William Gilbert, quite literally, wrote the book on magnetism. Prior to his work, which culminated in his epic treatise, De Magnete, the effects of magnetism were the stuff of myth and magic. Gilbert changed all that when he showed systematically that magnetism is a form of energy that acts with attractive or repulsive force, depending on the alignment (or polarity) of the objects involved.
Gilbert's scientific insights also peeled back the mystery of the Earth's magnetic field. By likening the Earth to a large magnet, he was able to explain why suspended lengths of magnetized iron automatically aligned in a north-south direction. Until then, people attributed the phenomenon to one of the stars in the Big Dipper or an undiscovered iron-capped mountain range somewhere in the far North.
Magnetism was just one of Gilbert's interests, however. He also investigated the “amber effect.” As far back as the ancient Greeks, people knew that certain materials, such as amber, developed attractive or repulsive forces when rubbed with a piece of fur. Gilbert called this peculiar property “electricity,” and experimentally proved it was of a different nature than magnetism.
Gilbert tested many substances, classifying them as “electrics” or “non-electrics” depending on whether or not they built up charge when rubbed. He explained this tribo-electrification as the removal of a fluid, or “humour,” which then left an “effluvium,” or atmosphere, surrounding the treated substance. Centuries before its discovery, Gilbert sensed the existence of positive and negative charge and its associated electric field.