The original form of the device is just a glass bottle partially filled with water, with a metal wire passing through a cork closing it. The role of the outer plate is provided by the hand of the experimenter. Soon John Bevis found (in 1747) that it was possible to coat the exterior of the jar with metal foil, and he also found that he could achieve the same effect by using a plate of glass with metal foil on both sides. These developments inspired William Watson in the same year to have a jar made with a metal foil lining both inside and outside, dropping the use of water.