Leyden Jar - Storage of The Charge

Storage of The Charge

It was initially believed that the charge was stored in the water in early Leyden jars. Franklin's investigations led him to conclude that the charge was stored in the glass, not in the water, as others had assumed. A popular experiment which seems to demonstrate this involves taking one apart after it has been charged and showing that the charge is stored on the dielectric, not the plates. The first documented instance of this demonstration is in a 1749 letter by Franklin. Franklin designed a "dissectible" Leyden jar, which was widely used in demonstrations. The jar in the demonstration is constructed out of a glass cup nested between two fairly snugly fitting metal cups. When the jar is charged with a high voltage and carefully dismantled, it is discovered that all the parts may be freely handled without discharging the jar. If the pieces are re-assembled, a large spark may still be obtained.

This demonstration appears to suggest that capacitors store their charge inside their dielectric. This theory was taught throughout the 1800s. However, this phenomenon is a special effect caused by the high voltage on the Leyden jar.In the dissectible Leyden jar, charge is transferred to the surface of the glass cup by corona discharge when the jar is disassembled; this is the source of the residual charge after the jar is reassembled. Handling the cup while disassembled does not provide enough contact to remove all the surface charge. Soda glass is hygroscopic and forms a partially conductive coating on its surface, which holds the charge. Addenbrook (1922) found that in a dissectible jar made of paraffin wax, or glass baked to remove moisture, the charge remained on the metal plates. Zeleny (1944) confirmed these results and observed the corona charge transfer.In capacitors generally, the charge is not stored in the dielectric, but on the inside surfaces of the plates, as can be observed from capacitors that can function with a vacuum between their plates.