The Physics
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Voltage of a Mercury Light

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Lighting Systems. HID DC Operations. "Open circuit voltage must be high enough to start and sustain the particular lamp type. At room temperature, 250 volts DC is usually sufficient for low-voltage (135V) types, while 325 volts DC will be needed for high-voltage (265V) types." 135V- 265V
Gilway Technical Lamp Engineering Catalog. Technical Information Edition 10198. "The HBO 50 W/AC is a lamp type, which is generally used in microscopy and is designed for operation in alternating current mode only and is offered in two operating voltage groups." 34V-39V
Goldwasser, Samuel M. Fluorescent Lamp Basics. Fluorescent Lamps, Ballasts, and Fixtures. 1994-2003. "When power is first applied, a high voltage(several hundred volts) is needed to initiate the discharge. However, once this takes place, a much lower voltage-usually under 100V for tubes under 30watts, 100 to 175votls for 30 watts or more- is needed to maintain it." 100–175 V

A mercury vapor lamp is gaseous lamp that consists of two bulbs, one within the other. The inner bulb is called the arc tube, which is made of quartz and the outer bulb serves to protect the arc tube. The arc tube contains argon gas and a small amount of pure memory.

Mercury lamps need a certain voltage to start up the arcs, which have an electrode on each end. Once there is a full voltage running from the starting electrode to the main electrode, the argon gas begins to become ionized by the electrons and an arc is formed between two electrodes, like a bridge. The heat produced from the arc vaporizes the mercury droplets, which become ionized as well. The mercury vapor now carries current within the arc. As the current continues to increase to its full potential, a ballast, which can be found in a mercury vapor lamp, serves as a resistor. So as the current is increasing, the ballast reduces the supply voltage simultaneously to keep the mercury-vapor lamp running under stable operation. As a result of the decreased voltage, a glow between the two electrodes is emitted within the arc tube. Emission from the mercury arc is limited to four particular wavelengths. For example, it can emit greenish-blue light as well as ultraviolet rays.

A mercury vapor lamp lasts longer than electric lights of similar wattages. But, mercury vapor lamps require five to seven minutes for it to reach its full brightness.

Dorothy Soo -- 2005