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1945 Theatre Catalog, 4th Edition, Page 335 (311)

1945 Theatre Catalog, 4th Edition
1945 Theatre Catalog
1945 Theatre Catalog, 4th Edition, Page 335
Page 335

1945 Theatre Catalog, 4th Edition, Page 335

fused. The shorter the focal length of the projection lens, the closer it will be to the film, and the more of this scattered light will enter it and be focused upon the screen. If a very large reflector is used in the arc lamp in an effort to collect more light from the arc, and a long-focal-length lens is used at the same time, the light reflected by the edges of the mirror may enter the aperture at such a steep angle as to cross the lens housing completely and strike the side before reaching the lens. Any such light will not enter the lens, unless it is reilected from the side of the lens housing, and will not contribute to producing illumination on the screen. However, it will heat up the aperture plate and film. Making the reflector so large as in the instance cited would be an extreme case, but will serve as an example of how lack of proper coordination between units may fail to produce the desired result, and also introduce other undesired effects.

Another problem that must be solved is that of an arc lamp with unsteady carbon feed, this is a hazard in the projection room and a detriment to good picture quality. Many a yellow or blue tinted screen is caused by improper carbon feed. The general purpose RCA Enarc high intensity are lamp utilizes a very flexible and fool-proof carbon feed mechanism. The motor speed which controls the feed of both positive and negative carbons is regulated by a potentiometer. In addition to this over-all speed control, a separate adjustment of the negative feed can be made with relation to the speed of positiVe carbon feed. These two adjustments make it possible to accurately adjust and regularly sustain the feed of both positive and negative carbons so that a uniformly brilliant and white screen is consistently produced.

Current Converters

The arc power is usually obtained from the ac. power line, so some form of conversion device is required. Motor generators and rectifiers are available for this service. Each has certain definite advantages and limitations. The Century-Roth motor generators now sold by RCA have been noted for years for their

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sturdy construction and unfailing operation. Certain types do not require d.c. ballast resistors, thus eliminating ballast power loss.

The Baldor tube rectifier and the Westinghouse copper-oxide rectifier need no introduction, since both have enjoyed a reputation for utmost excellence during many years.

The choice of units will depend upon local conditions. The electrical efficiency of the rectifier will generally somewhat exceed that of the motor generator, thus

reducing a.c. current drain. On the other hand, the rectifier has no storage action, so it must pass on to the arc lamp any fluctuation or momentary interruption of the ac. supply. The motor generator will tend to smooth out these interruptions because of its flywheel effect, so if power supply fluctuations exist, the small extra cost of operating a motor generator may be well worthwhile in avoiding momentarily dark screens.

If a rectifier is to be used, the type


1945 Theatre Catalog, 4th Edition, Page 335