Drawing of a magnetic field by French philosopher René Descartes, from his Principia Philosophiae, 1644. This was one of the first drawings of the concept of a magnetic field. It shows the magnetic field of the Earth (D) attracting several round lodestones (I, K, L, M, N) and illustrates his theory of magnetism. Descartes proposed that magnetic attraction was caused by the circulation of tiny helical particles, "threaded parts" (shown), which circulated through parallel threaded pores in magnets, in through the South pole (A), out through the North pole (B), and then through the space around the magnet (G, H) back to the South pole. Oppositely threaded particles circulated in the opposite direction. When the "threaded parts" came near a lodestone or piece of iron, they passed through its pores, causing magnetic force. Alterations to image: cropped, and cloned out intruding text around edges of drawing