The Moving picture world (November 1926-December 1926)

618 MOVING PICTURE WORLD December 25, 1926 Bluebook School Answers 542 and 543 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. 524 — Wtat is the resistance of a 60-ainpere, 58-volt arc, and what would be the difference in amperage when the arc is burning normally at 58 volts and when the carbons are frozen, assuming line voltage to be no? The following sent acceptable answers : W. C. Budge, Springfield Gardens, N. Y. ; Charles E. Curie, Chattanooga, Tenn. ; C. H. Hanover, Burlington, Iowa; G. L. Doe, "Bill" Doe, John Doe and Charles Doe, all of Chicago ; E. Fergodo, Livermore, Calif. ; Walter Munn, Iron Mountain, Mich.; G. R. Hahn, Memphis, Tenn.; Gilbert Atkindon, Cleveland, Ohio; Frank Dudiak, Fairmont, W. Va.; T. R. Bankerton, VVentzville, Mo.; Albert Tonnerson, St. Louis, Mo.; Albert Hancock, Dallas, Texas; D. G. Henderson, Quincy, 111. ; Charles C. Colby, Santa Fe, New Mex. ; Andrew T. Boylson, Halifax, Nova Scotia, and Tony Domini, San Juan, Porto Rico. There is little to choose between the answers of Budge, all the Does, Hanover, Curie, Dudiak, Livermore, Munn, Atkindon and some others. They all said the same thing, in much the same way. Curie, hov;ever, sets the matter forth very briefly, hence I think he wins for publication, as extended explanations would be of slight value. He says : The resistance of a 60 ampere, 58 volt arc would be (amperes divided by volts equals resistance) 58 divided by 60 equals 0.9666 ohms. The resistance of the rheostat would be (110 minus 58) divided by 60 equals 0.8666 ohms. The proof is that it requires 1.8333 ohms resistance to pass 60 amperes under 110 volts pressure, and 0.9666 plus 0.8666 equals 1.S333 (and as many more threes as you care for) ohms. With the carbons frozen we would have the arc resistance eliminated, and would have only the rheostatic resistance remaining, and since we have seen it has 0.8666 ohms resistance, we have but to divide 110 by 0.8066 to get 126.9 amperes as the resultant amperage. Hanover, John and G. L. Doe, Budge, Atkindon, Bankerton and Hancock added this bit of important data : This Increased amperage would almost immediately overheat the rheostat coils, hence set up a higher resistance, thus reducing the actual amperage below that indicated by the calculation. Hanover adds : There would be considerable resistance at the carbon tips even If frozen, as contact would not be any good. Atkindon, Hanover and Hancock give this finishing touch : Just what the ultimate amperage would be could only be determined if the actual exact temperature of the rheostate were a grid and Its temperature co-efflcient be known. Question No. 534— <What do you understand to be meant by the "fixed" and "variable" resistance in a rheostat, and in just what manner is the resistance of a variable rheostat 'varied? Just what happen* when you move the adjustment handle? Al the above, plus Al Lehman, Glenside, Pa.; Thomas T. Alberts, Brookfield, Mo.; .\. T. Bowers, Erie, Pa., and John AUerton, New York City, did all right on this one. Budge says : A fixed resistance rheostat has but two binding posts. The current enters the coils or grids through one of them, and Is forced to pass through the entire length of the resistance element to reach the other or "exit" binding post. Such a unit Is designed to deliver a certain, fixed number of amperes when opposed to the voltage for which it was designed. A variable resistance rheostat is one in which the coils or grids are tapped at different points, so that by means of a suitable mechanism the current may be made to pass through the entire resistance element, or to only pass thro'igh a portion of it. When the adjustment handle of a variable resistance is moved, the effect is to "cut out" or "cut in" a certain, predetermined amount of the total resistance element, and thus increase or decrease the total resistance and amperage, which latter is, of course, dependent upon the amount of resistance opposed to the voltage. A very good answer, but Hanover adds this important item which is a point overlooked by Brother P'Udge : In all variable or adjustable rheostats Study Increases That Qray Matter LL. BALL, projectionist. Empress Theatre, Arma. Kansas, says: "Attached • find my first attempt at answering Bluebook School questions. Hope it will "get by." The "school" certainly has been a big help to me. Keep it up, F. H., because our gray matter is constantly on the increase." The human brain, unlike most other things, improves rapidly with use. One especially encouraging thing in connection with the school is that a surprising number of men answer the questions week after week after week, without hardly a break, and almost never correctly enough to be included in the list published. Yet these same men often say the school has helped them immensely, and it unquestionably has too, because they have done something they never did before, viz: studied the technical matters of their profession and learned that they really did lack lots of knowledge. They there is a certain definite amount of fixed resistance, which cannot be cut out and which Is suflicient, if the rheostat be used on a voltage not in excess of that for which it Is designed, to limit the flow to the capacity of the rheostat coils or grids when all resistance handled by the adjustment device is cut out. See, Brother Budge and the rest of you, how easy it is to overlook some important item. No doubt you all knew that just as well as Hanover did, but it is just such things that more often than not influence my decisions when it comes to publication. I used Brother Budge's answer this time because it was absolutely correct, as far as it went, and it went as far as did the reply of any one else except Hanover and Bankerton, and all the latter said was : "Of course it is understood that all adjustable rheostats have a certain amount of fixed resistance." What's in a Name? P. S. — In looking over the answers again I note that Munn brought out one or two additional points, but unfortunately they are not germane to the question as asked. Qoshl Just Look Who's Returning EVERY once in a while a letter comes which causes the editorial heart to skip from one to seven and a half beats. You will all remember that for a long while a certain live wire up in South Dakota was a regular and darned good correspondent of this department. Yep, it's Bennewitz I'm talking about. We all thought he had crawled into a hole and pulled the lid down even tighter than Gray did, but here comes a letter saying that he has been enormously busy, but will be with us again in the department after the first of the year. All right. Old Timer, the departmental calf will be put on a corn diet immediately, and fatten against the time of your appearance. Glad to have you back, and that's no loose talk either. You were one of the family, darn y' and we all missed you a lot. Handbook of Projection Notice The last minute additions of extra equipment matter has held up publication of the new Fifth Edition HANDBOOK OF PROJECTION longer than we anticipated. It is rapi<Uy nearing completion now, however, and our readers will be advised through the pages of Moving Picture World just as soon as the book is ready. TTiose who wish to receive copies as quickly as possible should send in their orders now. The price of this two volume set with limp leather binding will be $6M. Make your checks or money orders payable to Chalmers Publishing Co., Address: — 516 Fifth Avenue, New York City, N. Y.