 John Gray also invented a belt machine about 1890.Another more complicated belt machine was invented during 1903 by Juan Burboa A more immediate inspiration for Van de Graaff was a generator W. F. G. Swann was developing during the 1920s in which charge was transported to an electrode by falling metal balls, thus returning to the principle of the Kelvin water dropper.The reason that the charge extracted from the belt moves to the outside of the sphere electrode, though it already has a high charge of the same polarity, is explained by the Faraday ice pail experiment.