A classroom jumping ring

Abstract

We present the design of a compact alternating current jumping ring apparatus which can electromagnetically launch conducting rings across a classroom. Jump energy and height are calculated for core and thin ring length, radius, thickness, and material. The effects of core saturation, permeability, hysteresis and demagnetizing field, ring shielding, and convective derivative are described. Rings with even small phase lag can pass by the magnetic pole of the primary in one-quarter of a cycle. Large ring size relative to skin depth delays the ring current close to the 180 deg of Lenz’s law. Contactless jump height measurement enables ring resistivity determination. Demonstrating the effects of voltage, frequency, conductivity, permeability, and geometry contributes to understanding electromagnetism in the classroom.

Measurements and mechanisms of Thomson’s jumping ring

“…12,17 The vertical pickup coil was calibrated to measure the ring current by replacing the ring with a single loop of #12 copper wire and passing a known current through it. 1͒.…”

“…11,12 The voltage-controlled jumping ring Fig. I, the naive explanation of the jumping ring ͑''solenoid magnet repels the induced ring magnet''͒ is accurate-but only when the electrical transient causes the ring to leave the solenoid in the first quarter cycle of the solenoid current.…”

“…Since, however, the demonstration is usually done with alternating current, the levitation requires either the inductive phase lag of the induced current in the ring to provide a time-averaged repulsive force, 1,5,11 or a sufficiently large electrical transient to induce a jump in the first quarter-cycle of the solenoid current. 12 The importance of the inductive phase lag was recognized from the outset by Thomson 1,13 and has been discussed more recently in a number of theoretical treatments. [14][15][16] The experimental record, however, is less complete and more contradictory.…”

“…Hall 18 used an apparatus identical to ours to explore the lifting force as a function of frequency over the range of 20-120 Hz, but did not examine the phases of the fields and current in the ring. Schneider and Ertel 12 have recently described a custom-built apparatus which operates in the inductive transient regime with a saturated iron core, using long tube-like rings.…”

PSSC Turbo Ring Flinger

“…1-3 There is also no shortage of instructions on how to construct one's own Thompson's jumping ring apparatus. 4,5 Although several incarnations of the jumping ring apparatus are available from educational scientific supply companies for around $300, here is another version of the jumping ring apparatus, one that is especially easy and economical to construct and that works especially well. Most college physics departments have disused (or at least, seldom used) PSSC 6 air core solenoids lying about.…”