Mercury Lamp Construction and Characteristics.
Mercury lamps typical of those used in industry are constructed with these essential parts: an arc tube made of quartz, containing argon and mercury vapor; activated tungsten electrodes; a glass bulb that encloses the arc tube, to shield the arc from drafts, prevent oxidation, and provide a surface for a phosphor coating; and a base which serves as an electrical connecting and holding device. These parts vary in size and shape for such different uses as general-lighting lamps (clear mercury, fluorescent mercury, tinted mercury, reflector and semi reflector), black light (ultraviolet) lamps, sunlamps, and photochemical lamps. The average rated life of the general-lighting lamps varies from 6,000 to 12,000 hours.
In operating a mercury lamp, once the arc is established in the arc tube the current through the mercury vapor tends to increase without control. A device, therefore, must be used with the lamp to control and limit the current through the lamp; this device is known as a ballast.
Advantages and Disadvantages.
Mercury lamps have a high luminous efficacy (2 to 3 times that of general-service incandescent lamps), have a long life, and are a compact size source for good light control. These obvious advantages may or may not be outweighed by such disadvantages as high lamp and equipment cost, bluish-green color of light, and slow restarting. Although the color of mercury light is bluish-green and lacking in red energy, this deficiency can be partially compensated for by using conjunction with the clear Lamps.
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