Measurements and mechanisms of Thomson’s jumping ring

Measurements of the phase delay of the current and force on a ring floated on a commonly available Thomson’s jumping ring apparatus were performed for phase angles from 12° to 88°. The force and phase data show excellent agreement with a linear inductive model. We find that the demonstration, as usually performed with large highly conducting rings, operates in the inductance-dominated regime at 60-Hz line frequency. Stroboscopic photographs of the jumping ring, for both room-temperature and 78-K rings, confirm that the same time-averaged inductive phase lag mechanism, not an electrical transient, accounts for the jump height. We introduce a simple room-temperature demonstration that illustrates the importance of the phase lag: Despite its greater weight, a stack of thin rings will float higher than a single ring as the inductive phase lag comes to dominate the parallel resistance of the combined rings. © 2000 American Association of Physics Teachers.