Motography (Apr-Dec 1911)

74 MOTOGRAPHY Vol. V, No. 5. accord ; although sometimes it is necessary to knock it off with a screw-driver or file. The whole lamp should be tilted backward about fifteen degrees from the vertical. This allows the crater to face the condenser lens. This can be done if the lamp is not too far forward, so that it may touch the front wall of the lamp-house. There is a great diversity of opinion in regard to the best method of setting the carbons for alternating current work. In alternating current the upper carbo'n becomes alternately positive and negative; with a 60-cycle current this change takes place 120 times every second. It would seem from this that a crater is formed on both carbons. More often the carbons burn flat, or the crater is very poorly defined. In alternating current most of the light comes from the white hot ends of the carbons. Until within the last two years most operators set the carbons for alternating current work as in Fig. 4. This setting gives a higher candle-power than if set as in Fig. 2, but the arc cannot be controlled. The great difficulty is to keep the crater in front of the carbons when using over thirty amperes, as it too often wanders to the back of the carbon. For this reason it is probably better to set the carbons the same way as for direct, except that the lower carbon should be about vertical and the upper one parallel to the rack Fig. 5. bars of the lamp. The whole lamp should be tilted up about 15 degrees from the vertical. As before stated I would advise operators to experiment as much as possible before being satisfied with their light. But one thing you must remember — it takes from five to fifteen minutes for a crater to form ; therefore do not change the setting of the carbons every few minutes and expect a first-class light. The right angle lamp used for stereopticon work with carbon set as in Fig 3 is very satisfactory, as the positive carbon is always in the axis of the optical system, but it cannot be used with a high amperage such as is required for moving picture work. No matter which way you set the carbons care must be taken to see that they are in line sideways. This is very important as many blue streaks on the curtain can be traced to the non-alignment of carbons sideways. Use the points of the carbons as guides in setting them. In focusing the light it is best to use the screen as a guide. After you have struck the arc and get a clear, even white light over the entire screen, turn the focusing screw of the lens, watching the spot on the screen for results. AVhen you get a sharp line between the spot and the rest of the screen your lens is correctly focused. In Fig 5. 8 is the perfect focus. At / the light is too much toward the right throwing a shadow to the right. At 2 the light is too much toward the left ; at 3 it is too high ; at 4 too low ; at 6 and 7 the light is too far away from the condenser lens, showing a blue shadow in the middle of the screen. If this shadow is near the edge of the picture as at -5, light is too near the lens. The spot of light on the film gate should be just large enough to cover the corners. All light that does not enter the aperture is lost, therefore get as small a spot as possible. The spot should be round. An oblong spot on the film gate indicates that the carbons are out of line sideways. Above all try for a clear white spot and do not stop until you get it. There are two kinds of carbons used for projection work, the solid and the cored carbons. Briefly the process of manufacture of carbons is as follows : The material is soot produced by burning resin or oils with an insufficient amount of oxygen. This material is very finely pulverized and the iron impurities removed with a magnet. A binder of refined tar is then added. The whole mixture passes through heavy polished rollers and is then formed with a power hammer into blocks about 12 by 15 inches. The blocks are then put into a hydraulic press of about 4,000 pounds pressure to the square inch, and forced through a die the size of the carbon being made. In making cored carbons a steel needle the size of the core is placed in the center of the die. The carbon rods are made in lengths of a yard. After leaving the press they are rolled on a perfectly straight, smooth surface, straightening themselves. They are then tied in bundles and baked in ovens for several days at a temperature of about 2,500 degrees. After they are cut into stock lengths, they are tested for imperfections and pointed. Those to be cored have the mixture forced into the core. After again drying them, they are ready for packing and shipment. For direct current a cored carbon is used on the positive lead, and a solid on the negative lead. For alternating current a cored carbon is used on both leads. Carbons Y§ of an inch in diameter are almost universally used. There are many makes of projecting arc lamps on the market at present, each having its good and bad points. The trouble with most arc lamps is that after a few months use the rack bars generally become shaky and it is extremely difficult to keep the carbons in line sideways, which is very necessary in. alternating work. The carbon clamps or holders should be removed frequently, and the sides which make contact with the carbons thoroughly cleaned and scraped out with a file sothat good contact with the carbons will be made and no heat generated in the connection. The lamps should be taken apart about every two weeks and the parts thoroughly cleaned ; then apply a mixture of graphite and vaseline. Be careful in using the pliers on the thumb screws which tighten the holder so that the wings will not be broken off. Look to the insulation between the carbon holders and the lamp body, as the mica becomes broken and must be renewed. A'Vhen the insulation is imperfect a leakage is afforded for the current, and the lamp-house is liable to become charged. Many inexperienced operators have quite an amount of trouble with the lead connections. To begin with the holes in the binding posts are never large enough to receive a No. 6 stranded wire. If the bare wires are wound around the binding post good contact cannot be made, and also the bare ends of the fine wires become charred from the intense heat of the