The transistor turns on when the initial current through the 100-ohm resistor creates a sufficient
voltage of base emitter.


Positive feedback from the secondary coil keeps the transistor on until the magnetic field in the transformer peaks.
When the rate of change of the magnetic field decreases, the feedback voltage drops, turning the transistor off.
A rapid collapse of the magnetic field induces a negative feedback voltage that ensures the transistor turns off completely.
The cycle repeats, creating oscillations that allow the neon bulb to light up from the induced voltage in the tertiary coil.


This dynamic behavior of feedback from the transformer ensures that the transistor does not stay permanently on, but instead oscillates between on and off states.