International projectionist (Oct 1931-Sept 1933)

September 1932 INTERNATIONAL PROJECTIONIST IS which contains arc supporting but not light emitting materials. The negative carbon is set at an angle below the axis of the positive. The positive carbon is slowly rotated. When the arc is burned under these conditions, it appears as in Figure 3, which is a cross-sectional view. The positive carbon forms a deep uniform crater, which is of nearly the same diameter as the electrode. Both of these conditions, deep crater and crater diameter equal to the electrode diameter, are characteristic of the high-intensity arc and are produced and maintained in the following way: The core containing the flame material becomes an excellent conductor at high temperatures and, therefore, almost all of the current enters the arc through the positive electrode core. Since the core burns away rapidly due to the high current density, the shell remains as a wall, which thus forms the deep crater. The electrode is consumed at a rate which is so rapid that the oxygen of the surrounding air has little opportunity of causing the electrode to spindle or taper on the outside. By means of shrouding the electrode from the air up to within an inch of the crater and running at this abnormal current density, the crater diameter is kept very close to the electrode diameter. The continyous rotation of the arc keeps this crater wall uniform. The shell burns away to a sharp lip with concave inner walls, making almost a perfect hemispherical cavity. This uniform crater is essential to the high-intensity arc, since it acts as the container for the brilliant positive flame. The percentage of flame material in the core, the current density and the shell and core ratio are so adjusted that the supply of the positive vapor is at all times sufficient to fill to overflowing this positive crater. The flame, in fact, bulges out in front of the crater and is only confined by the continuous pressure of the negative flam.e against it. The establishment of a considerable pressure exerted by the negative flame against the positive vapor is a large factor in the production of the increased brilliancy of the positive flame. The continuous opposing forces exerted against each other by these two flames have a tendency to cause the flames to push each other aside, so that both may pass by each other without pressure. Any ordinary solid negative electrode will immediately give an unsteady crossed flame type of arc. Such an arc condition gives less than 50 per cent, of the candlepower of the normal high-intensity arc. To prevent this condition it has been necessary to utilize a core in the negative electrode for the purpose of giving the negative flame such a directive force that it cannot be pushed aside by the counterpressure of the positive vapor. The 3 mm. soft core in the negative carbon makes a seat on which the negative flame tends to rest and from which it is only displaced with great difficulty. Voltage Drop There is considerable energy consumed at all arcs, the voltage drop is usually divided into three steps: (1) the cathode drop, (2) the drop through the arc flame, and (3) the anode drop. In the pure carbon arc, these are: cathode, 9 volts; arc flame, about 16 volts (depending on arc length) ; anode drop, 35 volts; total, 60 volts. In the high-intensity arc the cathode drop is 9 volts and the drop across the negative flame is 16 volts, the same as in the carbon arc, but there is a drop of 37 volts through the brilliant positive flame in the crater and a drop of 13 volts at the core face or anode drop. The whole voltage drop within the crater is therefore 37 + 13, or 50 volts. Consider the current as 150 amperes and there is then 7,500 watts of energy concentrated within the ^-inch crater. This great energy concentration is largely responsible for the"^ high temperature obtainable within the crater. The other important feature of the high-intensity arc is to have chemicals available in the crater which can form compounds having very high volatilizing temperatures. Carbon volatilizes at 3.700 degrees C. and hence cannot give more than 180 candlepower per square millimeter, but certain carbides, such as the rare earth carbides, have even higher volatilizing points than carbon and it is the formation of these carbides that makes possible brilliancies so much greater than the carbon crater in the high-intensity arc. Cerium fluoride or cerium oxide is usually used in high intensity electrodes. The carbides themselves are unstable and cannot be used directly. The temperature at the crater of the arc, however, is so great that a chemical reaction takes place between the carbon and the cerium compound which forms cerium carbide just at the core face in the bottom of tbt crater. Colloidal particles of this carbide make the bright flame in the crater and give a brilliancy of over 500 candlepower per square millimeter. The temperature of this flame is approximately 5,000 degrees C. The "How" of It When the flame has overflowed from the crater and comes in contact with the air, the cerium carbide is immediately burned by the oxygen into cerium oxide and carbon dioxide. This is the reason Fig. 3. — Cross-sectional view of normal highintensity arc, 150 amperes the high-intensity arc was not thoroughly understood for many years ; the source of light is a compound created only at the instant it is needed in the crater and destroyed again the instant it leaves the crater. In the motion-picture field, the highintensity arc has had a most interesting history. First, in the studios, it entered as a super floodlight of high actinic value. The power of these high-intensity floods was one of the causes of the rapid expansion of motion-picture studios from the small set to the huge sets that were built during the flourishing years of the silent motion pictures, 1922 to 1928. In 1921, one or two of these high-intensity floods was enough to over-illuminate the average set; but within a few years' time the sets grew to such enormous size that banks of fifty to seventy high-intensity arc floods were used on single sets, aggregating as high as 10,000 amperes in high-intensity floods alone. With the coming of the sound motionpicture, spectacular effects were set aside for the spoken word, and the noise of the arc also interfered with the voice recording. Consequently, the arcs fell into disuse, but turning to the projection end of the industry, we find the "Talkies" had the opposite effect on high-intensity arcs. High-intensity arcs were first tried in motion-picture projection about 1921. They were needed especially for the newer motion-picture houses which were being built, with seating capacities of 3,000 or over. The motion picture industry was just outgrowing the "movie house" and moving into the motion picture "palace." Fortunately, the highintensity arc arrived just in time to make possible clear projection in these large houses. It was possible to increase the lumen output of the motion picture projector three times by changing the source of light from the carbon arc to the highintensity arc, so the latter became well established in all of the larger houses. When, however, sound motion pictures arrived, there still was further demaijd for increase of light. This was because the screen had to be made porous in order to let the sound come through it from the loud speakers located behind it, and the porosity which allowed the sound to come through in one direction unfortunately allowed the light to fall through in the other direction, and.