<Li> The lamp is extinguished and no current flows . </Li> <Li> The lamp is operating with liquid amalgam in the tube . </Li> <Li> The lamp is operating with all amalgam evaporated . </Li> <P> The first and last states are stable, because the lamp resistance is weakly related to the voltage, but the second state is unstable . Any anomalous increase in current will cause an increase in power, causing an increase in amalgam temperature, which will cause a decrease in resistance, which will cause a further increase in current . This will create a runaway effect, and the lamp will jump to the high - current state (#3). Because actual lamps are not designed to handle this much power, this would result in catastrophic failure . Similarly, an anomalous drop in current will drive the lamp to extinction . It is the second state that is the desired operating state of the lamp, because a slow loss of the amalgam over time from a reservoir will have less effect on the characteristics of the lamp than a fully evaporated amalgam . The result is an average lamp life in excess of 20,000 hours . </P>

Working principle of high pressure sodium vapour lamp