A sodium vapour lamp is also called as the cold cathode low-pressure lamp which gives high luminous output about three times higher than the other lamp. For starting discharge through the lamp. it is essential that the striking voltage should be higher than the normal working voltage of the lamp. This high voltage taken from a high reactance transformer or auto-transformer which has poor voltage regulation. Thus when discharge in the lamp takes place, the lamp current increase due to the discrease in the resistance of the gas in the tube and output voltage of the auto-transformer fall. The lamp then continues to operate normally.