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More about Gilbert's Work
In addition of describing his own findings, Gilbert devoted long sections of his book to a critical examination of earlier writings about the magnet and the compass. For instance, he traced the claim that garlic robbed a magnet of its powers to "Plutarch and Claudius Ptolemy and all the copyists since their time," commenting "thus in philosophy many false and idle conjectures arise from fables and falsehoods."
Of his own experiments, the most important was conducted with a magnetized "terrella" ("little Earth"), a spherical magnet serving as a model for the Earth. By moving a small compass over the surface of the terrella, Gilbert reproduced the directional behavior of the compass; reputedly, he also demonstrated this in front of Queen Elizabeth and her court.
[In modern times, scientists have used magnetized terrellas inside vacuum chambers to mimic the effect of the Earth's magnetism on auroral electrons, cosmic ray particles and the solar wind. For one example, see here.]
In addition to studying magnets. Gilbert also looked into a vaguely similar phenomenon--the fact that certain materials, when lightly rubbed with cloth or fur, attracted light objects such as chaff. One such material was amber, a yellow fossilized resin called elektron by the ancient Greeks. From this Gilbert named such attraction the "electrick force," and from that came such words as electric charge, electricity, electrons and electronics. Gilbert even devised a pivoted lightweight needle--a "versorium" resembling a compass needle--to observe the direction of the electric force.
Gilbert ascribed the deviation of the compass needle from true north to the attraction of the continents, which tallied with observations in the Northern Atlantic--the needle veered eastwards near Europe, westwards near America. Noting that near the islands of Novaya Zemlaya, north of Russia, the compass needle pointed west of true north, Gilbert speculated that a "north-east passage" around Russia might exist, giving more direct access by sea to the spice islands of the Far East. Some decades earlier, Frobisher and Davis had sought in vain a similar "north-west passage" around the American continent.
The last of the 6 "books" into which Gilbert's work is divided deals with the motion of Earth in space and its possible connection to magnetism. Here Gilbert voiced complete support of Copernicus, "the Restorer of Astronomy," which made the book somewhat controversial. Galileo, who praised "De Magnete," obtained his copy of it as a gift from "a peripatetick philosopher of great fame, as I believe, to free his library of its contagion."