A mercury vapor light is one of the forerunners of other similar lights such as high pressure sodium lamps and even common fluorescent lights. The bulb does not have a filament that resists electrical current, like an incandescent bulb. Instead, it uses a small chamber of gas, including mercury vapor. Electricity is passed through this chamber, causing the gas to react and produce a glow as a result of electrons moving up and down energy levels.
The discharge tube is the glass chamber that holds the gas mixture that produces the light in a mercury vapor lamp. This tube is typically made from a thick fused silica material that can trap the gas but still allow the wavelengths of light pass through. If this tube is broken, the bulb can no longer function, and toxic mercury vapor is spread throughout the rest of the bulb.
Around the discharge tube is a much larger glass bulb. This bulb serves two functions. First, it protects the inner tube from any outside impact. Second, it filters out some of the rays the mercury vapor produces. The light from a mercury vapor lamp is a mixture of several different wavelengths, including dangerous UV radiation. The outer bulb has protective coatings that block these rays from extending outside of the bulb.
Electrodes are contact points for the electrical system. The current must jump through the discharge tube and then cycle back out from the bulb. There are four electrodes in the lamp. Two are primary electrodes, placed at either end of the tube so the current can jump from one to the other. The other two are auxiliary versions used if the first two fail.
It takes a very specific type of electrical current to keep a mercury vapor light lit. If the cycles or voltage are wrong, the bulb could refuse to light or burn out very quickly. The ballast is a small electrical device that automatically monitors and controls the current as it enters the bulb, but before it passes through the tube. The ballast changes the current until it is optimal for maintaining the bulb.
- Photo Credit light bulb image by Zbigniew Nowak from Fotolia.com