Inductor polarity

I just answered this SE EE question.

OP said: "Why then, when the current stops, does it produce a kickback with inverse polarity?"

Charging. As the voltage source charges the coil, current flows out of the positive end of the source and then enters the coil creating a positive voltage at its input. The coil thus acts as a load ("chargeable current source").

Discharging. When we open the switch, the coil begins acting as a current source that must produce the same current in the same direction. For this purpose, the coil reverses its polarity and begins to increase its voltage until it achieves its goal. But because the switch is open, it never succeeds, and its voltage theoretically reaches infinity.

RL differentiator. Usually, there is a resistor in the circuit that limits the current. Here, for example, is my illustration of such an RL differentiation circuit, which is a snapshot of a Flash movie frame (you can watch it by downloading the exe file):