In 1600 Gilbert published De magnete (On the Magnet, on Magnetic Bodies, and Concerning That Great Magnet, the Earth: A New Physiology), in Latin. The first major scientific work produced in England, it reflected a new attitude toward scientific investigation. Unlike most medieval thinkers, Gilbert was willing to rely on sense experience and his own observations and experiments rather than the authoritative opinion or deductive philosophy of others. In the treatise he not only collected and reviewed critically older knowledge on the behavior of the magnet and electrified bodies but described his own researches, which he had been conducting for 17 years.

In electrostatics Gilbert coined the word "electricity," greatly extended the number of known materials exhibiting electric attraction, and suggested that static electric attraction was due to a subtle electric effluvium emitted by electrified bodies. The greater bulk of the work, however, is devoted to magnetism. Although the compass had been known in Europe for at least 4 centuries, Gilbert's was the first important study on the detailed behavior of compass needles, their variation from true north, and the tendency of the north pole of the needle to dip. From experiments involving a spherical lodestone, the most powerful magnet then available, Gilbert concluded that the earth was a huge magnet, with a north and south magnetic pole coinciding with the rotational poles. The variation in compass readings from true north, he believed, was due to land masses.

Gilbert also speculated on the nature of magnetism, suggesting that magnetic bodies had a kind of soul which spontaneously attracted other bodies. He pointed out that gravity might be a sort of magnetism, or was at least analogous to it, and that the motions of the planets might well be explained by considering their mutual influence.

Gilbert's studies were so complete and comprehensive that as late as 1822 it was asserted that De magnete contained almost everything known about magnetism. Today the unit of magnetomotive force is called the gilbert.