Models made by Leopoldo Nobili to illustrate André-Marie Ampère's theory of magnetism. They are composed of two square-section wooden rods, covered with paper, with arrows drawn at right angles to the length and labeled "N" and "S" in ink. According to Ampère, the arrows indicate the direction of the current when an iron or steel bar is magnetized. They represent the sum of the electromagnetic currents around the molecules composing the bar. Provenance: Lorraine collections.