Leyden Jar

The Leyden jar is the first capacitor — a device that stores electric charge and holds it until you let it out. A glass jar is coated with metal foil inside and out; the two foil layers, kept apart by the glass, hold opposite charges that attract each other across the barrier and so pack in far more charge than a bare wire ever could. A rod through the lid carries charge down to the inner foil, and touching that rod and the outer foil at once dumps the whole store through you in a single violent snap.

Before the jar, static electricity was fleeting: you could strike a spark, but you could not keep one. Storable charge turned electricity into something you could accumulate, measure, compare, and release on command — and the jolt was strong enough to be felt through a room-long chain of people holding hands. In a single stroke it made electricity both a serious laboratory subject and a public sensation.

It was discovered twice, by accident, in 1745–46: by the Pomeranian cleric Ewald von Kleist, and independently at the University of Leiden, where Pieter van Musschenbroek's assistant Andreas Cunaeus took a shock so brutal that Musschenbroek wrote he would not repeat it "for the whole kingdom of France." The earliest jars were glass vials half-filled with water, gripped in the hand, with a wire pushed through the cork into the water; only later did experimenters replace the water and the hand with metal foil bonded to both faces of the glass. See the recipe for a modern, foil-lined version.

Quantitative electrical science. Benjamin Franklin used Leyden jars to prove lightning was electrical and to define positive and negative charge, and the abbé Nollet discharged one through a chain of 200 monks to show that electricity travels near-instantly. As a store of charge it is the direct ancestor of every capacitor built since, and it formed much of the accumulated understanding that Alessandro Volta drew on when he built the first battery.