Motion Picture Herald (Nov-Dec 1934)

Motion Picture Herald 17 December 15, 1934 MODERN PROJECTION PROJECTION • SOUND REPRODUCTION • ACOUSTICS PROJECTION WITH THE '^SUPREX ARC" What engineers find to be the best operating conditions for the new d.c. h i g h i n t e n s i t y carbon THE NEW developments in projection light sources representing the culmination of efforts to improve screen illumination in theatres vi^here this has been inadequate, were importantly considered, really for the first time from the point of view of operation, at the fall meeting of the Society of Motion Picture Engineers. They occupied the entire attention of the Projection Practice Committee, the report of which now becomes available, while D. B. Joy and E. R. Geib, National Carbon Company engineers, also discussed later phases of the new method, which is based upon the use of the so-called Suprex carbons for the production of a non-rotating high-intensity d. c. arc. The method itself has of course been generally discussed since its introduction, but both the Projection Practice Committee report and the Joy-Geib paper represent an intensive consideration of the method in the light of experience obtained from practical experience over a substantial period. As the committee report points out, the problem of inadequate illumination becomes particularly acute with the greater use of color, which is beginning a renascence after its retirement to the laboratory for further development. The committee report is based upon a series of practical tests, all made under actual operating conditions with the cooperation of manufacturers of motor-generators, single and three-phase rectifiers and arc lamps. The tests indicated that the consumption of the 6-mm. negative carbon was constant for currents between 40 and 50 amperes, the rate being 3^ inches an hour. However, the burning rate of the positive carbon varied with the current, the rates being 6% inches an hour at 40 amperes, \0j4, inches at 45 amperes, and 13^^ inches an hour at 50 amperes. For the 6.5-mm. negative carbon, the tests showed that the burning rate was 3% inches an hour at from 50 to 57 amperes (identical to the rate for the 6-mm. negative carbon at 40 to 50 amperes). At 65 amperes, however, the 6.5 burned at the rate of 4-}^ inches an hour. The 8-mm. positive carbon was found to burn at the rate of 6 inches an hour at 50 amperes, 8% inches an hour at 55 amperes, lOj/2 inches an hour at 60 amperes, and 13^ inches an hour at 65 amperes. The rate of consumption of the positive carbon is greatly affected by the current density, the tests disclosed. Best results in Suprex operation are to be obtained, it was found, by operating with the trims near the upper limits of their rated capacities. EFFECT OF ARC-SAP Other tests were made to find the exact effect of the arc-gap on the burning time and burning ratio of the carbons. These disclosed that if the arc-gap is increased while the arc current is held to the same value, the burning time remains unchanged — -the burning rate is practically independent of the arc-gap when the current remains constant. But if the arc-gap is increased, the current automatically decreases, and the positive carbon burns at a somewhat lower rate. Similarly, if the arc-gap is decreased, the arc current increases, and positive carbon burns at a higher rate. This demonstrates that since a change of current changes the burning time of the positive carbon while only slightly affecting the rate of the negative, change of arc-gap will change the current and therefore the ratio between the burning of positive and negative. "This change of ratio," points out the report, "tends to move the arc out of focus with the mirror. It is of the utmost importance, therefore, that the arc control mechanism be sensitive enough to hold the arc-gap constant (5/16 to 11/32 inch), and that the current also be held constant if frequent focusing of the arc is to be avoided." Tests were also made with 6 and 7-mm. carbon combinations to determine any differences in the voltage across the arc when various sources of power were used. In the tests the committee employed singleand three-phase rectifiers and motor-generators, and of course the same carbons, same lamp and same arc-gap. It was found that there was a difference of from SjA to 6 volts across the arc when using a singlephase rectifier, and of 5^ -volt with a threephase rectifier, as compared with results when a motor-generator was employed. This difference, it is pointed out, is due to the a.c. component of the rectified current. Applying the same tests to 6.5-mm. and 8-mm. carbon trim, the committee found that the d. c. arc voltage for a given arcgap depends upon the source of the current, while the voltage for the 6-mm. and 7-mm. carbons ranged between 30 at 40 amperes, and 35 at 50 amperes. For the 6.5-mm. and 8-mm. combination, the range was between 30 volts at 50 amperes, and 39 volts at 65 amperes. The investigation showed further that single-phase rectifiers do not deliver current of the same smoothness as the threephase rectifiers. In tests made to determine the visual effect of an alternating component upon the projected light under normal operating conditions and with the shutter running at 90 feet a minute, flicker resulted from the use of a single-phase rectifier, while no flicker could be noted with either the three-phase rectifier or motorgenerator. The committee also examined reflector mirrors in theatres where Suprex carbons had been in use for a substantial period and consistently found them pitted, indicating that it is necessary to watch for pitting and to replace pitted mirrors if proper illuminating is to be maintained. The committee recommends the use of a "miror guard," which is made of optical glass, has the same curvature as the mirror, and fits exactly inside it. SCREEN ILLUMINATION The findings of Messrs. Joy and Geib impinged at some points upon those of the Projection Practice Committee, but reveal other phases of the matter. They found that if the current is increased, but the arc length and the position of the arc with respect to the miror is held constant, there is a very definite increase in the screen light but very little change in light distribution. This increase in light is accompanied by an increase in crater depth and carbon consumption. If the arc current is too small, the crater is very shallow and the light is not uniform in color. If the current is too great, the consumption is excessive and the light is unsteady. Thus to obtain good distribution of light upon the screen in Suprex-type operation, it is necessary to