The motion picture projectionist (Nov 1929-Oct 1930)

32 Motion Picture Projectionist October, 1930 composition of the light sensitive element. The cell proper consists of two glass discs about three-quarters of an inch in diameter between which the photo-conductive surface is sealed. This latter in turn is mounted in a bakelite or metal case. The nature of the photo-conductive material cannot be disclosed at this time. Durability The extreme physical and electrical ruggedness of the X cell lies in the fact that no vacuum tubes or gases are used, thereby eliminating the possibilities of damage by breakage or ionization. Tests have shown that the X cell may be subjected to the direct sunlight for one thousand hours with the full voltage applied without any change in the electrical characteristics whatever. Under normal operating conditions this assures a long trouble free life. Sensitivity For motion picture sound work it will be interesting to note that with .4 lumens light flux (approximate amount of light obtained through the ordinary optical system) at 4.5 volts an output of one-half milliampere is obtained. This is sufficient to load up any three-stage power amplifier to capacity. In actual tests a SilverMarshall type 692 amplifier was used working directly from the X cell through an audio transformer. In this way ample volume was obtained for a twelve hundred-seat auditorium. In other tests using only a twostage Loftin White amplifier enough volume was obtained to fill a room thirty by sixty feet. This fact alone opens up great possibilities in the field of portable and home talking pictures. For those interested in relay work, it will be seen that the X cell will operate without any amplification whatever a relay up to 5 milliamperes on 22.5 volts with 51.2 lumen light flux. Coupling Mediums The new X cell is adaptable to any means of coupling such as direct resistance, impedance or transformer. For motion picture work the X cells are manufactured with a low impedance to permit transformer coupling. With the latter type a two-transformer fader unit can be located between the projectors and leads from this run anywhere in the room to the power amplifier. Two small 4.5 volt C batteries can be located in the transformer box to supply the polarizing voltage for the cells. This makes sound-on-film as simple as disc reproduction, eliminating battery-operated head amplifiers which are a constant source of worry. Further information on this new cell development may be had by addressing Pacific Research Laboratories, 1489 West Washington St., Los Angeles, Calif. New Changeover Adapter for Rear Shutter Projectors With the introduction of the rear shutter it was found that certain changeovers that were attached to the rear of the projector head would not fit there any longer because the manufacturers of rear shutters had not allowed enough space between the rear shutter housing and the projector for the box to fit in. This necessitated changes not in the rear shutter but in the changeover attachment. A special adapter for the projection head to overcome this difficulty is now being made by the W. & W. Specialty Co., 159 West 21st St., New York. This special adapter permits the changeover box to be fitted right in, and at a very minimum expense. With this new adapter it is not necessary either to make troublesome alterations or to purchase a new changeover when changing shutters on existing heads or buying a new rear shutter projector. This new adapter has no parts that will crack or break under ordinary usage, and it can be attached within a very few minutes, no drilling or tapping being necessary. The projectionist simply screws the adapter to the head — where the box originally was — and then screws the box down upon the adapter; then reverses the leads in the box and everything is set to run. These adapters are now being used extensively on the Fox Circuit. Present A. C. Heater Tubes Withstand Intense Heat Present-day heater type A. C. tubes mark an achievement of the highest order in the development of severe service materials. For one thing, the insulator tubing in most tubes must have a fusion point above 2270 deg. C, as contrasted with 1820 deg. which is the fusion point of porcelain. What is more, the insulating tubing must not warp, shrink, break, decompose or interact with the tungsten wire at this temperature. It must remain a good dielectric at glowing temperatures. Meanwhile, the tiny holes throughout the length of the tubing no larger than the lead of an ordinary lead pencil — holes hardly visible to the naked eye — must be accurate in size and evenly spaced at all times. The short life of earlier A. C. tubes has been traced to several causes. In the first place, materials originally employed for the heater insulator were quite unsatisfactory. The severity of the service conditions was not realized. Impurities resulted in brokendown insulation. There was frequently a chemical interaction between insulating material and tungsten wire. A common cause of failure was the fusing of insulator and wire, with early breakage of both tubing and wire, due to unequal rates of expansion and contraction. Porce lains, fused quartz, alumina and other materials were tried in turn, only to prove incapable of fulfilling the extremely trying conditions. Magnesia Great Aid The remarkable life and performance of present-day heater type tubes, according to Henry L. Crowley of West Orange, N. J., a pioneer worker and manufacturer of synthetic ceramics, are due to the introduction of magnesia. This material eliminates the heretofore critical exhaust conditions and high shrinkage in tube production, and provides the desired operating conditions. The crolite magnesia insulated tubes now available on the market have a life of several thousand hours, and, what is more, are capable of withstanding severe voltage overloads without materially decreasing their useful life, thus solving the problem of fluctuating line voltage faced in many sections of the country. In fact, it is now possible to obtain heater type tubes superior to the filament type, due to the successful solution of the insulation problem. New De Forest P.E. Cell Greatly increased sensitivity is the outstanding feature of the new De Forest caesium type photo-electric cell, aside from extreme compactness. This cell, in a — 99 size glass bulb, with the standard four-prong base, has an output of 35 to 75 microamperes per lumen, or several times the output of the larger potassium photo-electric cells heretofore extensively employed. As an example of its sensitivity, an automobile headlight bulb a foot away from the cell, shining through a %-inch hole, will result in an output of 4 to 7 microamperes. To obtain a corresponding output from the potassium type cell, a large size incandescent lamp of many times as much candle-power would have to be employed as the light source. The new De Forest caesium photoelectric cell has been employed for several months past in the radiovision pick-up equipment of the De Forest experimental transmitting station, W2XCD, maintained at Passaic, N. J. Blue Seal Products Co. Has Fine Equipment Record The Contner-Blue Seal Universal Lens Adaptor and Aperture System for projecting standard and movietone film the full screen ar,ea was introduced to the trade in June, 1929. This device has met with great success to the extent that many thousands of installations have been made throughout the world. Many larger theatre circuits have adopted them as standard equipment, as they have many other Blue Seal products. A new and important improvement