The Moving picture world (July 1925-August 1925)

July 4, 1925 MOVING PICTURE WORLD Bluebook School — Answers 286-290 97 Question No. 286 — What is a "ground"? Describe some of its effects under various conditions. A. L. Fell, Collingswood, New Jersey ; T. R. Guimond, Mobile, Alabama; W. C. Budge, Springfield Gardens, Long Island; Leslie E. Yates, Waco, Texas; G. W. Bennewitz, Sioux Falls, So. Dakota; C. H. Hanover, Burlington, Iowa; Thoreau and Clark, Vancouver, British Columbia; G. L. Doe, Chicago, Illinois ; H. E. Hurlbutt, Oskosh, Wisconsin ; Charles C. Colby, Santa Fe, New Mexico; and Glenn G. Wallace, Muskegon, Michigan, all sent correct replies to this one, though two or three of them might have been better without hurting anything. Fell says : This term is used to describe any electrical connection of an electrical conductor to any conducting medium other than that to which it belongs or should be joined, which may carry current to opposite polarity in a manner not desired. A ground is a current carrying connection of such high resistance that it is not a short circuit, but which nevertheless enables or permits current to reach opposite polarity without traveling an intended path. Urn, Brother Fell. Not so good, but it "gets by" at a pinch. Your meaning is all right I think, but your wording needs some repairs, and you don't describe effects. Bennewitz says : In an electrical circuit a ground (except those which are intentionally made, such as, for instance, the permanent grounding of the neutral in an Edison 3-wire system) is an unintentional current carrying connection having so much resistance that it does not constitute a short circuit, but which will permit of a small amount of current reaching a conductor of opposite polarity without following its allotted path. It is a connection of one or both polarities with earth, or may be a connection of part of the wiring of an electrical energy delivering or consuming device with a part of its metal frame, even though the device itself be thoroughly insulated, from earth. *. In a projector arc lamp if one or both of the carbon arms be grounded to the frame of the lamp, there may and probably will be current leakage. If the ground be a heavy one, the current thus "stolen" from the arc itself will cause it to work inefficiently. The leakage may be sufficient to greatly impair the screen illumination. In a rheostat which is, as a whole, insulated from earth, one or more of the coils or grids may become grounded to the rheostat frame. If the ground be at only one place, then it should not affect the working of the resistance if the ground from earth be complete, but if the coil or grid be grounded in two places, or if the bank of coils or grids be grounded in two places to the frame, then the frame may act as a shunt which will cut out a portion of the resistance, and thus increase the current flow at the arc, and increased current may load the carbons above capacity and cause penciling. It does not necessarily follow that there will be current leakage through a ground, but a ground— other than an intentional one— is dangerous and should be eliminated. Better, but also "not so good." Now you chaps listen to Hanover for a minute. He says : The term ground is used to designate two or three separate, entirely distinct things, as follows: First, in an Edison 3-wire system the neutral wire is "grounded," not only at the power stations, but if it be an A. c. svstem, then also on the step-down side (I say step-0'jwn, for that is almost invariably what it is— the side the power consuming devices are on) of every transformer. Effect: Without such grounding there would always be the danger of conduit, etc., becoming charged with the highest voltage the system carries, with possibility of unpleasant, or even dangerous shocks, and or fire. With the neutral wire and the conduit grounded, all this of course cannot take place. The same thing is true of the frames of generators and motors, which usually are grounded for what amounts to the same reason. Second: The frame of a motion picture projector should be "grounded" to the metal of the projection room, because if it (the projector frame) be insulated therefrom and becomes charged with E. M. F. from the lamp, the lamp circuit or otherwise, there is danger of a spark which might fire the film, if a metal reel touch the magazine and frame of the room at the same time. With the two "grounded," which in this case means electrically joined, there can, of course, be no spark, even though the frame of the projection room be not grounded, which it always should be. Third: And now we arrive at the thing for which the term "Ground" most commonlly is used. When we speak of what has been described we say "Ground the neutral" or "Ground the projector," but when we say "there is a ground," we mean what I shall now set forth. A ground in this sense means an unintentional and usually accidental currentcarrying path between the opposite polarities of an electrical generator, somewhere on the distributing system, or perhaps at the generator itself. If such a path has low resistance, and carries sufficient current to immediately blow fuses, it usually is called a "short circuit." On the other hand, if it have such high resistance that the fuses stand the extra load and there is merely continuous current leakage, then it is called a "ground." Such a ground may be between adjacent wires or conductors, through metal, but with imperfect contact which offers high resistance. It may even be through wet wood or other substance which would insulate if dry, in which case it usually 'Soon dries up the moisture and cures itself. It may be between the positive