An interactive showing a superconducting Josephson junction as a ball in a tilted washboard potential, equivalent to a pendulum with applied torque. A slider sets the bias current; below the critical current the ball stays trapped (zero voltage), above it the ball runs and the phase winds (a voltage appears).

Josephson junction as washboard and pendulum Tilted cosine potential with a ball, and an equivalent pendulum, both driven by a bias-current slider. Energy landscape U(φ) = −cos φ − (I_b/I_c)·φ Pendulum settles at angle φ* The ball, the pendulum angle, and the Josephson phase φ are one and the same variable
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State
zero-voltage
Junction current I/I_c
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Voltage ∝ ⟨dφ/dt⟩
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