How a glass jar could hold a charge
A Leyden jar stored electricity by keeping opposite charges apart. Metal lining the inside and outside of the glass acted as two conductors; the glass between them was an insulator, a material that resists the movement of charge. When an electrostatic machine supplied charge to the inner lining, the outer lining acquired an opposite charge. The glass itself did not let the charges flow together, so energy accumulated in the arrangement.
That separation is the basic idea behind a capacitor, a device that stores energy in an electric field between conductors. Unlike a battery, a charged jar could not provide a steady current for long. It held a brief, concentrated release instead.
A person could complete the circuit
The jar was dangerous because its charge had nowhere to go until a conducting path connected the two sides. The nail reached the charged inner lining. Touching it while also connected—directly or indirectly—to the outer lining let charge move through the body from one side to the other. That sudden discharge produced the jolt.
The shock was not electricity appearing from nowhere when the nail was touched. The charge had been built up beforehand; contact supplied the missing route for it to rush around the circuit. A modern capacitor works on the same principle, though its size and construction determine how much energy it can store and release.
From startling trick to useful tool
The Leyden jar made it possible to collect and release more static electricity than a simple rubbed object could hold at once. That made electrical effects easier to demonstrate and investigate—and made the danger of handling a charged apparatus unmistakable. Its lasting importance lies in that shift: electricity could be stored, then released on demand, a principle that became essential to countless later devices.
