International projectionist (Jan-Dec 1935)

26 REVIEW NUMBER May 193S 'Ground' Testing Made Easy By M. D. O' BRIEN supply to the lamps, the slight depositing of carbon ash may in time build up to a current-carrying path having very little, if any, resistance value — and without being detected. This is possible because of the generator leads being entirely isolated from a "true ground." But should a ground develop on that polarity of a generator opposite to that which was formed in the lamp burner, not only will serious damage to the lamp result but the generator may be temporarily put out of service by reason of its fields or armature being burned out. In the , event that a theatre is equipped with but one generator, there can be only one result — complete shutdown. The foregoing emphasizes anew the urgent necessity for constant inspection and cleaning by the projectionist of the lamp burners and the lamphouse. A majority of projection rooms today are equipped with a portable voltmeter. Ten minutes of your time with this handy little instrument is all that is required to determine positively whether current-carrying parts are ground-free. A handy little ground detector which may be permanently installed in the projection room can be made up of two OF ALL projection troubles none is more difficult to solve and more productive of unnecessary expense than the "ground." A ground will begin to build itself up in the most unlooked-for places and when least expected. Arc lamps are a .common source of grounds. Here the constant consumption of carbons creates and deposits over the entire lamp a certain amount of current-carrying ash which adheres like talcum powder and covers both metal parts and insulated joints. When a slight coating of carbon ash stretches from current-carrying metal across mica and fibre insulators to the frame of the lamphouse, a path is established for the flow of current which in its early stages has a reasonable amount of resistance. In systems employing straight D.C. as a source of supply, however — and particularly where a grounded neutral is part of the system — this slight ground can set up sufficient arcing from current-carrying parts to the grounded frame of the lamphouse to seriously burn and melt the metal and even destroy the insulating properties of mica. Such a condition would constitute a "dead short-circuit" (depending upon which polarity of the lamp is grounded) , and would necessitate complete disassembly of the lamp burner to replace defective parts and insulators. rp0 FmD thg ]ight factor o{ yQur arC) that ig the important thingj Decause by ref bupposing this should occur during A multiply the illuminated area of your erence to this figure you can determine a performance: JNeedless to say, the lamp sockets and two lamps. Connect the two lamp sockets in series across the positive and negative busses of the generator supply" panel. Then run a lead from the center of the two sockets to a good mechanical ground (water pipe, etc. ) . Insert a clear 25-watt Mazda-type filament lamp in each socket. Under normal conditions both lamps will burn with equal brilliancy. But should a ground develop on one leg of the service, that lamp which is connected to the opposite polarity will bum more brightly than the other. By the simple process of opening one projector, or spot, switch at a time it is a comparatively easy matter to determine which circuit is grounded. Should all switches be opened and a ground still be evident in the detector, this will indicate that the generator or its circuit is grounded. If some such device as this is utilized, you will enjoy the advantage of a continuous ground detector — constantly in full view. A "swinging ground" will be indicated through this device by an intermittent change in the brilliancy of the two lamps. If so desired, a fuse may be inserted ahead of each lamp to serve as a further precautionary measure. However, this is not absolutely necessary. The ground detector device described in the foregoing is not suitable for use on 3-wire, grounded neutral systems. Arc Light Factor Calculation resultant interruption of the performance with its attendant refund and inevitable loss of prestige would seriously injure the goodwill which had been built up by a theatre over a long period of time. Repair expense is, of course, an important factor, but this item cannot be compared in importance with probable loss of prestige. In the case of a theatre employing motor generators as a source of power From I. P., May, 1932 lultiply the illuminated area ot your screen in square feet by the average intensity in foot-candles to which that screen is raised, and you will arrive at the lumens being projected thereon. For example, you would be using 3,740 lumens to get 10 foot-candles on a screen 22 feet by 17 feet. Supposing that your arc was using 75 amperes at 55 arc volts, your wattage would be 4,125, and you would therefore be obtaining approximately .9 lumen on the screen for every watt consumed in the arc. Now, it is this ratio of lumens to watts Relation of Electron Flow to Current Flow (Continued from preceding page) electrons. But the positive charge is nothing but a lack of electrons on the atom. Definitive Terms Now, if we must have something to pin our imagination to in order to understand electron flow and current flow, think of the motion of the electrons as the electron flow, and the motion of the positive charge from atom to atom as the current flow. That is, as a matter of definition, let "current flow" mean the motion of the positive charge around the circuit, and "electron flow" mean the motion of the electron around the circuit. Current, flowing from positive to negative, is measured in amperes, which is just another way of saying that it is measured in coulombs-per-second. The coulomb is the unit of electrical quality. It takes about 6,290,000,000,000,000,000 electrons to equal one coulomb, so if that many electrons passed a given point in the circuit in one second of time it would be equivalent to one coulomb-per-second passing that point, or as we usually say, the current is one ampere. Since the more electrons there are passing the point per second the more amperes there are flowing, it can readily be seen that the so-called current flowing from positive to negative depends entirely on the flow of the electrons from negative to positive, both as regards the direction of flow and the value of the current in amperes. figure you the electrical efficiency or light factor of your carbons. Supposing, in the example we have just taken, that you fit another pair of carbons, and find that for the same arc wattage, you can obtain 12 foot-candles. Your lumens will now be 4,488, and your light factor will be 1.0 lumen per watt. Conversely, if you found that with the new carbons you could still get 10 foot-candles for an expenditure of only 70 amps., 50 arc volts, your wattage would be 3,500, and your light factor at 3,740 lumens would again be approximately 1.0 lumen per watt. Having therefore determined the light factor of your carbons, the next thing to consider is burning rate. If you burned two pairs of carbons of different make but of the same size for an hour, the difference in burning rate might not be very marked, but the slower-burning carbon may well give you an extra two or three-reel run, and leave no appreciable waste end. Naturally, the most accurate check of screen illumination is to be had through use of a photometer, but very few theatres have such a device. In the absence of such equipment, however, the foregoing information will serve the purpose admirably. Many projectionists try to estimate costs merely by a comparison of carbon prices, without regard for the factors of burning time and "juice" costs, which are absolutely necessary for any fair evaluation of costs. From I. P., September, 1933