Moving Picture News (Jan-Dec 1911)

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THE MOVING PICTURE NEWS 19 you would know your main fuse has blown, or else that the power has been shut off for some cause. Sketch No. 2 shows another proper way to connect up a moving picture lamp or ordinarj^ arc lamp. This sketch is designed to show j-ou how you can have two sets of fuses to protect your lamp, so that when one fuse blows you cut in on another set by simply throwing the double pole switch over, without waiting to install another fuse. Mr. Bennett advises a 75-ampere switch for all operating room circuits, and the advice is good. He also advises that no wire smaller than a No. 6 be used, while heavier wire is still better. Wire your booth, he says, with the proper size wire in the start, and then you won't have to be changing from time to time. "I am aware," sa3^s jNIr. Bennett, '"that j-ou may not need heavy wire, but never get the idea that you can have your wires too big, because in this you are mistaken." All of which the editor of this department will cheerfully endorse. Mr. Bennett further says: "Remember at all times that the size of wire required is dependent on the following, viz.: The distance your booth is from the point where the service wires enter the building. The number of machines to be installed in the booth, whether you are to use alternating or direct current and what voltage you are going to use. If i'ou use the so-called current savers or choke coils, then your rules change a little." 1 do not quite follow Neighbor Bennett in all of this. The voltage has nothing to do with the size of w^ires, except where a transformer is being used. Neither does it make any difference whether alternating or direct current is used, unless it be that a transformer is used, or, at least, I know of no material difference. The necessary size of wire will be governed bj the number of amperes the lamps in the booth will consume and, as Neighbor Bennett says, the distance of the booth from the supph' wires, or main switch board. We are indebted to Neighbor Bennett for the excellent sketches, another one of wliich will appear in the near future. What I mean by a transformer influencing sizes is that larger wires are necessary on the secondary than on the primary, due to drop in voltage and increased amperage. T cannot help but to rise to a point of order, when I see such a statement given by a man who has heretofore claimed to be a well-versed expert on both electrical and mechanical problems relating to the projecting machine. I would advise him to get a book, or take a course in some correspondence school that teaches elementary electricity. I shall endeavor to enlighten him on the following, that the voltage plaj-s an important part in the size of wire used. It is well known that in overcoming resistance electrical energy is wasted, and, as it costs money to develop or buy electrical energy, it is evident that such waste must be kept down to a minimum. Tliis maj be done by decreasing the resistance of the conductors, or what amounts to the same thing, or increasing the size of the conductors. The effect of resistance is to cause a drop in the voltage in the supply circuit, and the energy lost in overcoming this resistance appears in the form of heat. A few illustrations may suffice to show what an important bearing this question has in the size of wiring. A two-wire circuit supplies a current of 50 amperes to an arc lamp at a distance of 200 feet from the meter. The drop allowed is five per cent and the voltage of the circuit is 110 volts. What size wire should be used? Substituting the value given above, in formula, the size of the wire is found to be 2200 X 50 X 200 A= =40,000 cir. mils., or a No. 4 wire. 110 X 5 If this energy were used for ten hours a day for 300 days and the cost of the energy was eight cents per kw. hour, the total j'earlv cost would be 50 X 110 X 10 'x 300 X 0.08 —=$1320.00. 1000 Of this five per cent, or $66.00, would be lost yearlj' due to the drop. The cost of 400 feet No. 4 double braided, rubber covered wire would be about $21.00, so that the loss would be over three times the cost of the wire, and without anj- calculations it is easily seen that the system of wiring is not economical. We should use a large wire, thereby decreasing the voltage drop (about a No. 2 B. & S.). From the above it will be seen that although our wire, a No. 4, is of sufficient size to carry 50 amperes at 110 volts, we are having a drop in voltage of five per cent, or 22 volts, therefore, it will be seen that voltage is an important factor in regard to the size of wire. Our esteemed contemporary also says: What I mean hy a transformer influencing the sizes is that larger wires are necessary on the secondary than on the primary, due to the decrease in voltage and increase in amperage. (If that was all the designers and builders of transformers had to contend with, it v/ould be a cinch.) It is like taking candy from a kid, showing him how little he knows about electricity. It is a shame to do it. Well here goes his second piece of candy: The very important effects of electromagnetic inertia on the flow of alternating currents makes them appear to be more complex than direct currents. The point is so important that it must be given very careful attention. When a direct current is passed through an incandescent lamp, the amount of power expended by the passage of the electric current through the lamp filament, which is converted into heat and light, is equal to current X volts. In the same way, when alternating current is passed through an incandescent lamp, the amount of power which is expended in the lamp filament, and converted into heat and light is also equal to current X volts. We, therefore, see that an incandescent lamp which is intended to give 16 candle power at a pressure of say 110 volts will be equally efficient when it is connected to a constant pressure circuit which furnishes it continuous current at an uniform pressure of 110 volts, or when it is connected to a circuit which furnishes alternating current at an eft'ective pressure of 110 volts. Now suppose we take 200 feet of No. 7 B. & S. gauge insulated copper wire. Its resistance is almost exactly onetenth of an ohm at ordinary temperatures, and it, therefore, requires only one-tenth of a volt to send one ampere of direct current through it. This is true whether the wire is stretched out straight, wound in a simple coil, or wound around an iron core, since the resistance of the wire at a given temperature depends only upon its length, cross section and material, and none of these are altered by coiling or winding up the wire. To send one ampere of alternating current of, say a frequency of 125 periods per second (15,000 alternations per minute), through this wire when stretched out straight requires a tenth of a volt effective pressure, or the same as in the caseof a direct current. Now, if this wire is coiled up (as a transformer winding) a greater pressure than onetenth of a volt will be required to send one ampere through the wire, while if it is wound on a big laminated iron core there may be as much as 100 volts of even more required to send one ampere through the wire. We know that tlie resistance of the wire is not changed by coiling it up or by winding it on an iron core, so that the actual resistance is one-tenth of an ohm all the time. This is proved bj^ the fact that coiling the wire and winding it around an iron core does not change the amount of pressure required to send one ampere of direct current through it. This is where such things as current lag and impedence have to be dealt with. An alternating current changes all the time, so that it never has a steady value, and the effect of self-induction is, therefore, felt by it at all times. While the current is rising, self-induction tends to hold it back or keep it from rising, and when the current is falling self-induction still tends to keep it from changing. The result is that in a circuit having self-induction an alternating current is always retarded a certain amount behind the alternating pressure which sets it up. The current is said to have a lag. This same retardation causes the maximum value of the current to be smaller than it would be were there no self-induction effects. The effective current in an alternating circuit is equal to the effective electrical pressure applied to the circuit divided by the impedence of the circuit. The impedence is a combination of the true resistance of a wire composing the circuit with the effect due to self-induction. The true resistance of the wire depends only upon its length, cross section and material, while the effect of selfinduction depends upon the magnetic effect of the different parts of the circuit, and upon the frequency of the current. When a direct current flows through a circuit, the true resistance of the circuit only need be taken, but when an alternating current flows through the same circuit the impedence comes into account. From this it will be seen that our esteemed contemporary is up a tall tree with his electrical knowledge, when he says the size of wire used or a transformer depends only on the decrease in voltage and increase in amperage of the secondar}-. We sincerely hope he won't have a mental short circuit, or blow many fuses after reading this (because we are after him for fusing up too heavy on his feeders of electrical knowledge). It is a wonder his consumers (the operators) haven't got wise to the unreliable quality of juice (information) he has been sending out to them. His mental generator seems to be of poor electrical design going like the mischief but not developing any current. My boy, you had better rewind your generator or you will have an awful burn out one of these days.