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Storing Electricity — The Leyden Jar
For the first time, a substantial static charge could be kept and released later
The early friction machines solved one problem: they allowed experimenters to generate static electricity repeatedly. Discoveries about conductors and insulators solved another problem: charge could be carried where it was wanted and kept from escaping too quickly.
The next challenge was storage.
The first attempts did not look like a modern capacitor
Early investigators did not know what part of a storage device would actually hold the electrical effect. A reasonable idea was that electricity might somehow be collected in water.
An early arrangement therefore used a glass bottle or jar containing water. A metal wire or rod passed through the stopper and contacted the water. The experimenter held the outside of the glass container while connecting the inner conductor to an electrostatic machine.
| Part of the early experiment | What it did in modern terms |
|---|---|
| Water or conducting material inside the jar | Formed one conducting surface. |
| The experimenter's hand around the outside | Acted as the second conducting surface and connected it approximately to ground through the body. |
| Glass wall of the jar | Kept the two conductors separated; today we call this the dielectric. |
| Metal wire or rod | Provided a connection to the inside conductor. |
A discovery memorable for the shock
During 1745–1746, experiments in Germany and the Netherlands independently led to the discovery that this jar arrangement could store a surprisingly powerful electrical effect. Ewald Georg von Kleist in Pomerania and investigators associated with Pieter van Musschenbroek at Leiden are both part of the discovery story.
When the charged inner conductor was touched while the experimenter was also connected to the outside of the jar, the stored charge could discharge through the body. The resulting shock was far stronger than the little static sparks experimenters were accustomed to.
The water was not the important part
At first it was natural to imagine that electricity itself was being stored in the water. Further experiments showed that water was not essential.
The effective arrangement was really:
Later Leyden jars commonly used metal foil on both the inside and outside of the glass jar. A metal rod connected to the inner foil, while the outer foil was held or connected to ground.
In modern electrical language
A Leyden jar is a capacitor.
- The inner metal coating is one plate.
- The outer metal coating is the other plate.
- The glass between them is the dielectric.
- Opposite charges accumulate on the two conducting surfaces.
- Energy is stored in the electric field associated with the separated charges.
Measurement trail 3 — capacitance is here, but its unit must wait
Today we describe a Leyden jar by its capacitance: how much separated charge it can hold for a given electrical potential difference. The modern unit is the farad (F).
But we will deliberately not define one farad yet. Its definition requires two quantities we have not introduced in our measurement trail: the coulomb of charge and the volt of potential difference. Once those are established, we can return to the Leyden jar and the farad will make immediate sense.
Why two conductors?
This is an important point. A Leyden jar is not simply a bottle full of excess electricity. It works because charge is separated across an insulating barrier.
As charge is placed on the inner conductor, the electrical field affects charge on the outer conductor. If the outer conductor has a path to Earth, charge can move to or from it while the jar is being charged. The glass prevents the two sides from simply recombining.
When a conducting path is finally provided between the inner and outer conductors, charge moves rapidly and the stored energy is released.
Charge it — isolate it — discharge it
- Connect the inner conductor to an electrostatic generator.
- Allow the generator to build separated charge on the two sides of the glass.
- Remove the charging connection.
- The jar can retain a substantial electrical condition for a time.
- Connect the inner and outer conductors together.
- A rapid discharge can produce a spark and a strong shock.
A dramatic increase in what experimenters could do
Before the Leyden jar, an experimenter largely had to use charge as it was being generated. After the Leyden jar, charge from many turns of a friction machine could be accumulated and then released in one event.
That made possible much more dramatic experiments:
- larger sparks;
- stronger shocks;
- discharges through longer conducting paths;
- comparisons between different jar sizes and arrangements;
- and experiments involving several jars connected together.
generate energy now, store it, and release it later.
Several jars could be combined
Experimenters soon found that multiple Leyden jars could be connected together to increase the amount of stored electrical effect. Groups of jars became important research and demonstration apparatus.
This is also where a familiar electrical word begins to acquire a new meaning. Benjamin Franklin would later use the word battery for an arrangement of several Leyden jars, by analogy with a battery of artillery pieces acting together. The word would later be applied to groups of electrochemical cells as well.
What the Leyden jar did not yet solve
The jar stored static electricity, but it did not provide a steady continuous current. After discharge it had to be charged again.
Researchers still disagreed about what electricity actually was, how many kinds existed, how charge moved, and how the enormous electrical effects of thunderstorms related to the sparks made in a laboratory.