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MANAGERS AND PROJECTIONISTS 831
located as to distance from the mirror surface, will deliver maximum results.
The crater shown at B, Fig. 315A, bevels backward away from the mirror, for the reason that the negative tip is a trifle too high with relation to the positive. The remedy is to lower it gradually, until the point is found where the crater burns as nearly as possible square, as shown at A, Fig. 315A. It will, of course, not be found possible to force the crater to burn absolutely square, as shown at A, because of the action of the hot gasses which rise and tend to burn off the upper edge of the crater, but by careful work a very close approximation to the condition shown at A, Fig. 315A, may be had.
It is of course understood that the carbons must be in exact line with each other sidewise, but that, we believe, is taken care of in the lamp itself when it is made.
IMPORTANT CAUTION. — NEVER PERMIT THE NEGATIVE CARBON TO BE SO LOW THAT THE CRATER BEVELS DOWNWARD, opposite to the condition shown in B, Fig. 315A.
METAL COATED CARBONS.— Avoid them when using reflector arc lamps, as they are likely to pit the mirror surface, and any injury of that sort will reduce its reflective power in exact proportion to the area of the surface thus ruined.
CARBON SIZES.— See page 400y2.
IMPORTANT— WARNING.— With high intensity and ordinary arc lamps it has been possible to get fairly good results, though at the expense of great loss in efficiency, by using carbons far from the correct size (diameter) for the amperage used.
THIS CANNOT BE DONE WITH THE REFLECTOR TYPE LAMP, WHICH IS VERY SENSITIVE TO CORRECT CARBON DIAMETERS FOR ANY GIVEN AMPERAGE. UNLESS YOU KEEP YOUR CARBON DIAMETERS VERY CLOSE INDEED TO THEIR FULL AMPERAGE RATING YOU MAY EXPECT HEAVY FALLING OFF IN EFFICIENCY— IN SCREEN ILLUMINATION.
CLEANING MIRRORS— IMPORTANT.— Surely no argument is necessary to convince you that a mirror surface fogged with dirt cannot possibly be expected to give maximum results in reflection. The surface of a mirror must be perfectly clean in order to develop its full power in light reflection, therefore it follows that the efficiency of the reflector type arc lamp is directly in proportion to the percentage of the total light its reflector receives and the efficiency of its surface as a reflector.