Hunting the Magnetic Field

Petrus Perigrinus de Maricourt, a French physicist and mathematician wrote the first descriptions of the properties of magnets, now known as the Epistola de Magnete in 1269.  He summarized what was known at the time concerning the use of the compass, writing “that while the investigator in this subject must understand nature and not be ignorant of the celestial motions, he must also be very diligent in the use of his own hands, so that through the operation of this stone he may show wonderful effects.” (translation by J. Gimpel, 1976).  This is still true today, particularly for those of us who study the ancient magnetic field through ‘accidental’ records in geological and archaeological materials.  In this lecture I will review efforts to use ‘our own hands’ to understand the structure of the time averaged Earth’s magnetic field over the last five million years, using both directions (obtained with compasses!) and intensities.