Resumen
Galvani discovered that contact with metals could produce muscular contractions in frogs. He interpreted the phenomenon as due to an invisible fluid, called ?animal electricity?. Volta was convinced that those phenomena were due to common electricity produced by the contact between two different metals. As an evidence of his points of view he showed that weak electricity produced by a pair of metals could be amplified with an apparatus called condenser, allowing its detection in electrometers. However, there was no general acceptance of his ideas, because all the evidences provided were indirect. The invention of the pile allowed Volta to produce effects that were much stronger. Nevertheless, some well-grounded doubts subsisted concerning the identity between electricity produced by friction and the effects produced by piles. This paper describes that historical episode and discusses the methodological issue of the foundations of claims of identity two similar phenomena.