Contributions
A physician trained at Cambridge, William Gilbert practised in London, where he was president of the Royal College of Physicians before becoming, in 1601, personal physician to Elizabeth I. But his fame rests on a single, masterly scientific work: "De Magnete" (1600), the first great treatise of experimental physics published in England. The fruit of seventeen years of research, it sets out, with method on display, the systematic study of the magnet and of electricity. Its revolutionary thesis: the Earth itself is a giant magnet, which explains the orientation of the compass and solves a riddle vital to navigation. To prove it he fashioned magnetised spheres, the "terrella" or little earths, and observed how a needle behaved across their surface. Gilbert also distinguished magnetism from the attraction produced by rubbed amber, to which he gave — from the Greek elektron — the name that would become "electricity."
Significance
Gilbert holds a founding place in the history of science: he was among the first in northern Europe to replace the authority of the Ancients with proof by repeated, controlled experiment. Where the scholastic tradition reasoned over texts, he questioned matter itself, forging an approach the next generation would raise into a method. His intellectual audacity went further still: from his magnetic studies he rejected part of Aristotle's cosmology, held that the stars were not fixed to a single sphere but scattered at unequal distances, and was open to the idea that the Earth turns on its axis. By making the globe a magnet, he linked terrestrial physics and celestial mechanics in an unprecedented way. His influence was immediate and considerable: Galileo praised "De Magnete" and Kepler drew on it for his speculations on a magnetic force governing planetary motion. The shadow in the work lies in its own limits: Gilbert still ascribed a "soul" to the magnet, a legacy of ancient animism, and erred in trying to make magnetism the cause of planetary motion. Yet even these errors were fruitful, for they sought a physical cause where others rested content with mysteries.
Influence
"De Magnete" founded a school: Galileo and Kepler read and cited it, and it became a model of experimental physics for the seventeenth century. Gilbert bequeathed to science the word "electricity" and the idea of the Earth as a magnet, founding the study of geomagnetism so crucial to navigation. He had inherited the medical tradition of Cambridge and the scientific humanism of his time, but drew above all on the practical knowledge of navigators and compass-makers, whose observations he gathered; the rediscovery of Copernicus and the experimental curiosity of the Renaissance shaped his thought.
Anecdote
To demonstrate that the Earth is a magnet, Gilbert resorted to a scale model of striking elegance: he cut spheres from natural lodestone, which he called "terrella," little earths, and moved a tiny magnetised needle over their surface. The needle oriented and dipped exactly as a compass does at the different latitudes of the globe. With this experimenter's gesture Gilbert held a miniature Earth in his hands and proved, before the eyes of anyone willing to see, a truth that centuries of speculation had been unable to establish.