The motion picture projectionist (Nov 1931-Jan 1933)

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November, 1931 Motion Picture Projectionist 19 Principles of the Electric Arc In the article which follows the author gives a brief outline of the history and the development of the electric arc and discusses the properties and the principles underlying the arc phenomenon. The article does not restrict itself to the arc as known today in motion picture projection, but is somewhat more general in scope. Certain facts are presented concerning the color effects produced by impregnating the arc carbons with various mineral salts. The information supplied should prove helpful to the projectionist and perhaps suggest to him avenues along which interesting and profitable research may be conducted. — The Editor. THE phenomenon of the electric arc was first demonstrated by Sir Humphrey Davy in 1800 when he exhibited to the Royal Institute an apparatus by means of which a continuous spark was produced in a gap between two pointed pieces of charcoal whether the electrodes were exposed to the open air or immersed in water or other liquid. Continuing his research Davy succeeded in the year 1808 in producing an arc nearly four inches in length. Power for this arc was obtained from a battery composed of 2,000 elements. In 1843 carbon conductors, formed by the destructive distillation of coal, were used by Foucault. Later various substances were introduced into the carbon to increase the length of the arc and to steady it. Requirements for Arc The first essential required for the production of an electric arc is an electric current of sufficient tension to force its way across the gap or opening in which the arc is to be produced. Unless there is a very large difference in potential between these terminals, there must first be contact between the two carbons — that is, the arc must be "struck." The action produced in striking the arc is as follows: When the two carbons are brought together an electric current flows in the circuit. As they are separated, a minute spark is produced and a part of the carbon is volatilized permitting the current to pass from one electrode to the other by using this volatilized matter as a conducting medium. The heat thus produced is so intense that it is necessary to employ electrodes composed of highly refractory material in order to prevent their melting and to avoid too rapid vaporization. It has been found that carbon is the substance which best meets these requirements. An arc can be produced from either alternating or direct current. A By Arthur W. Schneider pressure of approximately 45 volts is required to maintain an arc between carbons exposed to the open air. If the carbon are enclosed so as to prevent the escape of the carbon vapor, as in the case of certain modern lamps used for street illumination and other purposes, a pressure of about 75 volts is needed. The current varies with the size of the carbons and with the applied electromotive force. When direct current is used the negative carbon assumes a pointed shape, but it is consumed only one half as rapidly as the positive carbon during the operation of the arc. A crater is formed in the positive carbon. Both carbons are incadescent at their tips and from this source considerable light is emitted. However, 85 percent of the light comes from the crater of the positive carbon. The arc itself contributes about 5 percent of the light and the remaining 10 percent comes from the tip of the negative carbon. Some commercial positive carbons are constructed with a hard shell and a soft core to facilitate the formation of a perfect crater. The effect of alternating current upon the carbons is quite different from the action to be observed when direct current is applied. No crater is formed on either carbon. The alternating current arc is much less efficient than the direct current arc in that it is extinguished for every reversal of the current. This means that if an alternating current of 60 cycles is applied, the arc will go out 120 times per second or once for every reversal or alternation of the current. Form and Color There are many factors which influence the shape and the color of an electric arc. If it is placed in a magnetic field, the tendency will be for the arc to spread out in a disc shape. This effect is produced by the fact that the electrons present in the arc vapor are attracted to the poles of the magnet. The magnetic field of the earth also influences the shape which an electric arc assumes. It is for this reason that the arc takes on its peculiar bow shape when the electrodes are set up in a vertical position. Incidentally, it is from its bow shape that the arc derives its name. The color of the arc is determined by the composition of its electrodes. In general, however, its li<rht resembles sunlight, but is richer in violet rays. A color analysis of the electric arc shows that its central portion is of a violet hue. This is the vapor of the carbon, which is rendered incandescent at the crater. Surrounding this violet colored region is a nonluminous area where a dark flame indicates that the oxygen of the external air is being combined with the carbon, and carbon monoxide is being further oxidized forming carbon dioxide. The so-called "flaming arc" is produced by impregnating the carbon electrodes with certain mineral salts. Its appearance is quite different from that of the common carbon arc. The arc flame itself is rendered intensely brilliant by the luminosity of its metallic vapors due to the great heat of the arc, and only a small portion of the light comes from the crater. Such arcs give from three to four times the illumination of the ordinary arc. The color of the arc depends upon the character of the salts used. It is usually, however, a dull orange color. A luminous arc lamp used extensively today is so constructed that an arc is maintained between a copper electrode and a magnetite electrode. There are also other substances which can be used as electrodes between which an arc can be established. It is in this class that most vacuum arcs may be included, such as the CooperHewitt mercury lamp. Structure of Carbons The ordinary carbon arc utilizes long cylindrical electrodes prepared from petroleum coke, gas coke, or lampblack. The raw materials are successively crushed, roasted, powdered, and bolted, then mixed with hot pitch. This compound is allowed to cool, and is then powdered, reheated and formed into molds or forced through dies into cylindrical forms, and is finally baked. The better grades of carbons are usually forced. In some cases the cored carbon mentioned in the earlier section of this paper is produced by filling the axial hole in the carbon with a mixture of powdered carbon and salt. This mixture volatilizes more readily than the basic carbon and improves the steadiness of the arc. In addition to this, it brings about the formation of a better crater. Various Types of Lamps In the "flaming" arc, as has been previously observed, the carbons are strongly impregnated with mineral salts. Calcium salts are used when an orange light is desired and barium salts are employed to produce a white light. Another type of arc lamp is the metallic oxide or magnetite arc. In this arc the positive electrode is made of copper, which remains inert and is not consumed to maintain the arc. (Continued on page 41)