A sodium vapour lamp is a lamp that uses sodium to create light. It can come in a high pressure or low pressure format. High pressure lamps have more components than low pressure and contain other substances such as mercury. The lamp produces a clarity of light that creates vivid colour from objects illuminated by it. The tube of a high pressure sodium light is generally made out of aluminium oxide, due to its resistance to the high pressure, and xenon, that is used as a starter for the light because it won't react with the other gases. Voltage runs to the light through a ballast, which regulates the current.
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The arc of gas that extends from one end of the tube to the other is created when ignited, and is made up of metallic sodium and mercury vapour. The temperature of the vapour is controlled by the power supplied to the lamp. With higher power comes higher temperatures and thus higher pressure in the tube, which creates more light. The ballast is an inductive ballast that helps regulate this power by keeping the current constant, instead of the voltage.
The inductive ballast is made up of a coiled wire. The coil creates a magnetic field inside when a current is applied. It stores the energy of the current in the magnetic field it creates. This way it controls the output current that continues on to the light. The store of energy also helps in the starting of the light when it is off, by sending an extra volt of electricity when the current first starts into the coil.