In 1893, at the World’s Columbian Exposition in Chicago, Nikola Tesla stood a copper egg upright on its end without touching it. No glue, no tricks, no flattened base. A rotating magnetic field did the work that Columbus once did with a tap. The legend says Columbus silenced his critics by standing an egg on its tip after cracking the shell flat. Tesla wanted to silence his own critics, the ones who doubted alternating current, and he did it with physics instead of force. Beneath a wooden platform sat a toroidal iron core wound with coils. Tesla fed them two-phase AC: two currents offset by 90 degrees in time, through windings offset by 90 degrees in space. The result is the heart of his entire legacy, a magnetic field of constant magnitude that sweeps in a circle at synchronous speed, ω = 2πf. The copper egg felt that sweeping field as changing flux. By Faraday’s law, the change drove eddy currents in the metal. By Lenz’s law, those currents fought the change, and the interaction p...