A leyden jar is a mechanism which can store electric charge. In the early 1700's people who experimented with electrcity had no way of storing the high-voltage electric currents that they produced. In the 1750's this a way of storing these electric currents was found. The leyden jar was the solution. Named for the University
of Leyden in Holland, the leyden jar allowed elctric currents to be moved to other places to be used. People began using leyden jars in the construction of frictional static machines to make longer and larger sparks.
In the mid 1850's, Lord Kelvin of England first observed the way that the discharged electricity oscillated when passed into an inductor. The subsequent work of scientists Mahlon Loomis, James Clerk Maxwell and Nikola Tesla led to the creation of radio, television and communications that we use today.
We now know that Ben Franklin's early experiments with leyden jars were actually producing radio waves.
Are they related to a capacitor?
Leyden jars are related to capacitors in that they are both made to store electricity, static electricity to be specific. They are both often used in scientific experiments.
What is a capacitator?
A capacitor is much like a leyden jar in that it consists of one container with two oppositely charged ends, much like a battery. Static electricty can be both caught in the jar and stored until both ends are touched, which would cause the capacitor to let its stored electricity free.