Two terrellae

A terrella (‘little earth’) is a sphere made of a magnetized substance, used to simulate the magnetic field of the earth. In the 16th century the English physician William Gilbert used a terrella made out of lodestone (a naturally-occurring magnetized mineral) to demonstrate that the earth is magnetic, and to explain how mariners’ compasses work. The Society used several terrellae in experimental demonstrations at meetings in the 17th century. Sir Christopher Wren donated one that was the same size as the larger of these two examples (bottom image).