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Operation

An amalgam of metallic sodium and mercury lies at the coolest part of the lamp and provides the sodium and mercury vapour in which the arc is drawn. The higher the temperature of the amalgam, the higher will be the mercury and sodium vapour pressures the lamp. It will cause a decrease in the electrical resistance of the lamp. They are generally three modes of operation:

1. The lamp is extinguished and no current flows - the lamp resistance is weakly related to the voltage so this state is stable.
2. The lamp is operating with liquid amalgam in the tube - it’s unstable. Any anomalous increase in current will cause an increase of power, causing an increase in amalgam temperature. It will cause a decrease in resistance which will cause a further increase in current. The lamp will jump to the high-current state as it will create a runaway effect.
3. The lamp is operating with amalgam evaporated - as in the first case, it’s a stable state.

To supply a nearly constant current to the lamp, it is powered by an AC voltage source with an inductive “ballast”. The ballast is usually inductive rather than resistive which minimizes resistive losses.

The light from the lamp consists of atomic emission lines of mercury and sodium but is dominated by the sodium D-line emission