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THE UGHT SOURCE 375
ceives such hot particles as the arc may throw toward the mirror from time to time; it is theguard that becomes pitted and must be replaced in due course. The advantage of using mirror guards is that they cost much less than mirrors.
(53) All parts of the lamp are exposed to very high temperatures. Metals used for the carbon jaws and other exposed parts are selected accordingly. Lubricants (where required) must be such as to withstand the arc temperatures.
(54) All lamps are provided with means by which the projectionist can observe the arc while it is burning, and with devices by which he can accurately control the posi tion of the arc in case of any slight misfunctioning of the feed mechanism. Small tinted glass windows are built into the door of the lamphouse. In addition there is often a small mirror or prism, and a marked card. A reflection of the arc is thrown on the marked card. If the arc does not occupy exactly the correct position with reference to the markings the projectionsit adjusts its position by means of the external controls with which every lamp is equipped.
(55) Many, in fact most, lamps are fitted with ammeters, so the projectionist can observe the amount of current he is using. He can adjust the currentconsumed, within limits, by slightly changing the length of the arc gap, using the external controls of his lamp for that purpose.
(56) D.C. arcs derive their current from (a) a d.c. power line, in which case the voltage is usually reduced by a series resistor known (see page 143) as a ballast resistor; (b) from a motor-generator, or (c) from a rectifier. A.C. arcs are not at present in very widespread use, but there is a type of a.c. arc that is operated at 96 cycles, the current being derived from a special motor-generator. Other types of a.c. arcs may be supplied at line frequency through a transformer.
Straight High Intensity Light Source
(57) By "straight high intensity" we mean the high in