Projection engineering (Sept 1929-Nov 1930)

Page 2,0 Projection Engineering, September, 1921) It is apparent that the motion of the center of each cross-link is the average of the motions of its two ends. Each of the first two cross-links, therefore, averages the motions of two of the gear-layers, and the third crosslink averages the motions of the first two cross-links, driving the oil cup with the average motion of all four gear-layers. Elimination of Backlash Because the deflections with which this apparatus is expected to operate are very small, it is essential that no motion be lost by "backlash" in pivots. For this reason, and to minimize pivotal friction, flat reed-springs are used for all joints. Since the linkage cannot be constructed in a single horizontal plane but must be built in several planes, it is subject to warping forces which tend to produce velocity errors. To avoid these, the linkage is extensively braced (Fig. 8). In the development of this filter, reliance upon theory had to be supplemented by measurements of the effectiveness of various models. Since smooth rotation of the turntable was in view, fluctuation in turntable speed was the performance to be directly measured. This was accomplished stroboscopically. On the rim of the turntable 216 accurately spaced grooves were cut. A disc, with six radial slots, was connected by a rigid drive to the shaft of the synchronous motor. The disc was so placed that the grooves on the rotating turntable could be observed one by one by a microscope looking through the slots in the rotating disc. Observed through this apparatus, the groove on the edge of the rim appears to stand still when the speed of the turntable is exactly "* + = Av. Turntable Vel. Max. Turntable Vel. = Min. Turntable Vel. : Angular Velocity of Vibration = 2 s f. 2 = Velocity Variation R 200 = Velocity Var. in per cent. U) T CO 200 = 200 0 U) , R wx II = Amplitude in Radians When = I, zoo = 200 0 w, to. R Fig. 9. Method of converting stroboscope micrometer reading to per cent velocity variation. The curve of large radius represents the rim of the turntable, and the curve of small radius represents the familiar diagram of harmonic motion. At the prolection of the end of the small circle's radius is a groove on the turntable rim. The quantity read on the micrometer is 2i — the maximum motion of a groove. Fig. 7. Schematic diagram of linkage.' The bell-cranks are pivoted at themechanism's center, but are not attached to the shaft. The third cross-link, between the ends of the bell-cranks, is perpendicular to the plane of the diagram. From its center extends the only member which has a rigid connection at the center of the mechanism, to the collar which drives the oil cup one thirty-sixth the speed of the motor. A small error in turntable speed causes the image of the groove to change position momentarily. The amount of this shift can be read in thousandths of an inch on a filar micrometer placed in the eyepiece of the microscope. From this reading the per cent velocity variation can be calculated (Fig. 9). The model finally developed drives the turntable with remarkable constancy. Of the velocity variations from the two major sources of error ■ — from varying loads, at one cycle per revolution, and from varying gear-spacing, at four cycles per revolution— the former has been reduced to 0.04 per cent and the latter to a point below the limit of measurement. Supplemented by suitably modeled drives for reproducing machines, recording drives of this type provide ample insurance against maltreatment of sounds by driving machinery. Projecting Films on Metal Prisms New Metallic Screen Developed Abroad Materially Reduces Arc Current Required LAMINATIONS of gold and other metals, together with pigments of various reflective properties are being offered as a means of achieving a super-reflective screen. Engineers, working for eight years in this country and abroad, have evolved a screen which, it is said, has reflective properties so high that amperage, which sometimes has been increased with sound, may now be reduced to less than one-half and still give projection as good as or better than before. This, at least, is the case with the Film Guild Cinema, which is the first New York theatre to install the new screen. "From 25 amperes, the current has been reduced to 12y2 amperes," Symond Gould, director of the theatre, declared. "That is as low as the pres ent rheostat will go, but I believe that from five to ten amperes will be sufficient for any house up to a thousand seat capacity using this type of screen." A three-inch strip of this surface, seen through a magnifying glass, looks like a blaze of diamonds. The pigments and the metals on its granulated face build up myriads of prisms. These reflect light from one to the other. When one considers that there are many of them to the square inch, one may get some idea of the highly reflective properties of the whole. The prisms give an equal reflection on each side. The screen is made in both perforated and unperforated form. The saving in current, for the smallest theatre, is said to be at least ten dollars a day, increasing proportionately with the larger houses. The screen is non-oxidizing and weatherproof. It may be washed and scrubbed. It won't peel and it won't turn black. As proof of this, it is cited that a piece of it left out of doors for years at a time, was subjected to tests which indicated that it was just as good, if not better, than before — better perhaps, because it has a tendency to whiten with age. The absence of aluminum in its make-up is responsible for its non-oxidizing properties. The screen is made in a graded series, so that each theatre may be fitted with a surface having the exact reflective angle to give every patron an evenly illuminated picture. This will be done with the least light waste possible.