Close the switch to switch on the light.
When energised, current flows through the ballast and through the tube's heater filaments through the Starter switch, represented by the lower relay.
When the starter switch heats up, it opens to disconnect the heaters, and the interrupted ballast current causes a voltage spike.
The voltage spike causes an arc through the tube, represented by the upper relay. The arc's UV emission reacts with the phosphor coating inside the tube.
