Meeting Information

Peregrinus Versus the Pole Model: The Physics and History of Magnetism

October 18, 2017
The American Center for Physics
College Park, MD

Date: October 18, 2017

Speaker: Dr. Jonathan Keohane, Hampden-Sydney College, Farmville, VA

Topic: Peregrinus Versus the Pole Model: The Physics and History of Magnetism

Time and Location: 1:00 p.m., with Q&A to follow in a 1st floor conference room at the American Center for Physics (, 1 Physics Ellipse, College Park, MD-- off River Rd., between Kenilworth Ave. and Paint Branch Parkway.

Abstract: In 1264, the French Pope Urban IV gave southwestern Italy to the French prince Charles of Anjou. There was a catch, however; Charles would have to take it by force. Despite the valiant efforts of a band of Muslim archers from the town of Lucera, he won the Battle of Benevento in 1266. Two years later, the pretender mayor incited Lucera to revolt against their new king. Thus began the year-long siege of Lucera, during which Sir Peter of Maricourt engaged in an extensive study of magnetism, which was soon published as "Epistola de Magnete by Petrus Peregrinus."

Consider a bar magnet with a north and a south pole. If we break it into two pieces, we get two smaller bar magnets each of which possesses its own north and south magnetic pole. Therefore, Peregrinus reasoned, this could be done no matter how small the magnet. Therefore, neither northness, nor southness, can be a property of matter. This principle of Peregrinus, now enshrined as one of Maxwell's four field equations, had a competitor, the pole model. The pole model had greater success in modeling magnetic matter until well into the twentieth century, when Heisenberg explained ferromagnetism quantum mechanically. By this time, scientists and engineers had been successfully applying the pole model to ferromagnetic materials for decades. Thus, many geologists and engineers still use the pole model to characterize magnetic materials today.

In this talk, I will give an overview of the history of magnetism, with an eye toward how to best combat physical misconceptions brought about by the pole model. I will also argue that physicists should refer to the four Maxwell's equations as: Gauss's Law, Faraday's Law, the Maxwell-Ampere Law, and Peregrinus' Principle.

Biography: Dr. Keohane received his BS in physics from Yale University in 1988 and his Ph.D. in astrophysics, with a minor in science education, from the University of Minnesota in 1998, after spending his final three years as a predoctoral fellow at the Laboratory for High Energy Astrophysics at NASA/Goddard Space Flight Center, where he completed his Ph.D. thesis titled, High Energy Particle Acceleration and Turbulent Magnetic Field Amplification in Shell-Type Supernova Remnants. Dr. Keohane has written multiple papers on observations of supernova remnants, using data ranging from the radio to the x-ray regions of the electromagnetic spectrum.

Dr. Keohane has been teaching physics and astronomy since the fall of 1988 when he enrolled in a teacher education program at the University of Oregon, and subsequently taught high school physics at Jordan High School in Durham NC, while also teaching astronomy to gifted middle school students with the Duke TIP Program. At the University of Minnesota, he coauthored the introductory astronomy laboratory manual, and at NASA/GSFC he was a founding member of their education and public outreach group. Dr. Keohane continued his teaching and research work as an instructor at the North Carolina School of Science and Mathematics in Durham NC, and as a postdoctoral researcher at the Spitzer Science Center in Pasadena CA. He has been on the faculty at Hampden-Sydney College in Farmville VA for the past thirteen years.