mercury lamps, metal halide lamps, and high-pressure sodium lamps. All high-intensity-discharge lamps produce  light  from  an  arc  tube  that  is  usually  contained in an outer glass bulb. Figure 6-14 shows the basic configuration of a HID lamp. In these lamps, a material, such as sodium, mercury, or metal halide, is added to the arc tube. In design, the lamp has three electrodes—one acting as a cathode and the other as an anode with the other electrode being used for starting. The arc tube contains small amounts of pure argon gas, halide salts, sodium, and  vapor  to  aid  in  starting.  Free  electrons  are accelerated by the starting voltage. In this state of acceleration, these electrons strike atoms and displace other  electrons  from  their  normal  atomic  positions. Once the discharge begins, the enclosed arc becomes the  light  source. Commercial  companies  that  produce  these  light bulbs claim a 100-percent increase in lamp life over tungsten filament bulbs that produce the same amount of light. The power in watts required to operate these lamps is less than one half of that required for filament lamps.   The initial cost of the components for lights is Figure 6-14.—HID lamp configuration. substantially  greater  as  these  lights  will  require  ballasts; however, this cost can be made up later by the savings of energy  costs.  The  selection  of  lighting  fixtures  will depend on budgeted dollars for new installation projects versus maintenance dollars. Most discharge lighting fixtures are supplied with the  required  ballast  installed  in  the  fixture.  In  some cases   ballasts,   usually   called   transformers,   are externally installed. High Pressure Mercury Lamps This lamp consists of a quartz arc tube sealed within an outer glass jacket or bulb. The inner arc tube is made of quartz to withstand the high temperatures, resulting when the lamp builds up to normal wattage. Two main electronemissive electrodes are located at opposite ends of the tube; these are made of coiled tungsten wire. Near the upper main electrode is a third, or starting, electrode  in  series  with  a  ballasting  resistor  and connected to the lower main-electrode lead wire. The arc tube in the mercury lamp contains a small amount of pure argon gas that is vaporized. When voltage is applied, an electric field is set up between the starting  electrode  and  the  adjacent  main  electrode.  This ionizing potential causes current to flow, and, as the main  arc  strikes,  the  heat  generated  gradually  vaporizes the mercury. When the arc tube is filled with mercury vapor, it creates a low-resistance path for current to flow between the main electrodes. When this takes place, the starting electrode and its high-resistance path become automatically inactive. Once  the  discharge  begins,  the  enclosed  arc becomes a light source with one electrode acting as a cathode and the other as an anode. The electrodes will exchange functions as the ac supply changes polarity. The quantity of mercury in the arc tube is carefully measured  to  maintain  quite  an  exact  vapor  pressure under  design  conditions  of  operation.  This  pressure differs with wattage sizes, depending on arc-tube dimensions, voltage-current relationships, and various other design factors. Efficient operation requires the maintenance of a high temperature of the arc tube.  For this reason, the arc tube is enclosed in an outer bulb made of heat-resistant glass that makes the arc tube less subject to surrounding temperature  or  cooling  by  air  circulation.  About half an atmosphere  of  nitrogen  is  introduced  into  the  space between the arc tube and the outer bulb. The operating pressure for most mercury lamps is in the range of two 6-13