Through the eighteenth century electricity was a matter of demonstration: friction machines and Leyden jars accumulated charge that discharged in a single spark. Measurement was nearly impossible, because the apparatus had to be built before the event was over.

The argument began with a frog's leg. Luigi Galvani observed a severed leg contracting when touched by two different metals and attributed it to an 'animal electricity' peculiar to living tissue. Volta disagreed: what produced the electricity was the contact of two dissimilar metals, with the tissue serving only as conductor and indicator. To prove it he built an apparatus containing nothing living — zinc and copper discs stacked with brine-soaked card between them. Reported to the Royal Society in 1800, this 'pile' was the first source of continuous current.

Consequences came quickly. That same year William Nicholson and Anthony Carlisle used the current to split water into hydrogen and oxygen, founding electrochemistry; Humphry Davy isolated potassium and sodium by the same means in 1807. Without continuous current there is no Ørsted deflecting a magnetic needle in 1820, no Faraday induction, no telegraph. The unit of potential carries Volta's name today — and Galvani was not simply wrong: that the nervous system really is electrical was confirmed a century later, and each man founded a field.