🧲 Rikitake and Bullard Disc Dynamos: Self-Excitation and Polarity Reversals (2D)

Two disc dynamos feed the coil of each other and their currents wander, flip sign and fall back, imitating the irregular reversals of the geomagnetic field. Choose the loss rate and the asymmetry, and read a polarity record, the current traces and the phase portrait with its fixed points. A single Bullard disc shows the growth above the threshold speed, the saturation and the small oscillations around it. Four in-page numerical checks.