When a considerable quantity of static electricity is required an electric machine is used. The earliest machines which were constructed in the seventeenth and eighteenth centuries worked by friction. For example, Otto von Guericke used a rotating
sulphur-ball to which a dry hand was applied to produce the friction. Later experimenters used glass spheres or discs which were rotated against pads made of wool or leather.
These friction machines were not very satisfactory, and after Volta’s invention of the electrophorus, scientists of the nineteenth century turned their attention to induction machines. Very good machines of this type were made by the German workers Holtz, Voss, and Toepler, but the most efficient one was designed in 1883 by an Englishman named James Wimshurst, who was a consulting engineer to the Board of Trade. The Wimshurst machine, illustrated in Fig. 32.15 is often used in elementary science laboratories. The action of this machine is described in more advanced textbooks.
About 1930 Van de Graaff in America developed a highly efficient electrostatic generator which works on a different principle. Van de Graaff machines have been used a great deal in nuclear research (page 550). Simply Van de GraatT machines are available for elementary purposes but, normally, they provide charge of one kind only Fig. 32.16. The Wimshurst gives both kinds.