William Gilbert was an English physician, natural philosopher and physicist most noted for his seminal scientific work De Magnete, Magneticisque Corporibus, et de Magno Magnete Tellure. The book in which Gilbert elucidated several of his experiments with a small magnetised model ball called terrella representing the Earth, and gave full account on his research on magnetic bodies and electrical attractions, became a huge success and influenced many contemporary writers. Considered discoverer of the Earth's magnetic field, Gilbert through his experiments, concluded that the Earth acts as a giant magnet because of which compasses point north. He was the first one who argued that the centre of the Earth was iron. It is believed that terrella was invented by Gilbert while he investigated on magnetism. Reputed as inventor of the word electricity, Gilbert also considered static electricity produced by amber and referred to the phenomenon by the adjective electricus. He invented versorium, the first electroscope, in the form of a pivoted needle. It was the first instrument that was used to detect presence of static electric charge. Besides making his mark as a pioneer researcher into magnetism, Gilbert, an MD from Cambridge, also made his name as a physician and served as royal physician of Queen Elizabeth I and thereafter her successor King James I. Gilbert was a Fellow of the Royal College of Physicians and was later elected as President of the college.