Jumpring

Jumpring, a young armoured knight, watches a glowing ring hover above an iron-cored coil.

The Physics

In 1887 Elihu Thomson showed that a metal ring slipped over the iron core of a coil leaps into the air when AC is switched on. The changing flux induces a current in the ring; by Lenz's law it opposes the change, and because the ring current lags behind, the net force on average pushes the ring away. With steady AC a ring can float.

F ∝ −I_coil I_ring

• I_coil: current in the coil (A)
• I_ring: induced current in the ring (A)

Cut the ring and nothing happens: no closed path, no current. Cool it in liquid nitrogen and it flies higher, as its resistance drops.

Honest caveat: The repulsion relies on the phase lag from the ring's inductance; a purely resistive ring would feel no average force.

On the sticker

Jumpring holds his hand by a cyan ring of light floating above a glowing coil. Printed text: JUMPRING. Pixel-art kiss-cut vinyl sticker, white, in 2″, 3″, 4″ and 6″ sizes.