Fig. 1. A hydrogen atom contains a proton that spins on its own axis and an electron in orbit around the nucleus. The rotating nucleus behaves as a magnetic dipole, similar to the bar magnet illustrated on the left. The magnetic moment of the nucleus, denoted by the arrows, is determined by the sum of its magnetic forces. (Dortzbach RK, Kronish JW, Gentry LR: Magnetic resonance imaging of the orbit. Part I. Physical principles. Ophthal Plast Reconstr Surg 5:152, 1989)