International projectionist (Oct 1931-Sept 1933)

Record Details:

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THE NEW ALTERNATING CURRENT PROJECTION ARC D. B. Joy and A. C. Downes ENGINEERING DEPARTMENT, NATIONAL CARBON COMPANY IN projecting motion pictures, several grades or types of carbon and several makes or designs of lamp are used. But all the grades of carbons produce arcs of only three fundamental types ; and all the lamps can be placed in one of three classes if the very few installations still using lamps of the condenser type, burning either direct-current, neutral-cored carbons or the white-flame, alternating-current special carbons, be omitted. The three carbon arc types are (1) the highintensity, rare-earth, cored-carbon, direct-current arc; (2) the plain neutralcored, direct-current arc, and (3) the so-called special white-flame, alternating-current projector arc. The three lamp classes and the carbons burned in them are shown in Table I. Both Classes I and II use the directcurrent, high-intensity arc, which produces the brilliant light of blue-white sunlight quality universally considered desirable in the theater. In the range of current used in these two classes, there is no gap of any magnitude, so that the theaters using these highintensity arcs can easily arrive at the level of screen illumination best suited to their particular conditions. There is a large drop in current value, however, between the high Snpplementing the article on the new A.C. carbon arc which appeared in our April issue is the accompanying very much more detailed exposition of this subject which was presented to the Spring, 1932, meeting of the S.M.P.E, Of particular interest are the various tables which tend to sustain the claims, relative to economy and efficiency, which have been advanced in favor of this new arc, a development of the National Carbon Co. — Editor. intensity arcs of Class II and the lowintensity arcs of Class III; and the color of the light from the lowintensity installations, while appearing a brilliant white viewed by itself, is yellowish-white when compared with the high-intensity sources of Classes I and II. A large number of the theaters using the low-intensity light sources desire the same blue-white light of the high-intensity sources, but to obtain this desirable light color would require a change to more expensive lamp equipment and higher power cost. Bridging the Gap To bridge the gap between the highintensity and low-intensity arcs, and at the same time to give the small theater the advantage of the bluewhite light enjoyed by the large theaters, has been the object of researches in the laboratories of the National Carbon Co. for several years. The results of this long research show that it is possible to fill the gap between the high-intensity and low-intensity sources with an arc giving a light color very similar to that of the highintensity arcs, which will also provide a number of other advantages to theater owners of Class III, who outnumber the high-intensity classes by two to one. The desired result has been accomplished by means of an alternatingcurrent arc burned on the secondary of a specially designed transformer, without the ballast resistance, always necessary with direct current, and also without the motor-generator set or rectifier now required in the vast majority of theaters of all classes. This alternating-current arc is a modified white-flame arc with specially designed carbons containing compounds of the cerium rare earth group of elements. The accompanying Figs. 1 to 4 show the differences in the light sources of various carbon arcs and why this new alternating-current arc should be of great value in projection. Fig. 1 is a front and side view of a high-intensity arc, showing very clearly the very brilliant, concentrated light source at the positive crater and TABLE I Classes of Projector Lamps and Carbons Class Kind of Carbon Type of Lamp Current Used (Amps.) Per Cent of Carbons Used I 11-mm. high-intensity positive carbon 13.6-mm. " 16-mm. ^Vsz" to Vs" Orotip negative carbon Condenser lenses, rotating positive carbon 85-150 d-c. 15 II 9-mm. high-intensity positive carbon Vie" Orotip negative carbon Reflecting mirror, rotating positive carbon 65-85 d-c. 18 III 7-10-mm. reflecting arc negative carbon l()-14-mm. " " positive Reflecting rriirror, non-rotating positive carbon 16-42 60 Miscellaneous: White-Flame A-c. Special Low-Intensity D-c. Arcs with Condenser Lenses 7 [20]