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 Lent'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. (C) 1998 American Association of Physics Teachers. [References: 12]
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