Motography (Jan-Jun 1915)

480 MOTOGRAPHY Vol. XIII, No. 13. slightly over 3 seconds between flashes, which would be 4 feet apart. As a matter of fact you can time the flashes and tell what size racks the manufacturer used. UNEVEN SHRINKAGE. There are at least a dozen other defects, most of which are due to uneven shrinkage, with which everyone has to contend who adheres to the old rack and drum method, but extreme care has worked wonders, and fairly uniform films are now the rule, rather than the exception. When the exhibitor receives the tin boxes which contain the reels of a newly released "special feature" he passes them on to his "operator," who is the man behind the projection machine. The operator takes reel No. 1 and after removing it from the can, puts it in the upper "magazine," M, Figure 19, pulling out about three feet of "leader" to be "threaded" through the "gate," G, at the back of the machine between the light and the lens. From here it goes to the lower magazine, M1, where the end of the leader is fastened to the spool in the center. This spool is made to revolve while the film runs through Figure 21. The "head" of a projector. the machine so that all slack will be taken up, and the film "rewound" automatically as fast as it leaves the upper spool. After threading up it is only necessary to "frame" the picture by moving the handle H, and this is usually done after the light is turned on and the "carbons" adjusted. These carbons are in the "lamp-house," P, Figure 19, and when they are "lit" a wise operator never looks directly at them, but peeks through the dark glass or mica, E, near the handle of the door to the lamphouse, to see whether they are in proper adjustment. ADJUSTING THE ARC. There are several different adjustments to be made from time to time as the carbons burn. These are made by turning the round handles J, one of which adjusts the distance between the carbons, another moves both of them up and down, another moves them forward or back, while still another adjusts them sideways. And this is riot all, for in some there is an individual adjustment for each carbon, and a tilting device to get the angle just right, so that the full light from the "crater" will be sure to go through the film and thence to the screen. The crater is simply the little spot of boiling carbon near the point of the upper or positive carbon. It is only about % of an inch across, but hot enough and bright enough to be very interesting and important. Its temperature is somewhere between 6,000° and 9,000° Fahrenheit, more than enough to melt any thermometer ever made, and its brightness for projecting motion pictures is often from 10,000 to 15,000 candle power, or 1,000 times brighter than an ordinary gas flame, and 100 times brighter than a Wellsbach mantle. The electricity that passes from one carbon to the other, making the "arc," goes first through the "Rheostat," R, Figure 19, because if it went to the carbons directly from the service mains it would be too strong and the arc would sputter and give poor light. THE RHEOSTAT. This rheostat is also known as a "resistance coil" and is simply a series of coils of wire which resist the passage of the electric current. Iron and German silver are much used, but not copper, because copper is too good a "conductor." On the other hand, copper is just the thing to use after we have reduced the "voltage" of the current to the proper intensity, and wish to transmit it, without further loss, to the arc. So copper is nearly always used for conductors, and iron for ordinary coarse resistances. Cost has something to do with these choices, for while genuine pure silver conducts more freely than copper, its cost is out of all proportion to the advantages. German silver (which is not silver at all, but an alloy of copper, zinc and nickel) is not nearly so expensive and is often used in place of iron for resistances. The action of a rheostat is much like the effect of a very small pipe or tube on the passage of a fluid such as water or gas. The smaller and rougher the pipe the more the "pressure" will be reduced, and the less fluid will get through. So with the electric current or fluid, the "voltage" corresponds to the pressure of water, and the smaller the wire and the coarser the material (electrically speaking) the less "juice" will get through. Iron is, so to speak, "rough," and copper, "smooth," to the passage of the current. ELECTRICAL TERMS. In this connection it may be well to explain a few other electrical terms. "Amperage" corresponds to the rate of flow of other fluids, just as voltage corresponds to pressure, and to say a current has so many "amperes" means that so many "units" of electric "juice" will get by in a certain time at a certain voltage, but if the voltage goes up more units will pass. Therefore, in order to designate how much actual quantity is consumed, or how many units pass over a wire, or through some device in a stated time, we have another measure which is termed a "watt," and for commercial purposes it is so small that they bunch them in thousand lots, and call them "kilowatts." A watt is an ampere multiplied by a volt, so that a current of 100 volts pressure, and a volume of 10 amperes would be called a kilowatt, or 1,000 watts. For purposes of charging consumers of electricity according to total quantities used, meters are made which record the numbers of kilowatt-hours, or units, which correspond to the number of gallons of water which go through your water meter. "Voltage," then, means pressure or intensity; "amperage" means capacity, or the size and "smoothness" of the conduction ; "wattage" means rate of flow, and is