History Continues: William Gilbert

Published the celebrated treatise entitled De Magnete(On the magnet) in 1600. This was the first comprehensive study of magnetism and took 17 years to complete. Gilbert dedicated it to those who look for knowledge "not only in books but in things themselves." The growing interest in compass navigation may have influenced Gilbert somewhat because he wrote De Magnete at the time the English were preparing to meet the Spanish Armada. Gilbert lived from 1544 to 1603, roughly the same period as Johannes Kepler. In 1600, when De Magnete was published, Giordano Bruno was burned at the stake in Rome because he believed in the Copernican theory. It was also the year in which Johannes Kepler set out to join Tycho Brahe in Prague.

Gilbert, a student of medicine, received his M.D. at Cambridge University in 1569, and by themid-1570s was a prominent physician in London. In 1600 he became president of the Royal College of Physicians and was appointed as personal physician to Queen Elizabeth I. When she died in 1603, her only personal legacy was a grant to Gilbert to pursue his hobby, physics, but he had little time to enjoy it because he was a victim of the plague a few months later.

For his work on magnets, Gilbert became known as the "Father of Magnetism." He discovered various methods for producing and strengthening magnets. For example, he found that when a steel rod was stroked by a natural magnet the rod itself became a magnet, and that an ion bar aligned in the magnetic field of the earth for along period of time gradually developed magnetic properties of its own. In addition, he observed that the magnetism of a piece of material was destroyed when the material was sufficiently heated.

One of Gilbert's major discoveries (he credited himself with 21) was that the earth is a huge magnet, a connection that Peregrinus failed to make. He proved that a compass needle swings north because of the magnetism of the earth itself and not - as some believed - because of a star in the Big Dipper or a mysterious range of iron-capped mountains in the North. Using a spherical magnet and magnetic needle that was free to rotate in a vertical plane that included the magnetic poles of the sphere, he found that the needle dipped below the horizontal (the tangent plane to the sphere) at different angles, depending on its position on the sphere. Gilbert realized that lines joining points of constant magnetic declination (the angle between the magnetic needle and the horizontal) were also lines of constant latitude on a sphere. Impressed with his discovery, he suggested an application to navigation. Although navigators used compasses at sea, they knew that variations in the earth's magnetism often caused a compass to be unreliable. Gilbert thought circles indicating constant magnetic dip on the earth might be more reliable. However, navigators soon found that the dip along latitude lines varied considerably, and so the idea was abandoned.

Although he is chiefly noted for his work in magnetism, Gilbert made many important contributions to the science of electricity, ranging from the invention of the electroscope to the study of conductors and insulators. To him we owe the term "electricity." He also left a large manuscript devoted to speculative work in general science, which was published posthumously in 1651.

Galileo said De Magnete made Gilbert "great to a degree that is enviable." The inscription on Gilbert's tomb is more modest. It reads: "He composed a book, concerning the magnet, celebrated among foreigners and among those engaged in nautical affairs."

William Gilbert set out to debunk magical notions of magnetism, yet in building an intellectual bridge between natural philosophy and emerging sciences, he did not completely abandon reference to the occult. For example, he believed that an invisible "orb of virtue" surrounds a magnet and extends in all directions around it. Other magnets and pieces of iron react to this orb of virtue and move or rotatein response. Magnets within the orb are attracted whereas those outside are unaffected. The source of the orb remained a mystery.

Source - University of Dallas, Department of Physics