Abstract

In his Tractatus de proportionibus, Thomas Bradwardine describes some devices consisting of a magnet and pieces of iron, in order to put his rule of motion to the test. These devices, or similar ones, are also found in the Questiones super libros Physicorum of Nicole Oresme, and in three works by Blasius of Parma, namely the Questiones de ponderibus, the Questiones super libros Physicorum and the Questiones circa Tractatum proportionum. In this paper, I describe these devices, while examining the theory of magnetism that is in use and I analyse how the arguments on magnets demarcate the field of application of the rule of motion.