series, sealed in a coating of sulphur, and housed under a glass dome to keep out the dust. Its fame has secured it a place in the Guinness Book of Records. This device closely resembles the one described by George John Singer in his textbook of 1814 (figure 3).2 The dry pile, although it became to be regarded as a novelty, also had its brief moment of glory in the debate concerning the relationship between “classical ” high-tension electrostatic experiments and Volta’s low-tension experiments with the voltaic pile. The debate over the dry pile highlights the nature of scientific evidence and the key role of instruments in this debate. 2. The Origins of the Dry Pile The dry pile grew out of the debate between those that ascribed the electrical behaviour of the “wet ” voltaic pile either to “contact tension ” (the “Volta effect”) or “chemical action”. Helge Kragh and Nahum Kipnis have described in detail this lengthy controversy with its ramifications in the first3 and in the present volume of the Nuova Voltiana series. Instrumental evidence figured largely in this debate, as it did in Volta’s original discovery of his pile. A key aspect that made the contact theory popular was that it kept the explanation of the pile in familiar territory, that is, in terms of the high-tension static electricity of the phenomena associated with the frictional electrical machine. In thes