Catching and storing electricity is harder than catching fireflies, but both often involve jars! The first device invented that could acquire electric charge and store it until a scientist wanted to use it for an experiment or demonstration was a Leyden jar. Named for one of the universities (University of Leiden) at which it was first used for research, the Leyden jar was developed independently in the middle of the 18th century by E. Georg von Kleist in Germany and by Pieter van Musschenbroek in the Netherlands.
Leyden jars evolved over time into more complex, more efficient devices, but the earliest versions were very basic. These consisted of glass bell jars coated with metal foil on their inner and outer surfaces. The jars were filled with water (or sometimes beer!) and featured a stopper with a wire running through it. The free end of the wire allowed the device to be hooked up to an electrostatic generator, which served as a quick and easy source of electricity. The somewhat more advanced model shown here features an exterior metal sphere linked to the inside of the Leyden jar by a brass rod terminating in a chain rather than a simple wire lead.
At first people thought the liquid in the Leyden jar stored electricity, but through experimentation scientists discovered that water was not necessary. Later models were empty, but still worked, because the basic requirement for a functioning Leyden jar is the presence of two conductors separated by an insulator. Typically two layers of metal foil serve as the conductors in a Leyden jar and the glass is the insulator, so there was no need for water (also a conductor). When used with an electrostatic generator, charge flows from the generator to the metal sphere or other lead in a Leyden jar and builds up in the interior layer of conductive foil because it is blocked from further movement by the insulating glass. At the same time, the outer foil layer builds up an opposite charge from the ground because positively charged particles are attracted to the negative particles accumulating inside the jar. Until released in some way, the equal but opposite charges on the opposing sides of a Leyden jar remain in place.
To release the electricity in a Leyden jar, a pathway must be provided along which the stored charge can travel. A wire, rod or even the human hand can be used for this. In fact, one of Benjamin Franklin’s favorite electrical demonstrations involved many people standing in a circle holding hands. One would be asked to touch the top of a charged Leyden jar, while the person at the end of the chain was asked to touch the bottom of the same jar with his free hand. Electricity flowed through every member of the group, each feeling an accompanying shock. Such demonstrations were quite dangerous, but were nevertheless popular.
As the first capacitors, Leyden jars could also be used for more practical endeavors. Knowledge of the jars quickly spread to laboratories around the globe, facilitating more electrical research than had been possible. This research also became mobile: Thanks to “electricity in a bottle,” scientists could take their electrical experiments outside and on the road. Later capacitors served as important components in electronics, such as lights and radio.