History of electrochemistry
In 1789, Franz Aepinus developed a device with the properties of a "condenser" (now known as a capacitor.) The Aepinus condenser was the first capacitor developed after the Leyden jar, and was used to demonstrate conduction and induction. The device was constructed so that the space between two plates could be adjusted, and the glass dielectric separating the two plates could be removed or replaced with other materials.
Experiments and Observations on Electricity
Franklin's first experiments explained in this first set of letters to Collinson that the Leyden jar, a type of capacitor, had equal and opposite charges on the inner and outer conductors. This was shown in illustrations in his book. The significant Franklin theory introduced to Collinson in a letter dated April 29, 1747, was the concept of the "dissectible condenser." Today we call this distribution of charges in a device, a capacitor. He observed that a charge was built up on both sides between a piece of nonconducting material. In the case of the Leyden jar this was glass (nonconducting matter) and on each side of it was a metal material (conducting matter). These opposite metal layers had the exact opposite electric charges (positive and negative). He observed that when the wire on the inside of the jar was "electrised" positively by a certain amount, that the outer conductor simultaneously became "electrised" negatively in the same proportion.