of one circuit and the negative of another joined to the same generator, but situated a considerable distance apart, the path being through water or gas pipes, or even through, earth. The effect of grounds of this character is waste of electrical energy, unnecessary loading of both generator and wires, and possible overload which will lower the performance of the power using devices through overload of the wires supplying them. In fact the possible effects are so many and diverse that it would require a great deal of space to describe them in detail. Grounds set up fire hazard, or may do so at any rate. In projection, a ground in the projector lamp using A. C. will or may magnetize the entire lamphouse and base, and the magnetic action will or may injure the performance of the arc. Of course if the EX U lamp become grounded in such a way that there is heavy current leakage, the arc will receive just that much less current, and the screen illumination will suffer. Grounded rheostats, etc., also cause trouble and may cause waste. There, Gentlemen, that's a darned good answer, if you ask me; also I think Dobson and Fell and most of the rest of you knew all that quite as well as Hanover did, only you (and I'm not blaming you so much this hot weather) just did not gear up your think works. You ran on low, because it was easier on the ole think engine. Well, anyhow we got a good and very complete answer, though I will say that Doe, of Chicago, was a close second to Hanover, while Thoreau and Clark gave evidence of a good understanding of the matter, and Hurlbutt did very well. Question No. 287— What is meant by "Magnetic Density?" Bennewitz, Hanover, Doe, Thoreau and Clark, Walter Johnson, Champaign, 111.; Wallace Colby, and Guimond all made replies which I would not say are incorrect. Gosh, though, I'm stuck. Bennewitz says: Magnetic density is the number of lines .of force per unit area passing through any substance through a plane at right angles to the direction of the flux lines. In practical work in the United States of America, magnetic flux is measured in number of lines per square inch, and is usually designated by the capital letter B. That "at right angles to the direction of the flux lines" is one too many for me, nor can I find a definition in which that is included, after half an hour of diligent search. I have never heard of that twist to magnetic density, and have always considered it as just plain "the number of lines per unit area of cross section." If I am in error, please cite me to the authority who says so, Brother Bennewitz. Thoreau and Clark say : Magnetic density is a term used to indicate the density of saturation, or number of lines of magnetic force passing through a unit area of cross section. In other words, the amount of magentism of cross sectional area in a magnet. Hurlbutt says : Magnetic density is the number of lines of magnetic force passing through a magnetic field per unit area. This also is known as magnetic flux, and is measured by a unit called the "Maxwell," which is the amount of magnetism, or number of lines of magnetic force passing through every square centimeter of the field. Oh hel-elujah! I gotcha 1 Bennewitz says: "through a plane at right angles etc." I'm a dumbell. It is the plane which it at right angles to the lines of force. Gosh ! I 'spose I ought to rub all that out, and not let you know what a dumbdora I can be sometimes, but I won't. Bennewitz is right, and his wording is correct too. I'm it! Well, maybe it'll make some of you whom I have verbally spanked feel good to know that one is coming to me too, so there'll be no rubbin' done. Question No. 288 — What is the difference between single and 2-phase current7 Everybody named made good. Doe says : A single phase current is a plain A. C. current which gives the lines maximum power producing force — maximum voltage and amperage— once for each alternation, or twice for each cycle, and leaves them theoretically, if not actually absolutely without either E. M. F. or amperage an equal number of times in the same period of time. Two-phase current is, in effect, two similar single-phase currents joined in such way that when the E. M. F. of one is at maximum, the E. M. F. of the other is at zero, and vice versa. It is analogous to the steam locomotive engine. Disable one side (one engine) and twice in each revolution of the drivers the single engine on the other side will not be pulling an ounce; also twice in every revolution it will be pulling with all its power. Such action produces a lunging, jerking pull. With the other engine working, however, since they are coupled to the drivers, and through the driving axels with each other in such manner that when one engine in "on center" the other is pulling at maximum power, the resultant applied power is to all Intents and purposes steady. It is the same in somewhat lesser actual effect because of the relative rapidity of movement, with single and two-phase currents. The single phase produces a jerky, intermittent power application, whereas twophase produces an almost steady pull. I really don't see how that could be improved upon very much, so we'll just let it go at that. Question No. 289 — What is meant by the "Ratio of Transformation?" Wallace says : Transformers are for the purpose of either "stepping up" the voltage and lowering the amperage or "stepping down" the voltage and increasing the amperage. This is done by the number of turns in the primary coils with relation to the number in the secondary coil. If it is a "step up" transformer the secondary coil will have a greater number of coils than the primary and if it is a "step down" the primary will have the greater