The leyden jar
We are now in a position to understand why Musschenbroek received so severe a shock from the Leyden jar described on page 389. His hand, holding the outside of the bottle, acted as an earthed conductor separated by a glass dielectric from the conducting water inside. The arrangement thus formed a capacitor. Later on, the use of water inside the jar was abandoned and the inside and outside of the jar were covered with lead foil to serve as the two conductors. The outer covering is automatically earthed when the bottle stands on a table. Fig. 33.8 (b) illustrates the construction of a modern form of Leyden jar. Leyden jars were used a great deal in eighteenth-century electrical experiments.
During this period, electrical demonstrations became a popular form of entertainment. Rats and mice were killed and gunpowder and alcohol ignited by electric spark discharges. On one occasion in France the Abbe Nollet demonstrated the conducting properties of the human body by discharging a Leyden jar through a battalion of Guardsmen joined hand to hand. On a subsequent occasion the same experiment was repeated at a monastery in Paris. This time the human chain was formed by obedient monks. It is reported that they all leapt some distance into the air.
Nowadays, Leyden jars are rarely found in use outside elementary teaching laboratories, but they were used in the early days of wireless telegraphy in connection with morse transmitters.