Moving Picture World (Mar-May 1927)

762 MOVING PICTURE WORLD April 23. 1927 Bluebook School Answers 560 and 561 Note: — This “School is designed to arouse interest in the study of those many problems which constantly arise in motion picture projection, AND to cause men to really study the Bluebook and assimilate the vast amount of knowledge contained within its covers. Question No. 560 — Suppose you were sent out to put on a show in a country place. Arriving late you found the current to be 110 volt D.C., but that no rheostats had been provided. .You were able to secure three 50volt, 90-ampere rheostats. The highest amperage available from the local power lines is fifty. What would you do ? Really I was a bit surprised at the rather weird assortment of answers to this rather simple problem. Some very competent men made a basic error in that they did not stop to consider that rheostats such as described, found in a country town, naturally would not be rheostats built to work with a projection arc, but just what they are called, Viz 50volt, 90 ampere rheostats — rheostats which will pass that amperage when connected directly to that voltage, hence these men very greatly underestimated or under calculated the resistance they would offer. That “catch” was deliberately planned and you fell for it. I blush for you ! There were only three men who made good on this one. One of them is brother W. R. Gwynn, of Longmont, Colorado, whose answer I shall publish, C. H. Hanover, Burlington, Iowa and Allan Gengenbeck, New Orleans, La. In addition G. L. Doe, Chicago, Ills., W. C. Budge, Springfield Gardens, N. Y., Frank Dudiak, Fairmont, Va., and S. F. Cooley, Manchester, N. H., had the right general idea, but were careless in their “figgerin.’ or guessed at the arc voltage. Brother Gwynn says : ‘‘Before deciding how the three available rheostats should be connected to deliver sufficient current for a good light without exceeding the maximum capacity of the supply service, it will be of advantage to know just what resistance is necessary at the rheostats to prevent a flow of current in excess of 50 ampere, the maximum amount of current available as stated in the problem. After the necessary resistance figure has been determined and the resistance of each rheostat has also beqn calculated, a glance at the resultant figures will give the clue as to which of five possible connections will be nearest to the result desired. It must not be forgotten that the arc itself will offer resistance, and this must be taken into consideration in calculating the resistance necessary in the rheostat hook-up. With the above points in mind, the problem may be approached as follows: What is the total resistance necessary to limit the current flow of the circuit to 50 amperes when the pressure is 110 volts? llOv 11 Applying Ohms law: R = = — or 50a 5 2.20 ohms, which is the total resistance required. The real question is: of the 2.20 ohms re sistance necessary to limit the current to 50 amperes, how many will be supplied by the arc itself? Turning to page 400 of the Bluebook, we find that 56 volts is approximately the correct pressure for a D.C. arc of 50 amperes. Again applying Ohms law it is found that: 56v R = = 1.12 ohms, which is the arc re 50a sistance, and subtracting the arc resistance (1.12 ohms) from the total resistance (2.20 ohms) it is found that the rheostat must supply a resistance of 1.08 ohms. What is the resistance of a 50 volt, 90 am pere rheostat? 50v 5 R — or — ohms 90a 9 The question now is, shall the rheostats be hooked in a series of three, multiple of three, one series and two multiple, two in series, or two in multiple? Since slightly over 1 ohm resistance is all that is required of the rheostat, and since each of them has a resistance of 5/9 ohms, or slightly more than yz of an ohm, it is apparent that a series of three would cut the current below the amperage maximum available. Three in multiple would offer 1/3 the resistance of one rheostat or 5/27. ohms, which is far below the resistance required; two in multiple would offer half the resistance of one rheostat or 5/18 ohms which also is less than the resistance required of the rheostats. Out of the five possible connections only two have promise of giving the result desired (1.08 ohms). These are the two in series connection and the two in multiple and one in series connection. Of these two, the two in series connection looks the more promising and will be worth working out. “Resistance in series equals the sum of the separate resistance unit so connected,” hence 5 10 R = 2 x — = — or 1.19 ohms. 9 9 It has already been determined that the resistance of the arc is 1.12 ohms. Adding the resistance offered by the two rheostats in series to the resistance of the arc, it is found that the circuit now offers a total resistance of 2.27 ohms. Again applying ohms law it is found that this resistance will pass a current of 48.89 amperes as follows: lOOv C = = 48.89 amperes 2.27a Note: Don’t freeze the arcon such a hook up, since the resistance offered by this member of the resistance family when it is burning is a vital factor. Its elimination would quickly set off the fireworks. As to the three rheostats in a series-multiple hookup, it will be found that such a combination will pass 56 plus amperes, which is slightly over the maximum permitted in the problem.” Question No. 561 — Explain just why two 110 volt rheostats cannot be used in multiple on 110 volts. WHY PAY MORE? Roll Tickets Your Own Special Wording 1 0O.OOOforSI 5.50 10,000 for $4.50, 20,000 for $7.50 50,000 for $10.00 Standard Rolls of 2,000 KEYSTONE TICKET CO. Dept. W„ SHAMOKIN, PA. The Union Label if you want it. Have been printing Roll Tickets for 10 years and no better can be had at any price. This was obviously a miscabobble. “On 220 volts” was what was meant. Brothei George Lawrence, Sackville, N. B., S. F. Cooley, and W. B. Budge were the only ones who seem to have sensed this. Lawrence says : “I see no reason why they could not be used in multiple on 110 volts, but if it were attempted to connect them in multiple on 220, their resistance elements would be overloaded to such an extent that they would burn out very quickly, meanwhile delivering an amperage far above their normal capacity. Title Referendum I WAS a bit amused to see a monthly paper with a small circulation recently seriously proposing something this department considered and discarded as impractical years ago. It, with much less than a thousand paid circulation, proposed to “conduct a nation-wide referendum” on the question: Are you in favor of the term ‘projectionist.’ ? Well, well, well! This little paper proposes to jump in after all these years and undertake to conduct a “nation-wide” referendum (Canada apparently entirely ignored) covering about 18,000 theatres and say, anyhow, 25,000 machine operators and motion picture projectionists with less than a thousand circulation. Of course, the Society of Motion Picture Engineers has, after due deliberation, officially adopted the title this editor proposes to question, but after all, what is a mere authoritative body such as the Society as compared with a “nation-wide” referendum. How Did She Swim So Long ? BRO JAMEsON, St. Louis, Mo., asks: “Some time ago I projected a picture in which a woman swam under water longer than any one could possibly do so. It was a tank with a glass side. Will you please tell me how it was done.” I don’t know, Friend Jameson, but here is one method. A motion picture is taken of a glass tank in which the water is kept in movement There is no one in the tank. Next, a swimmer is either placed on a strip of black velvet on the floor of the studio, where she simulates the motions of swimming while the same length of film is again exposed, or the swimmer is suspended by a suitable, invisible set of fine wires and simulates the motions of swimming while he or she is manipulated by stagehands from a platform above, the camera being, of course, directly over the swimmer in either case. When the film is developed, through double exposure, we have the swimmer apparently in a tank of water. I am not certain whether the swimmer or the tank picture is shot first, or whether it makes any difference which is shot first. I am not a cinematographer or a studio man.