International projectionist (Jan 1959-Dec 1960)

NOTE: Reprints of this article are available at a nominal cost. Minimum order quantity: 100. and accordingly produce light and other forms of radiation only by their effect upon the matter which they encounter. During their passage from the tip of one carbon to the tip of the other, impelled by the positive charge of the positive carbon, the electrons encounter various regions of electrical resistance where friction turns some of the energy of their flow into heat and visible radiation. The pointed tip of the negative carbon is heated by the resistance which the electrons meet as they pass from the highly conductive solid carbon to the less conductive gases of the arc stream. The arc stream, itself, has a very high temperature (6000° C = 11,000° F) because of frequent collisions of electrons with gas atoms. Only a small percentage of the electrons smash into the gas atoms "head on," however, and the total amount of heat in the arc stream (calories) is less than the amount of heat generated in the positive crater. Crater "Anode Layer" The anatomy of the positive crater, even in the case of a low-intensity arc, is rather complex. If the electrons passed directly from the arc stream to the more conductive positive carbon, it would be difficult to understand how the crater could get so hot. Once arrived in the positive crater, the bullet-like electrons encounter a dense, relatively cool (3600° C = 6500° F) film of carbon gas that offers considerable resistance to their passage. This gaseous skin is called the anode layer, an "anode" being a positively-charged electrode. (A negatively-charged electrode is a "cathode.") The electrons are slowed down by friction; and the energy which they lose is absorbed by the anode layer and turned into heat. It may seem strange that the anode layer should be cooler than the crater floor and arc stream, but such is the case. The anode layer has a relativelylow heat-retaining capacity, so it transfers the heat generated by the electrons to its surroundings, especially to the solid crater floor which it covers like a thin atmosphere. The solid carbon then gets hotter and hotter until it attains a temperature of 3900° C = 7000° F at the surface of the crater floor. * At this high temperature solid carbon readily "sublimes," or turns into gas without first melting to a liquid. Many substances, such as naphthalene moth balls and camphor, sublime even at room temperature! Now, the vaporized carbon passes into the anode layer where it transfers electron-generated heat to the crater floor. It then evaporates into the tail-flame of the arc and cools the remaining anode gases as it does so. The heated carbon vapor of the tailflame burns to carbon-dioxide gas the moment it comes in contact with the cxygen of the air, but the heat of this combustion is very small compared to the heat of the arc itself. The anode layer of dense carbon gas may be considered as a sort of heat-transferring mechanism which turns the kinetic energy of electrons into heat and then transfers the heat energy to the solid positive carbon. The crater thereby becomes hot enough to radiate a white light which is bright enough to make possible the projection of motion pictures on large theatre screens. Principle of H-I Arc The Beck, or H-I, arc works on the same basic principle as a L-I arc. Contrary to popular opinion, the temperatures prevailing in a H-I arc are no higher than those in a L-I arc. The only difference is the addition to the core of the H-I positive carbon special light -producing ingredients which form an intensely brilliant ball-like flame emitting approximately three times more light than the white-hot crater of solid carbon! This remark * The positive crater of o carbon arc is not as hot as the surface of the sun, as has sometimes been stated. The sun's surface temperature is 6000°C = 11,000° F and has a "color temperature" matched by the less hot H-I arc. able light-producing core material consists of compressed powdered carbon with which rare-earth compounds* are mixed. Remember the old-fashioned Welsbach gas mantles used for household illumination many years ago? A bare gas flame gives out very little light despite its high temperature. (Unlike hot solids, hot gases are poor radiators of light.) The bare gas flame is analogous to a L-I arc. But when a mantle is placed in the flame, the rare-earth compounds with which it has been impregnated are excited by heat into brilliant luminescense. Somewhat the same process takes place in the "whiteflame" and H-I arcs. The chief electrical peculiarity of the H-I arc is the effect of the luminescent rare-earth atoms on the resistance of the relatively cool anode layer of carbon gas which, as we have seen, plays a very important part in the burning of any carbon arc. In fact, without the electrical resistance of the anode layer, the electrons would pass from the arc stream into the highly conductive positive carbon without changing much of the electron-energy into heat and light. H-I Current Requirements Rare-earth atoms are fairly good conductors of electricity when excited by high temperatures. The resistance of the anode layer is accordingly lowered and the transfer of energy less efficient than in the L-I arc using plain carbons. To maintain a fair amount of voltage-drop in the anode layer of the H-I arc, therefore, the voltage of a * The rare earths comprise a group of 15 iron-gray metals having similar physical and chemical properties. Cerium and lanthanum are the most abundant. Thorium and such rarer aluminum-like metals as scandium and yttrium are associated with the rare earths. You very likely have some rare-earth metals in your pocket. Cigarette-lighter flints are rare-earth alloys. Tail flame Bright gas ball Delineating the anatomy of a high-intensity carbon arc. HI Positive Crater floor Anode gas layer Arc stream INTERNATIONAL PROJECTIONIST • JANUARY 1959