Plate 2 from Nova Reperta
by Johannes Stradanus

The Magnetic Earth

 

In De magnete, William Gilbert describes numerous experiments with the magnet or lodestone. Gilbert's treatise is a key text in the development of the experiment as a means of discovering truths about the natural world. In twentieth-century scholarship, De magnete is the focal point of a discussion about the origins of the methodology that became central to the experimental philosophy in the seventeenth century. One issue involves the extent to which Gilbert derived his methods and views from earlier treatises such as the thirteenth-century "Letter on the Magnet" by Pierre de Maricourt, first published in 1558, and The Newe Attractive by the retired mariner and compass maker, Robert Norman, first published in 1581.

For most of his experiments Gilbert chose a natural lodestone, which he shaped into a sphere by turning the stone on a lathe. He called this round stone a terrella, or little earth. The many images of this "little earth" (of which the image shown here is the first) thoroughly prepare the reader to accept Gilbert's conclusion that the earth itself is a magnet. Click Here for a Larger ViewThe illustration shows how to find the poles of the terrella similar to the earth's poles. Gilbert explains how to do this using an instrument called a versorium. The versorium is a small compass needle made with a piece of magnetized iron that can freely turn on its base. The versorium is placed on various spots on the terrella and the point to which it turns is marked. Gilbert explains that after marking a number of points, you can discover the point at which all the points converge (A and B) which are the poles. A versorium placed near but not on the terrella (as D) will point directly at the pole.

Nine years later, Galileo created strikingly earth-like images of the moon which he claimed to have viewed through his new instrument, the telescope. Galileo was acquainted with Gilbert's De magnete and may have noticed how effectively Gilbert used visual images of magnets that looked like small earths to help persuade his readers that the earth itself was actually a magnet.

Pamela O. Long
Washington, DC

Suggested Reading

Freudenthal, Gad. "Theory of Matter and Cosmology in William Gilbert's De Magnete." Isis 74 (1983): 22-37.

Henry, John. "Animism and Empiricism: Copernican Physics and the Origins of William Gilbert's Experimental Method." Journal of the History of Ideas 62 (January 2001): 99-119.

Pumfrey, Steven. "William Gilbert's Magnetical Philosophy, 1580-1674: The Creation and Dissolution of a Discipline." Ph.D. diss., University of London, Warburg Institute, 1987.

Zilsel, Edgar. "The Origins of William Gilbert's Scientific Method." Journal of the History of Ideas 2 (January 1941): 1-32.