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July, 1934] HIGH-INTENSITY ARC AND LIGHT 41 as close limits as is practicable by the feeding mechanism, and that this feeding mechanism be kept in adjustment by the projectionist. In addition to the foregoing factors, which are directly related to the control of the arc, certain other conditions may affect the steadi- ness of the screen illumination. It is assumed that any magnetic fields which might cause the arc to burn unsteadily have been taken care of in designing the lamp. Therefore, if blowing of the arc is observed, evidenced by a fluttering of the light on the screen, the drafts in the lamp house should be given some attention. It was noted in one theater installation that the rear shutter was so designed as to draw the air from the projection lamp through the light shaft of the lamp. This caused a blowing of the arc around the forward carbon and a noticeable disturbance in the light on the screen. A characteristic of the high-intensity a-c. arc which, although not directly connected with the light on the screen, is a distinct advantage, TABLE I Thermocouple Temperature at Film Aperture Carbons and Lamp Light on Screen Temp, in °C. . (Relative) at Aperture 12-mm. X 8-mm. S.R.A. d-c. Carbons 30 Amps, in Mirror Arc Lamp 56 525 8-mm. H.-I. a-c. Carbons 80 Amps, in H.-I. Mirror 100 630 Arc Lamp (Full light) 8-mm. H.-I. a-c. Carbons 80 Amps, in H.-I. Mirror Arc Lamp 56 425 (Light and heat reduced by wire screen between lamp and aperture) is the comparatively low temperature at the film aperture. Table I compares the relative screen illuminations obtained with a low- intensity d-c. mirror arc and the high-intensity a-c. arc, and the corresponding temperatures at the film gate. The temperature was recorded by a thermocouple placed at the aperture. No shutter was used between the lamp and the projector, and the projector was not running. Although producing almost twice as much light on the screen as the low-intensity d-c. mirror arc, the high-intensity a-c. arc caused a temperature at the film gate of only 630 °C. as compared with 525 °C for the d-c. arc. In order to compare film gate tem- peratures for a given screen illumination, a wire grating was placed between the high-intensity a-c. lamp and the aperture. The openings in the grating were of such size as to allow the same amount of light to fall upon the projection screen as from the unscreened low-inten-