Gilbert's terrella 'Earth' magnets. Page from 'De Magnete' (On the Magnet, 1600) by English physicist William Gilbert (1544-1603). The diagrams show Gilbert's terrella ('little Earth'), a magnetised sphere he used to model the Earth and its magnetic field. He cut the sphere in half and examined the varying direction of the magnetic forces (arrows) for two halves (top) and a single half (bottom). 'De Magnete' reported Gilbert's studies of magnetism and electricity. He theorised that the two forces were closely related. He also considered the Earth to be a spherical magnet. This first edition of 'De Magnete' consisted of 115 chapters in six books and 246 pages. This is page 136, chapter 9, book 3.