Priestley's globe electrical machine, c 1768.
© Science Museum / Science & Society Picture Library
In the 18th century, interest in electricity was very fashionable, and many people attended scientific lectures to see demonstrations of electrostatics, which used machines such as this one, designed by the British chemist Joseph Priestley (1733-1804). When the handle was turned, it caused a pad to rub against the bottom of the glas globe. The friction caused the glas to build up a positive charge, while the pad developed a negative charge. The charge could be conducted away from the bras back plate of the pad and used to make sparks.