The Moving picture world (November 1926-December 1926)

November 13, 1926 MOVING PICTURE WORLD 117 Bluehook School Answers Nos, 528 and 529 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. 528 — What effect has a rise in temperature on the resistance of metals and upon the resistance of carbon? W. C. Budge, Springfield Gardens, N. Y. ; C. H. Hanover, Burlington, la. ; Charles E. Curie, Chattanooga, Tenn. ; G. L. Doe, Chicago, 111., and Allan Gengenbeck, New Orleans, La., each gave a remarkably good answer to this question while "Bill" Doe, Chicago, 111. ; E. Fergodo, Livermore, Cal. ; Frank Dudiak, Fairmont, W. Va. ; Chas. C. Colby, Santa Fe, N. M.; T. R. Guimond, Mobile, Ala. ; F. D. Orenbacher, Truesdale, Mo.; Andrew T. Boylson, Halifax, Nova Scotia, and John Doe, Chicago, 111., all made correct replies. I believe Brother Curie's answer is perhaps the best for publication purposes. He says : "The resistance of all pure metals increases as the temperature of the metals is increase.l. The resistance of certain allo'ys, however, does not increase with increase in temperature, The increase in resistance for each degree increase in temperature, as measured in ohms, is what is known as the 'temperature coefBcient of resistance.' A coefficient is a multiplier. Such coefficients are determined experimentally for different metals. For all pure metals the coefficient is practically the same. It is .004 of an ohm per degree Centigrade, or .0023 of an ohm per degree Fahrenheit. The temperature coefficient of an alloy is usually less than the coefficients of the constituent parts of the alloy. "In a few words, in pure metals the increase and decrease in resistance is directly proportional to increase or decrease in temperature. Carbon, on the other hand, acts exactly opposite to metals in this respect. Its resistance decreases as temperature is increased, and vice versa." Some Alloys Useless An excellent answer but it should be supplemented by the statement that no alloys which have stationary resistance regardless of temperature fluctuations, are used in electrical work. In fact, I personally know of no such alloys, and would like to have citation of the authority from which Brother Curie obtained his data concerning them. Question No. 529 — Is resistance in metals directly proportional to the increase in temperature over normal ? The correct answers to this one were essentially the satiie as those for Question 528, but Brother Curie has provided a complete one, taking as his authorit}' the "Electrical Engineer's Handbook." Had I noticed Curie's very complete reply to this question in time, I would have used one of the answers of the men named as having supplied excellent replies to the previous question, thus distributing the "honors." Curie says : "The increase or decrease of resistance of all pure metals to eleotric current is directly proportional to increase or decrease of temperature. The following is only an approximate method of finding the resistance of a conductor at any ordinary temperature, but it is sufficiently accurate for all ordinary work. The formulas are: WHY PAY MORE? Roll Tickets Your Own Special Wording 100,000for$15.50 10,000 for $4.50, 20,000 for $7.50 50,000 for $10.00 Standard Rolls of 2,0D0 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 aaf price. Rn = Rc+ [axRc (Th — Tc)] (ohms) Tn — Tc = Rh — Ro (deg. C. or F.) a X Rc "Wherein Rn = resistance, in ohms, hot. Rc = resistance, in ohms, cold. Tn — temperature of conductor, hot, in degrees either C. or F., depending on which coefficient is selected. Tc — temperature of the conductor, cold, in degrees, a — the average temperature coefficient of the conductor material. "Example. — The resistance of a circular mil-foot of anealed copper is 9.59 ohms at .32 deg. P. What will be the resistance at 75 deg. F.?. ''Solution. — From the following table the coefficient is 0.0002,23. Substitute in the first formula: Rd = Rc + a X Rc (Tn — Tc) = 9.59 + [0.002,23 X 9.59 (75 — 32)] =9.59 +0.002,23 + 9.59x43 = 9.59 + 0.92 — 10.51 ohms at 75 degrees F. "The formulas given here assume that the temperature coefficient of resistance is constant for all temperatures. This assumption is not strictly true because the temperature coefficient for a metal decreases as the temperature increases. The reason for this is that the resistance of any conductor is greater at, for example, 35 deg. C. than it is at O deg. C. Hence, the proportional increase in resistance for each ohm, for each degree rise in temperature, will be less at 35 deg. C. than for each ohm at O deg. C. The values given of "a" referred to in the formulas and in the following table are average values; Commutator Sparking FROM a western town conies this request for information : "Please do not publish my name. This is a small town. The theatre only opened a short while since and I am trying hard to learn, studying the Bluebook and the Bluebook school. My trouble just now is that there is sparking on the commutator, at the ends of the brushes, and I cannot stop it. Will you please tell me what I ought to do?" You have told n',e very little — not even what sort of machine it is. There are two general types of the kind of commutator sparking you tell of. One of them is caused l)y a dirty commutator and the other by wrong adjustment of the brushes. If the sparks have a brilliant blue appearance and the commutator is clean and apparently in good order, then the trouble is probably due to wrong adjustment of the brushes and you will have to loosen the yoke carrying the brush holders (I can't tell you in detail how to do this, not knowing what sort of machine it is) and rotate the yoke slightly until the sparking ceases. If the commutator seems to be rough and dirty, and the sparks not a brilliant blue, but more of a subdued, reddish color, then the trouble may be remedied by cleaning the commutator, using 00 sandpaper folded and held lightly against its surface, while the machine is running, but preferably when no current is being used. When you have cleaned the commutator thoroughly, stop the machine and wipe off all dust, rubbing the bars lengthwise. Then apply a little vaseline to a square of canvas, in its center. Fold it over so that the vaseline is inside. Apply heat until the vaseline has impregnated the canvas. Use more judgment — more judgment than vaseline. You only want to get the thinest imaginable coating of vaseline on the commutator. Remember that. When the canvas is ready, start the machine and hold the canvas against the commutator, a moment, very slightly — just so as to lubricate the face of the bars very slightly. I would recommend that you follow instructions on pages 451 to 460. APPRO.XIMATE TEMPERATURE COEFFICIENTS OP CONDUCTORS Conductors Silver, pure annealed Copper, annealed Copper, hard drawn Aluminum, 97 per cent. pure. Zinc, very pure Iron wire Nickel Steel (wire) Phosphor-bronze German silver Platinoid Manganin • Average temperature coefficient per deg. C. between O deg. and 100 deg. C. 0.004,000 0.004,020 0.004,020 0.004,350 0.004,000 0.004, <>:io 0.000,220 0.004,030 0.000,640 (1.000,400 0.000,310 0.000,000 Average temperature coefficient per deg. F. between 32 deg. and 212 deg. F. 0.002,220 0.002,230 0.002,230 0.002,420 0.002,200 0.002,570 0.003,400 0.002.570 0.000,350 0.000.220 0.000,172 0.000.000