The Leyden jar is variously called a condenser or capacitor, and the reasons for those two names become obvious when one understands the logic of its operation. The earliest Leyden jars of the mid-eighteenth century consisted of a glass bottle fitted with a cork and filled with water. A copper wire, which was immersed in the water, ran through the cork and was held to a machine generating (let us say) negative charge. Today, we know that this means the generator is sending electrons flowing through the wire and the water, creating a negative charge on the inside of the glass. If the bottle were insulated from the earth, one would soon reach a point at which no more negative charge or electrons could be absorbed by the jar. But with an experimenter holding the outside of the bottle, the results were quite different. Since like charges repel, the force of the negative charges on the inside of the bottle would push electrons out from the experimenter's palm and the adjacent glass and ultimately into the earth. The positive charge remaining on the experimenter's hand and adjacent glass would pull on the electrons inside the glass, packing them together more tightly and making room for still more electrons and a greater negative charge.
Because the Leyden jar worked by squeezing the electrons or negative charge more closely together, this type of device later came to be called a condenser. (The term seems to have originated with Alessandro Volta in approximately 1780). Because the result of the process was an increase in the jar's capacity to absorb charge, the more modern term for such a device is a capacitor
But even without understanding how the Leyden jar worked, several improvements were made to it. Rather than using the glass itself as both the bearer of charge and the insulator between the two types of charge, William Watson lined the glass jar inside and out with metal foil; the glass then served principally as an insulator. The copper wire through which charge was entering was attached directly to the foil by a metal wire rather than by water.
In a letter of April 1748, "Franklin described some new experiments showing that a charged Leyden jar always has charges of opposite signs on the two conductors and that the charges are of the same magnitude." (I.B. Cohen, "Franklin," The Dictionary of Scientific Biography, p. 131) In that sense, Franklin said, experimenters did not "charge" and "discharge" the Leyden jar. It contained a certain amount of electrical fluid before "charging" and an equal amount afterward. "Charging" the jar simply meant redistributing the fluid, and "discharging" was required because the original equilibrium could not be restored by having electrical fluid pass through the bottle (nor, practically speaking, over the lip of the bottle). It could be restored only through a conducting connection between the outside and inside.
Using an early type of Leyden jar, without foil lining, Franklin then announced the most surprising discovery of all. "The whole force of the bottle, and power of giving a shock, is in the GLASS ITSELF." Franklin proved this by eliminating all the other candidates.
First, he set his Leyden jar on a glass insulator. After he had carefully removed the cork and wire, the jar could still be discharged by having the experimenter touch the outside and the water. Next, he carefully poured off the water from a charged Leyden jar into an empty uncharged jar resting on glass. This jar gave no indication of having received a power to shock. He then refilled the empty Leyden jar with an equal amount of pure water, and discovered that the jar retained the power to shock. Clearly, the glass itself must be implicated.
For a final experiment with Leyden jars, Franklin asked whether the charge on the jar was influenced by shape. This was not so absurd as it may seem. Franklin's first discovery, remember, had been that the shape of conductors had an effect on their ability to discharge an electrified body. So the question about the shape of the jar was hardly silly. To answer the question, Franklin constructed a condensor consisting of two lead plates separated by a flat sheet of glass. It produced the same effect as Leyden jar. Franklin then made a series of eleven such flat condensers and linked up the lead plates with a wire, creating what he was the first to called an "electrical-battery.