Nation projector carbons (1935)

National Projector Carbons avoid the above conditions, the 8 mm A.C. highintensity carbons at 80 amperes will exhibit good burning characteristics for arc lengths between 0.23inch and 0.36-inch, and from approximately 24 to 29 volts. There will, however, be a noticeable change of light intensity between those extreme limits, so that the permissible range of variation in arc length and voltage from moment to moment is much less than the complete range of satisfactory performance. At 75 amperes the arc lengths that will give good burning characteristics with the 8 mm carbons are essentially the same as those for 80 amperes, and the arc voltage is approximately one volt lower. The 7 mm A.C. high-intensity carbons are rated from 60 to 65 amperes. The corresponding conditions for good operation are an arc gap of 0.21-inch to 0.31inch, and an arc voltage of approximately 23 to 26 volts. One of the objectives of good projection is to maintain a light of uniform intensity and good distribution on the screen at all times. As has been pointed out in the case of other types of arcs, the light on the projection screen is dependent on both the arc itself and the position of the arc in relation to the optical system. This applies also to the A.C. highintensity arc. The lamps using this carbon ordinarily have a horizontal trim, with the carbon tip facing the back of the lamp held at the focus of an elliptical reflector that projects the light picked up from the carbon and arc onto the aperture and film. These lamps are provided with a device that throws an image of the arc on a white card, on which the correct arc length and arc position with respect to the mirror are indicated. They also have a method of feeding and adjusting the arc that keeps the carbons in the proper position with respect to each other and the optical system, provided the lamp is operated properly and 80