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PROJECTION ARCS 131
10 mm. carbon at 21 amperes. They also show that if the carbon is run below its rating, for example, 31 amperes on a 1 3 mm. carbon or 24 amperes on a 12 mm. carbon, there will be less light through the aperture plate opening because of the lower intrinsic brilliancy of the middle portion of the crater.
However, there are very important reasons for not using, for ordinary theatre projection, the smallest size carbons or the lowest current theoretically possible. The total light and the uniformity of the light on the screen depend largely on the position of the positive crater with respect to the focal point of the mirror. In practical projection it is very difficult to hold the positive crater within 0.025 inch of the correct focal point. This accuracy would be necessary to obtain the maximum screen light from a 10 mm. carbon at 21 amperes with a mirror arc system in common use.
The change in the light on the screen with the movement of the positive carbon along the axis of the reflector of a typical optical system for various currents and sizes of carbons is shown in Fig. 4 and Table IV. The movement of the crater away from the axis for
TABLE IV.
S. R. A. Positive Carbons Extent of Movement of Positive Crater along Axis
without Materially Decreasing Screen Light
Total Allowable Movement
Carbon Diameter 1 0 mm. 10 mm. 1 2 mm. 1 2 mm.
13 mm. 1 3 mm.
14 mm. 14 mm.
the various carbon sizes and currents without a material decrease in the screen light was not measured, but is illustrated in Fig. 1 and Fig. 5, where the aperture plate opening for a 6 to 1 magnification is compared with the crater diameters and intrinsic brilliancies. These data show the disadvantage of using a carbon below, or even near, its minimum current capacity rather than the next lowest size carbon near its maximum current capacity.
From these considerations, the use of a 13 mm. carbon at 40 amperes appears to be amply justified to guarantee uniformity of screen illumination. Even with this carbon and current, the positive crater must be held within 0.07 inch of the focal point of the reflector to maintain the screen illumination within 5 per cent of the possible maximum.
The limitations of the low intensity mirror arc optical systems heretofore available have been clearly defined. The fastest projection lens would pass a cone of light only approximately 20 degrees in total angular diameter. This practically fixed the magnification ratio of the reflector system at 6 to 1 and the angle of light taken by the reflector from the crater at 120 degrees. If the angle of the reflector were increased to take a greater angle of light from the
in Inches
along Axis for a
Change
of
No More than 5
Current
Per Cent
in
Screen Illumination
21
.06
24
.10
28
.10
34
.15
34
.11
45
.21
45
.18
55
.34