His primary scientific work—much inspired by earlier works of Robert Norman—was De Magnete, Magneticisque Corporibus, et de Magno Magnete Tellure (On the Magnet and Magnetic Bodies, and on the Great Magnet the Earth) published in 1600. In this work, he describes many of his experiments with his model Earth called the terrella. From these experiments, he concluded that the Earth was itself magnetic and that this was the reason compasses point north (previously, some believed that it was the pole star (Polaris) or a large magnetic island on the north pole that attracted the compass). He was the first to argue, correctly, that the centre of the Earth was iron, and he considered an important and related property of magnets was that they can be cut, each forming a new magnet with north and south poles.
In Book 6, Chapter 3, he argues in support of diurnal rotation, though he does not talk about heliocentrism, stating that it is an absurdity to think that the immense celestial spheres (doubting even that they exist) rotate daily, as opposed to the diurnal rotation of the much smaller Earth. He also posits that the “fixed” stars are at remote variable distances rather than fixed to an imaginary sphere. He states that situated “in thinnest aether, or in the most subtle fifth essence, or in vacuity – how shall the stars keep their places in the mighty swirl of these enormous spheres composed of a substance of which no one knows aught?”
The English word “electricity” was first used in 1646 by Sir Thomas Browne, derived from Gilbert’s 1600 New Latin electricus, meaning “like amber“. The term had been in use since the 13th century, but Gilbert was the first to use it to mean “like amber in its attractive properties”. He recognized that friction with these objects removed a so-called “effluvium”, which would cause the attraction effect in returning to the object, though he did not realize that this substance (electric charge) was universal to all materials.