was the first to employ the "prime conductor" in such machines, this consisting of an iron rod held in the hand of a person whose body was insulated by standing on a block of resin. In 1746, William Watson's machine had a large wheel turning several glass globes, with a sword and a gun barrel suspended from silk cords for its prime conductors. Johann Heinrich Winckler, professor of physics at Leipzig, substituted a leather cushion for the hand. During 1746, Jan Ingenhousz invented electric machines made of plate glass. Experiments with the electric machine were largely aided by the discovery of the Leyden Jar. This early form of the capacitor, with conductive coatings on either side of the glass, can accumulate a charge of electricity when connected with a source of electromotive force. The electric machine was soon further improved by Andrew (Andreas) Gordon, a Scotsman and professor at Erfurt, who substituted a glass cylinder in place of a glass globe; and by Giessing of Leipzig who added a "rubber" consisting of a cushion of woollen material. The collector, consisting of a series of metal points, was added to the machine by Benjamin Wilson about 1746, and in 1762, John Canton of England (also the inventor of the first pith-ball electroscope) improved the efficiency of electric machines by sprinkling an amalgam of tin over the surface of the rubber. In 1768, Jesse Ramsden constructed a widely used version of a plate electrical generator. In 1783, Dutch scientist Martin van Marum of Haarlem designed a large electrostatic machine of high quality with glass disks 1.65 meters in diameter for his experiments. Capable of producing voltage with either polarity, it was built under his supervision by John Cuthbertson of Amsterdam the following year. The generator is currently on display at the Teylers Museum in Haarlem. In 1785, N. Rouland constructed a silk-belted machine that rubbed two grounded tubes covered with hare fur. Edward Nairne developed an electrostatic generator for medical purposes in 1787 that had the ability to generate either positive or negative electricity, the first of these being collected from the prime conductor carrying the collecting points and the second from another prime conductor carrying the friction pad. The Winter machine possessed higher efficiency than earlier friction machines. In the 1830s, Georg Ohm possessed a machine similar to the Van Marum machine for his research (which is now at the Deutsches