King's electrical orrery, c 1770.
© Science Museum / Science & Society Picture Library
This type of instrument was devised by Erasmus King, a mid 18th century science lecturer. It was made by George Adams, instrument maker to the king. The bras balls represent the Sun, Moon and Earth. Charging the central column caused the supporting rods to become charged. The charge streamed off the small spikes causing the rods to rotate, thereby imitating the motion of the Moon, Earth and Sun.